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Title: Pyrometry - A Practical Treatise on the Measurement of High Temperatures
Author: Darling, Chas. R.
Language: English
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PYROMETRY
_BY THE SAME AUTHOR_
HEAT FOR ENGINEERS
A TREATISE ON HEAT WITH
SPECIAL REGARD TO ITS
PRACTICAL APPLICATIONS
Third Edition, revised, with 110 illustrations,
xiv + 430 pp. Demy 8vo. Price 12s. 6d. net.
LIQUID DROPS AND
GLOBULES
THEIR FORMATION AND MOVEMENT
THREE LECTURES DELIVERED
TO POPULAR AUDIENCES
With 43 illustrations, x + 84 pp. Crown 8vo,
cloth. Price 3s. net.
E. & F. N. SPON, LTD., HAYMARKET, LONDON, S.W. 1
PYROMETRY
A PRACTICAL TREATISE ON
THE MEASUREMENT OF HIGH
TEMPERATURES
BY
CHAS. R. DARLING
ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, DUBLIN; WHITWORTH
EXHIBITIONER; FELLOW OF THE INSTITUTE OF CHEMISTRY; FELLOW
OF THE PHYSICAL SOCIETY, ETC.
LECTURER IN PHYSICS AT THE CITY AND GUILDS TECHNICAL COLLEGE,
FINSBURY, E.C.
AUTHOR OF “HEAT FOR ENGINEERS”
SECOND EDITION, REVISED AND ENLARGED
SIXTY-NINE ILLUSTRATIONS
[Illustration]
_London_
E. & F. N. SPON, Ltd., 57 HAYMARKET, S.W. 1
_New York_
SPON & CHAMBERLAIN, 120 LIBERTY STREET
1920
Contents
PAGE
PREFACE TO SECOND EDITION ix
PREFACE TO FIRST EDITION xi
CHAP.
I. INTRODUCTION 1
II. STANDARDS OF TEMPERATURE 9
Absolute or Thermodynamic Scale—Constant Volume
Gas Thermometer—Fixed Points for Calibration—National
Physical Laboratory Scale—Temperatures
above the Present Limit of the Gas Thermometer.
III. THERMO-ELECTRIC PYROMETERS 20
General Principles—Metals used for Thermal Junctions—
Changes in Thermal Junctions when constantly
used—Electromotive Force developed by
Typical Junctions—Practical Forms of Thermocouples—
Liquid Element Thermocouples—Indicators for
Thermo-electric Pyrometers—Special Features of
Indicators—Standardizingof Indicators to read
Temperatures directly—Standardization by Fixed Points—
Standardization by Measurement of E.M.F.—Cold-Junction
Compensators—Constant Temperature Cold Junctions—
Special-Range Indicators—PotentiometerIndicators—
Recorders for Thermo-electric Pyrometers—The Thread
Recorder—The Siemens Recorder—Foster’s Recorder—
Paul’s Recorder—The Leeds-Northrup Recorder—
Control of Furnace Temperatures—Contact-Pen Recorders—
Installations of Thermo-electric Pyrometers—Management
of Thermo-electric Pyrometers—Laboratory Uses of
Temperatures Thermo-electric Pyrometers—Measurement of
Lower by the Thermo-electric Method—Measurement of Surface
of Surface Temperatures—Measurement of Low Temperatures—
Temperature of Steam, Exhaust Gases—Measurement of
Differences of Temperature—Advantages of the
Thermo-electric Method of Measuring Temperatures.
IV. RESISTANCE PYROMETERS 101
General Principles—Measurement of Resistance by
the Differential Galvanometer—Measurement of Resistance
by the Wheatstone Bridge—Relation between
Resistance of Platinum and Temperature—Changes
in Resistance of Platinum when constantly Heated—Terms
used in Resistance Pyrometry—Practical Forms
of Resistance Pyrometers—Indicators—Siemens’
Indicator—Whipple’s Indicator—The Harris Indicator—
The Leeds-Northrup Indicator—Siemens’ Differential
Indicator—Recorders for Resistance Pyrometers—The
Leeds-Northrup Recorder—Paul’s Recorder—Installation
of Resistance Pyrometers—Management of Resistance
Pyrometers—Special Uses of Resistance Pyrometers.
V. RADIATION PYROMETERS 134
General Principles—Practical Forms of Radiation
Pyrometers—Féry’s Mirror Pyrometer—Féry’s Spiral
Radiation Pyrometer—Foster’s Fixed-Focus Radiation
Pyrometer—Paul’s Radiation Pyrometer—Indicators
for Radiation Pyrometers—Calibration of Indicators—
Recorders—Management of Radiation Pyrometers—Special
Uses of Radiation Pyrometers.
VI. OPTICAL PYROMETERS 167
General Principles—Wien’s Law—Practical Forms of
Optical Pyrometers—Féry’s Optical Pyrometer—Le
Chatelier’s Optical Pyrometer—Wanner’s Pyrometer—
Cambridge Optical Pyrometer—Holborn-Kurlbaum
Pyrometer—Lovibond’s Pyrometer—Mesuré and
Nouel’s Pyrometer—Colour-extinction Pyrometers—
Management of Optical Pyrometers—Special Uses of
Optical Pyrometers.
VII. CALORIMETRIC PYROMETERS 195
General Principles—Practical Forms—Siemens’
Calorimetric or “Water” Pyrometer—Special Uses.
VIII. FUSION PYROMETERS 204
General Principles—Seger Pyramids or “Cones”—
Watkin’s Heat Recorder—“Sentinel” Pyrometers—
Stone’s Pyrometer—Fusible Metals—Fusible Pastes.
IX. MISCELLANEOUS APPLIANCES 211
Expansion and Contraction Pyrometers—Wedgwood’s
Pyrometer—Daniell’s Pyrometer—Northrup’s Molten
Tin Pyrometer—Vapour-Pressure Pyrometers—Water-Jet
Pyrometers—Pneumatic Pyrometers—Conduction
Pyrometers—Gas Pyrometers—Wiborgh’s Thermophones—
Joly’s Meldometer—Brearley’s Curve Tracer.
INDEX 222
Preface to the Second Edition
Since the publication of the first edition in 1911, a great extension
has been witnessed in the use of pyrometers in industrial processes
and laboratory work, to which development the author hopes his book
has contributed in some measure. During the stress occasioned by the
war, pyrometers have proved invaluable in many processes, and British
makers were fully able to meet the demands, owing to the status
attained in pre-war days. The increasing use of pyrometric appliances
renders necessary some book of reference which will provide the user
with information to enable him to get the best results out of his
instruments, and it is hoped that the present treatise meets this need.
In preparing the second edition, certain parts have been revised in
conformity with modern practice, and the later developments included.
The scope of the book remains as before.
The author desires to acknowledge the assistance he has received from
the British makers of pyrometers, all of whom have liberally provided
him with information of a most useful kind, of which he has availed
himself in the production of the present edition.
CHAS. R. DARLING.
WOOLWICH, 1920.
Preface to the First Edition
The present treatise has been founded on a course of Cantor Lectures
on “Industrial Pyrometry,” delivered by the author before the Royal
Society of Arts in the autumn of 1910. The practice of pyrometry in
recent years has progressed at a greater rate than the literature
bearing upon it; and the author is not aware of the existence of
any other book written in English which treats the subject from
the standpoint of the actual daily use of the instruments. In the
succeeding pages the exact measurement of temperature, as an end in
itself, is made subordinate to the practical utility of pyrometers
in controlling various operations; and consequently descriptions of
appliances of interest only theoretically have either been omitted, or
at the most briefly described. Nevertheless, the fundamental principles
are in all cases fully explained, as an understanding of these is
essential to the intelligent use of the appliances dealt with in the
book. When necessary, numerical examples are given to illustrate the
applications of the principles; and the reader who finds any difficulty
in following the various explanations—which of necessity involve an
understanding of many portions of the subject of heat—is referred to
the author’s treatise on “Heat for Engineers,” issued by the publishers
of the present volume.
With regard to temperature scales, the author has in the main employed
Centigrade degrees, but has recognised that the Fahrenheit degree is
still largely used, and has therefore frequently expressed temperatures
in terms of both scales.
The number of those who find it an advantage in their calling to
measure and control high temperatures is constantly increasing; and
the manufacture of pyrometric appliances now gives employment to
considerable numbers. The author trusts that the present treatise will
prove of service to all thus concerned, and also to those who pursue
the fascinating study of high temperature measurement from the purely
scientific standpoint.
In conclusion, the author expresses his thanks to the various firms,
mentioned in the text, who have loaned blocks for the purpose of
illustration, and who have furnished him with much valuable information.
CHAS. R. DARLING.
WOOLWICH, 1920.
PYROMETRY
CHAPTER I
INTRODUCTION
The term “pyrometer”—formerly applied to instruments designed to
measure the expansion of solids—is now used to describe any device
for determining temperatures beyond the upper limit of a mercury
thermometer. This limit, in the common form, is the boiling point of
mercury: 357° C. or 672° F. By leaving the bore of the tube full of
nitrogen or carbon dioxide prior to sealing, the pressure exerted by
the enclosed gas when the mercury expands prevents boiling; and with a
strong bulb of hard glass the readings may be extended to 550° C. or
1020° F. Above this temperature the hardest glass is distorted by the
high internal pressure, but, by substituting silica for glass, readings
as high as 700° C. or 1290° F. may be secured. Whilst such thermometers
are useful in laboratory processes they are too fragile for workshop
use; and if made of the length necessary in many cases in which the
temperature of furnaces is sought, the cost would be as great as that
of more durable and convenient appliances. No other instrument, however,
is so simple to read as the thermometer; and for this reason it is
used whenever the conditions are favourable. The latest proposal in
this direction is due to Northrup, who has constructed a thermometer
containing tin enclosed in a graphite envelope, which is capable of
reading up to 1500° C. or higher. This instrument is described on page
216.
The origin and development of the science of pyrometry furnish
a notable example of the value of the application of scientific
principles to industry. Sir Isaac Newton was the first to attempt
to measure the temperature of a fire by observing the time taken to
cool by a bar of iron withdrawn from the fire; but, although Newton’s
results were published in 1701, it was not until 1782 that a practical
instrument for measuring high temperatures was designed. In that year
Josiah Wedgwood, the famous potter, introduced an instrument based on
the progressive contraction undergone by clay when baked at increasing
temperatures, which he used in controlling his furnaces, finding it
much more reliable than the eye of the most experienced workman. This
apparatus (described on page 211) remained without a serious rival for
forty years, and its use has not yet been entirely abandoned.
The next step in advance was the introduction of the expansion
pyrometer by John Daniell in 1822. The elongation of a platinum rod,
encased in plumbago, was made to operate a magnifying device, which
moved a pointer over a scale divided so as to read temperatures
directly. Although inaccurate as compared with modern instruments, this
pyrometer was the first to give a continuous reading, and required no
personal attention. The expansion pyrometer—with different expanding
substances—is still used to a limited extent.
The year 1822 was also marked by Seebeck’s discovery of
thermo-electricity. The generation of a current of electricity by
a heated junction of two metals, increasing with the temperature,
appeared to afford a simple and satisfactory basis for a pyrometer,
and Becquerel constructed an instrument on these lines in 1826.
Pouillet and others also endeavoured to measure temperatures by the
thermo-electric method, but partly owing to the use of unsuitable
junctions, and partly to the lack of reliable galvanometers, these
workers failed to obtain concordant results. The method was for all
practical purposes abandoned until 1886, when its revival in reliable
form led to the enormous extension of the use of pyrometers witnessed
during recent years.
In 1828 Prinsep initiated the use of gas pyrometers, and enclosed the
gas in a gold bulb. Later workers used porcelain bulbs, on account of
greater infusibility, but modern research has shown that porcelain is
quite unsuitable for accurate measurements, being porous to certain
gases at high temperatures, even when glazed. Gas pyrometers are
of little use industrially, but are now used as standards for the
calibration of other pyrometers, the bulb being made of an alloy of
platinum and rhodium.
Calorimetric pyrometers, based on Regnault’s “method of mixtures,”
were first made for industrial purposes by Byström, who patented an
instrument of this type in 1862. This method has been widely applied,
and a simplified form of “water” pyrometer, made by Siemens, is at
present in daily use for industrial purposes. It is not capable,
however, of giving results of the degree of accuracy demanded by many
modern processes.
The resistance pyrometer was first described by Sir W. Siemens in 1871,
and was made by him for everyday use in furnaces. Many difficulties
were encountered before this method was placed on a satisfactory
footing, but continuous investigation by the firm of Siemens & Co.,
and also the valuable researches of Callendar and Griffiths, have
resulted in the production of reliable resistance pyrometers, which are
extensively used at the present time.
In 1872 Sir William Barrett made a discovery which indirectly led to
the present development of the science of pyrometry. Barrett observed
that iron and steel, on cooling down from a white heat, suddenly became
hotter at a definite point, owing to an internal molecular change; and
gave the name of “recalescence” to the phenomenon. Workers in steel
subsequently discovered that this property was intimately connected
with the hardening of the metal; thus Hadfield noticed that a sample of
steel containing 1·16 per cent. of carbon, when quenched just below the
change-point was not hardened, but when treated similarly at 15° C.
higher it became totally hard. The demand for accurate pyrometers in
the steel industry followed immediately on these discoveries, for even
the best-trained workman could not detect with the eye a difference in
temperature so small, and yet productive of such profound modification
of the properties of the finished steel. In this instance, as in many
others, the instruments were forthcoming to meet the demand.
The researches of Le Chatelier, published in 1886, marked a
great advance in the progress of pyrometry. He discovered that a
thermo-electric pyrometer, satisfactory in all respects, could be
made by using a junction of pure platinum with a rhodioplatinum
alloy, containing 10 per cent. of rhodium; a d’Arsonval moving-coil
galvanometer being used as indicator. This type of galvanometer,
which permits of an evenly-divided scale, is now universally employed
for this purpose, and has made thermo-electric pyrometers not only
practicable, but more convenient for general purposes than any other
type. Continuous progress has since been made in connection with this
method, which is now more extensively used than any other.
Attempts to deduce temperature from the luminosity of the heated body
were first made by Ed. Becquerel in 1863, but the method was not
successfully developed until 1892, when Le Chatelier introduced his
optical pyrometer. This instrument, being entirely external to the
hot source, enabled readings to be taken at temperatures far beyond
the melting point of platinum, which would obviously be the extreme
limit of a pyrometer in which platinum was used. The quantitative
distribution of energy in the spectrum has since been worked out by
Wien and Planck, who have furnished formula based on thermodynamic
reasoning, by the use of which optical pyrometers may now be calibrated
in terms of the thermodynamic scale of temperature. Other optical
pyrometers, referred to in the text, have been devised by Wanner,
Holborn and Kurlbaum, Féry, and others; and the highest attainable
temperatures can now be measured satisfactorily by optical means.
The invention of the total-radiation pyrometer by Féry in 1902 added
another valuable instrument to those already available. Based on the
fourth-power radiation law, discovered by Stefan and confirmed by
the mathematical investigations of Boltzmann, this pyrometer is of
great service in industrial operations at very high temperatures,
being entirely external, and capable of giving permanent records.
Modifications have been introduced by Foster and others, and the method
is now widely applied.
Recorders, for obtaining permanent evidence of the temperature of a
furnace at any time, were first made for thermo-electric pyrometers by
Holden and Roberts-Austen, and for resistance pyrometers by Callendar.
Numerous forms are now in use, and the value of the records obtained
has been abundantly proved.
For scientific purposes, all pyrometers are made to indicate Centigrade
degrees, 100 of which represent the temperature interval between
the melting-point of ice and the boiling point of water at 760 mm.
pressure, the ice-point being marked 0° and the steam-point 100°.
In industrial life, however, the Fahrenheit scale is often used in
English-speaking countries, the ice-point in this case being numbered
32° and the steam-point 212°; the interval being 180°. A single degree
on the Centigrade scale is therefore 1·8 times as large as a Fahrenheit
degree, but in finding the numbers on each scale which designate a
given temperature, the difference in the zero position on the two
scales must be taken into account. When it is desired to translate
readings on one scale into the corresponding numbers on the other, the
following formula may be used:—
(C. reading) (F. reading - 32)
———————————— = ————————————————— .
5 9
Thus by substituting in the above expression, 660° C. will be found to
correspond to 1220° F. and 1530° F. to 832° C.
It is greatly to be regretted that all pyrometers are not made to
indicate in Centigrade degrees, as confusion often arises through the
use of the two scales. An agreement on this point between instrument
makers would overcome the difficulty at once, as the Centigrade scale
is now so widely used that few purchasers would insist on Fahrenheit
markings.
It may be pointed out here that no single pyrometer is suited to every
purpose, and the choice of an instrument must be decided by the nature
of the work in hand. A pyrometer requiring skilled attention should not
be entrusted to an untrained man; and it may be taken for granted that
to obtain the most useful results intelligent supervision is necessary.
In the ensuing pages the advantages and drawbacks of each type will be
considered; but in all cases it is desirable, before making any large
outlay on pyrometers, to obtain a competent and impartial opinion as
to the kind best suited to the processes to be controlled. Catalogue
descriptions are not always trustworthy, and instances are not wanting
in which a large sum has been expended on instruments which, owing to
wrong choice, have proved practically useless. An instrument suited to
laboratory measurements is often a failure in the workshop, and all
possibilities of this kind should be considered before deciding upon
the type of pyrometer to be used.
CHAPTER II
STANDARDS OF TEMPERATURE
=The Absolute or Thermodynamic Scale of Temperatures.=—All
practical instruments for measuring temperatures are based on some
progressive physical change on the part of a substance or substances.
In a mercury thermometer, the alteration in the volume of the liquid
is used as a measure of hotness; and similarly the change in volume
or pressure on the part of a gas, or the variation in resistance to
electricity shown by a metal, and many other physical changes, may be
employed for this purpose. In connection with the measurement of high
temperatures, many different physical principles are relied upon in the
various instruments in use, and it is of the greatest importance that
all should read alike under the same conditions. This result would not
be attained if each instrument were judged by its own performances. In
the case of a mercury thermometer, for example, we may indicate the
amount of expansion between the temperatures of ice and steam at 76
centimetres pressure, representing 100° Centigrade, by _a_; and then
assume that an expansion of 2_a_ will signify a temperature of 200°,
and so on in proportion. Similarly, we may find the increase in
resistance manifested by platinum between the same two fixed points,
and indicate it by _r_, and then assume that an increase of 2_r_ will
correspond to 200°. If now we compare the two instruments, we find
that they do not agree, for on placing both in a space in which the
platinum instrument registered 200°, the mercury thermometer would show
203°. A similar, or even greater, discrepancy would be observed if
other physical changes were relied upon to furnish temperature scales
on these lines, and it is therefore highly desirable that a standard
independent of any physical property of matter should be used. Such a
standard is to be found in the thermodynamic scale of temperatures,
originally suggested by Lord Kelvin. This scale is based upon the
conversion of heat into work in a heat engine, a process which is
independent of the nature of the medium used. A temperature scale
founded on this conversion is therefore not connected with any physical
property of matter, and furnishes a standard of reference to which all
practical appliances for measuring temperatures may be compared.[1]
When readings are expressed in terms of this scale, it is customary to
use the letter K in conjunction with the number: thus 850° K would mean
850 degrees on the thermodynamic scale.
When existing instruments are compared with this standard, it is found
that a scale based on the assumption that the volume of a gas free
to expand, or the pressure of a confined gas, increases directly as
the temperature is in close agreement with the thermodynamic scale.
It may be proved that if the gas employed were “perfect,” a scale in
exact conformity with the standard described would be secured; and
gases which approach nearest in properties to a perfect gas, such
as hydrogen, nitrogen, and air, may therefore be used to produce a
practical standard, the indications of which are nearly identical with
the thermodynamic scale. If any other physical change be chosen, such
as the expansion of a solid, or the increase in resistance of a metal,
and a temperature scale be based on the supposition that the change
in question varies directly as the temperature, the results obtained
would differ considerably from the absolute standard. For this reason
the practical standard of temperature now universally adopted is an
instrument based on the properties of a suitable gas.
=The Constant Volume Gas Thermometer.=—In applying the properties
of a gas to practical temperature measurement, we may devise some
means of determining the increase in volume when the gas is allowed to
expand, or the increase in pressure of a confined gas may be observed.
The latter procedure is more convenient in practice, and the instrument
used for this purpose is known as the constant volume gas thermometer,
one form of which is shown in fig. 1. The gas is enclosed in a bulb B,
connected to a tube bent into a parallel branch, into the bend of which
is sealed a tap C, furnished with a drying cup. The extremity of the
parallel branch is connected to a piece of flexible tubing T, which
communicates with a mercury cistern which may be moved over a scale,
the rod G serving as a guide. In using this instrument the bulb B
is immersed in ice, and the tap C opened. When the temperature has
fallen to 0° C., the mercury is brought to the mark A by adjusting
the cistern, and the tap C then closed. The bulb B is now placed in
the space or medium of which the temperature is to be determined, and
expansion prevented by raising the cistern so as to keep the mercury
at A. When steady, the height of the mercury in the cistern above the
level of A is read off, and furnishes a clue to the temperature of B.
If the coefficient of pressure of the gas used (in this case, air) be
known, the temperature may be calculated from the equation
P_{1} = P_{0}(1 + _bt_),
where P_{1} is the pressure at _t_°; P_{0} the pressure at 0°; and _b_
the coefficient of pressure; that is, the increase in unit pressure
at 0° for a rise in temperature of 1°. Thus if P_{0} = 76 cms.; _b_ =
0·00367; height of mercury in cistern above A = 55·8 cms.; then
P_{1} = (76 + 55·8) = 131·8 cms.,
and by inserting these values in the above equation _t_ is found to be
200°. In the instrument described, P_{0} is equal to the height of the
barometer, since the tap C is open whilst the bulb is immersed in ice.
The coefficient of pressure may be determined by placing the bulb in
steam at a known temperature, and noting the increased pressure. In the
equation given, P_{1}, P_{0}, and _t_ are then known, and the value of
_b_ may be calculated.
[Illustration: FIG. 1.—CONSTANT VOLUME AIR THERMOMETER.]
In using this instrument for exact determinations of temperature,
allowance must be made for the expansion of the bulb, which causes a
lower pressure to be registered than would be noted if the bulb were
non-expansive. Again, the gas in the connecting tube is not at the same
temperature as that in the bulb; an error which may be practically
eliminated by making the bulb large and the bore of the tube small.
The temperature of the mercury column must also be allowed for, as the
density varies with the temperature. When the various corrections have
been made, readings of great accuracy may be secured.
When applied to the measurement of high temperatures, the bulb must
be made of a more infusible material than glass. Gold, porcelain,
platinum, and quartz have been used by different investigators, but
the most reliable material for temperatures exceeding 900° C. has
been found to be an alloy of platinum with 20 per cent. of rhodium.
The most suitable gas to use inside the bulb is nitrogen, which is
chemically inert towards the materials of the bulb, and is not absorbed
by the metals mechanically. When measuring high temperatures with this
instrument, a considerable pressure, amounting to 1 atmosphere for
every increase of 273 degrees above the ice point, is requisite to
prevent expansion of the nitrogen; and this pressure tends to distort
the bulb and so to falsify the indications. This trouble has been
overcome by Day, who surrounded the bulb by a second larger bulb, and
forced air or nitrogen into the intervening space until the pressure
on the exterior of the thermometer bulb was equal to that prevailing
in the interior. Even then it was not found possible to secure higher
readings than 1550° C., as the bulb commenced to alter in shape owing
to the softening of the material. This temperature represents the
highest yet measured on the gas scale; but by using a more refractory
material, such as fused zirconia, it may be found possible to extend
this range to 2000° C. or more. Experiments in this direction are
very desirable, in order that high-reading pyrometers may be checked
directly against the gas scale.
=Fixed Points for Calibration of Pyrometers.=—It is evident
that the gas thermometer is totally unsuited for use in workshops
or laboratories when a rapid determination of a high temperature is
required. Its function is to establish fixed points or temperature
standards, by means of which other instruments, more convenient to
use, may be graduated so as to agree with each other and with the gas
scale itself. The temperature scales of all modern pyrometers are thus
derived, directly or indirectly, from the gas thermometer. In the
table on next page, a number of fixed points, determined by various
observers, is given; the error, even at the highest temperatures,
probably not exceeding ±2° C.
In preparing the temperature scale of a pyrometer for practical
use, the instrument is subjected successively to a number of the
temperatures indicated in the table, and in this manner several fixed
points are established on its scale. The space between these points is
then suitably subdivided to represent intermediate temperatures.
TABLE OF FIXED POINTS.
──────────────────┬────────────────────┬───────┬──────
Substance. │Physical Condition. │ Deg. │ Deg.
│ │ Cent. │ Fahr.
──────────────────┼────────────────────┼───────┼───────
Water (ice) │ At Melting Point │ 0 │ 32
Water │ ” Boiling ” │ 100 │ 212
Aniline │ ” ” ” │ 184 │ 363
Naphthalene │ ” ” ” │ 218 │ 424
Tin │ ” Melting ” │ 232 │ 449
Lead │ ” ” ” │ 327 │ 620
Zinc │ ” ” ” │ 419 │ 786
Sulphur │ ” Boiling ” │ 445 │ 833
Antimony │ ” Melting ” │ 631 │ 1167
Aluminium │ ” ” ” │ 657 │ 1214
Common Salt │ ” ” ” │ 800 │ 1472
Silver (in air) │ ” ” ” │ 955 │ 1751
Silver (free from │ │ │
oxygen) │ ” ” ” │ 962 │ 1763
Gold │ ” ” ” │ 1064 │ 1947
Copper (in air) │ ” ” ” │ 1064 │ 1947
Copper (Graphite │ │ │
covered) │ ” ” ” │ 1084 │ 1983
Iron (pure) │ ” ” ” │ 1520 │ 2768
Palladium │ ” ” ” │ 1549 │ 2820
Platinum │ ” ” ” │ 1755 │ 3190
──────────────────┴────────────────────┴───────┴───────
It is necessary to point out that the figures given in the table refer
only to pure substances, and that relatively small quantities of
impurities may give rise to serious errors. The methods by which the
physical condition to which the temperatures refer may be realised in
practice will be described in the succeeding chapter.
=National Physical Laboratory Scale.=—Exact agreement with regard
to fixed points has not yet been arrived at in different countries, and
an effort to co-ordinate the work of the National Physical Laboratory,
the United States Bureau of Standards, and the Reichsanstalt, with a
view to the formation of an international scale, was interrupted by the
war. In 1916 the National Physical Laboratory adopted a set of fixed
points on the Centigrade thermodynamic scale, in conformity with which
all British pyrometers have since been standardised. It will be seen
that the figures differ very slightly from those given in the previous
table, which represent the average results of separate determinations
in different countries.
NATIONAL PHYSICAL LABORATORY SCALE (1916).
────────────────────┬──────────────────────────┬────────┬──────
Substance. │ Physical Condition. │ Deg. │ Deg.
│ │ Cent. │ Fahr.
────────────────────┼──────────────────────────┼────────┼──────
Water (ice) │ At Melting Point │ 0 │ 32
Water │ ” Boiling ” (760 mm.)│ 100 │ 212
Naphthalene │ ” ” ” ” │ 217·9 │ 424
Benzophenone │ ” ” ” ” │ 305·9 │ 582
Zinc │ ” Melting ” │ 419·4 │ 787
Antimony │ ” ” ” │ 630 │ 1166
Common Salt │ ” ” ” │ 801 │ 1474
Silver (in reducing │ │ │
atmosphere) │ ” ” ” │ 961 │ 1761
Gold │ ” ” ” │ 1063 │ 1945
Copper (in reducing │ │ │
atmosphere) │ ” ” ” │ 1083 │ 1982
────────────────────┴──────────────────────────┴────────┴──────
For higher temperatures the melting points of nickel (1452° C.) and
palladium (1549° C.) are employed, but the accuracy in these cases is
not so certain as with the substances named in the table. A useful
point, intermediate between copper and nickel, has been established
by E. Griffiths, and is obtained by heating nickel with an excess of
graphite, when a well-defined eutectic is formed which freezes at 1330°
C., or 2426° F.
=Temperatures above the Present Limit of the Gas Thermometer.=—As
it is not yet possible to compare an instrument directly with the
gas thermometer above 1550° C., all higher temperatures must be
arrived at by a process of extrapolation. By careful observation of a
physical change at temperatures up to the limit of 1550° C., the law
governing such change may be discovered; and assuming the law to hold
indefinitely, higher temperatures may be deduced by calculation. An
amount of uncertainty always attaches to this procedure, and in the
past some ludicrous figures have been given as the result of indefinite
extrapolation. Wedgwood, for example, by assuming the uniform
contraction of clay, gave 12001° C., or 21637° F., as the melting point
of wrought iron, whereas the correct figure is 1520° C., according
to the gas scale. Even in recent times, the extrapolation of the law
connecting the temperature of a thermal junction with the electromotive
force developed, obtained by comparison with the gas scale up to 1100°
C., led Harker to the conclusion that the melting point of platinum
was 1710° C., a figure 45 degrees lower than that now accepted. The
laws governing the radiation of energy at different temperatures,
however, appear to be capable of mathematical proof from thermodynamic
principles, and temperatures derived from these laws are in reality
expressed on the absolute or thermodynamic scale. Extrapolation of
these laws, when used to deduce temperatures by means of radiation
pyrometers, appears to be justified; but it is still desirable to
extend the gas scale as far as possible to check such instruments.
Assuming the radiation laws to hold, it is possible to determine the
highest temperatures procurable, such as that of the electric arc, with
a reasonable degree of certainty.
[1] For a fuller account of the thermodynamic scale, see the author’s
treatise _Heat for Engineers_, pp. 391-2.
CHAPTER III
THERMO-ELECTRIC PYROMETERS
=General Principles.=—Seebeck, in 1822, made the discovery that
when a junction of two dissimilar metals is heated an electromotive
force is set up at the junction, which gives rise to a current
of electricity when the heated junction forms part of a closed
circuit. Becquerel, in 1826, attempted to apply this discovery to
the measurement of high temperatures, it having been observed that
in general the E.M.F. increased as the temperature of the junction
was raised. No concordant results were obtained, and the same fate
befell the investigations of others who subsequently attempted to
produce pyrometers based on the Seebeck effect. These failures were
due to several causes, but chiefly to the non-existence of reliable
galvanometers, such as we now possess. It was not until 1886 that the
problem was satisfactorily solved by Le Chatelier of Paris.
Although any heated junction of metals will give rise to an
electromotive force, it does not follow that any pair, taken at random,
will be suited to the purposes of a pyrometer. A junction of iron and
copper, for example, gives rise to an E.M.F. which increases with the
temperature up to a certain point, beyond which the E.M.F. falls off
although the temperature rises, and finally reverses in direction—a
phenomenon to which the name of “thermo-electric inversion” has been
applied. Evidently, it would be impossible to measure temperatures
in this case from observations of the electromotive force produced,
and any couple chosen must be free from this deterrent property.
Moreover, the metals used must not undergo deterioration, or alteration
in thermo-electric properties, when subjected for a prolonged
period to the temperature it is desired to measure. These and other
considerations greatly restrict the choice of a suitable pair of
metals, which, to give satisfaction, should conform to the following
conditions:—
1. The E.M.F. developed by the junction should increase uniformly as
the temperature rises.
2. The melting point of either component should be well above the
highest temperature to be measured. An exception to this rule
occurs when the E.M.F. of fused materials is employed.
3. The thermo-electric value of the couple should not be altered by
prolonged heating.
4. The metals should be capable of being drawn into homogeneous
wires, so that a junction, wherever formed, may always give rise
to the same E.M.F. under given conditions.
It is a further advantage if the metals which fulfil the above
conditions are cheap and durable.
The exacting character of these requirements delayed the production
of a reliable thermo-electric pyrometer until 1886, when Le Chatelier
discovered that a junction formed of platinum as one metal, and an
alloy of 90 per cent. of platinum and 10 per cent. of rhodium as the
other, gave concordant results. In measuring the E.M.F. produced, Le
Chatelier took advantage of the moving-coil galvanometer introduced by
d’Arsonval, which possessed the advantages of an evenly-divided scale
and a dead-beat action. This happy combination of a suitable junction
with a simple and satisfactory indicator immediately established the
reliability of the thermo-electric method of measuring temperatures.
As platinum melts at 1755° C., and the rhodium alloy at a still higher
temperature, a means was thus provided of controlling most of the
industrial operations carried out in furnaces.
So far, the effect of heating the junction has been considered without
regard to the temperature of the remainder of the circuit, and it is
necessary, before describing the construction of practical instruments,
to consider the laws governing the thermo-electric circuit, the
simplest form of which is represented in fig. 2. One of the wires is
connected at both ends to separate pieces of the other wire, the free
ends of which are taken to the galvanometer Two junctions, A and B, are
thus formed, which evidently act in opposition; for if on heating A the
direction of current be from A to B, then on heating B the direction
will be from B to A. Hence if A and B were equally heated no current
would flow in the circuit, the arrangement being equivalent to two
cells of equal E.M.F. in opposition. Thermal junctions are formed at
each of the galvanometer terminals, but the currents to which they give
rise, when the temperature changes, are opposed and cancel each other.
The law which holds for this circuit may be expressed thus:—
“If in a thermo-electric circuit there be two junctions, A and B,
the electromotive force developed is proportional to the
_difference_ in temperature between A and B.”
[Illustration: FIG. 2.—TWO-JUNCTION THERMO-ELECTRIC CIRCUIT.]
It is customary to refer to the two junctions as the “hot” and “cold”
junctions; but it is important to remember that fluctuations in the
temperature of either will alter the reading on the galvanometer or
indicator.
A second law, which applies to all thermo-electric circuits, is that
“the E.M.F. developed is independent of the thickness of the wire.”
This does not mean that the deflection of the galvanometer is the
same whether thin or thick wires are used to form the junction. The
deflection depends upon the current flowing through the circuit, and
this, according to Ohm’s law, varies inversely as the total resistance
of the circuit. Consequently, the use of thin wires of a given kind
will tend to give a less deflection than in the case of thick wires, as
the resistance of the former will be greater, and unless the resistance
of the galvanometer be great compared with that of the junction, the
difference in deflection will be conspicuous. The E.M.F., however, is
the same under given conditions, whatever thickness of wire be used.
Reference to fig. 2 will show that in order to realise this circuit in
practice, one of the wires forming the couple must be used in the form
of leads to the galvanometer. This can readily be done if the material
of the wire is cheap; but if platinum or other expensive metal be
used, and the galvanometer be some yards distant, the question of cost
necessitates a compromise, and the circuit is then arranged as in fig.
3. The wires forming the hot junction are brought to brass terminals T
T, from which copper wires lead to the galvanometer G. This arrangement
results in three effective junctions, viz. the hot junction A to B; the
junction A to brass, and the junction B to brass. It will be seen that
the two junctions of copper to brass are in opposition, and cancel
each other for equal heating; and the same applies to the galvanometer
connections. A circuit thus composed of three separate junctions does
not permit of a simple expression for the net E.M.F. under varying
temperature conditions, and to avoid errors in readings care must be
taken to prevent any notable change of temperature at the terminals T T
in a practical instrument arranged as in the diagram.
[Illustration: FIG. 3.—THREE-JUNCTION THERMO-ELECTRIC CIRCUIT.]
A point of practical utility in thermo-electric work is the fact that
if a wire be interrupted by a length of other metal, as indicated at C
in fig. 3, no current will be set up in a circuit if both joints are
equally heated, as the electromotive forces generated at each junction
are in opposition. It is thus possible to interrupt a circuit by a
plug-key or switch, without introducing an error; always provided that
an even temperature prevails over the region containing the joints.
Another useful fact is that if two wires be brought into contact, they
may be fastened over the joint by soldering or using a third metal,
without alteration of thermo-electric value, except in rare cases. Thus
a copper-constantan or iron-constantan junction may suitably be united
by silver solder, using borax as a flux, thus avoiding the uncertainty
of contact which must always occur when the wires are merely twisted
together. Welding, however, is preferable to soldering.
=Metals used for Thermal Junctions.=—Until recent years
it was customary to employ a platinum-rhodioplatinum or
platinum-iridioplatinum junction for all temperatures beyond the
scope of the mercury thermometer. The almost prohibitive price of
these metals has caused investigations to be made with a view to
discovering cheaper substitutes, with successful results up to 1000°
C. or 1800° F., thus comprehending the range of temperatures employed
in many industrial processes. Above this temperature the platinum
series of metals are still used for accurate working, but it will be
of great advantage if the range measurable by cheap or “base” metals
can be further extended. Promise in this direction is afforded by
the properties of fused metals when used in thermal junctions. An
investigation by the author has shown that in general the E.M.F.
developed by a junction does not undergo any sudden change when one
or both metals melt, but continues as if fusion had not occurred. By
making arrangements to maintain the continuity of the circuit after
fusion, it may be possible to read temperatures approximating to the
boiling points of metals such as copper and tin, both of which are over
2000° C. The base metals are not so durable as platinum and kindred
metals, but as the cost of replacement is negligible, this drawback is
of little importance. Moreover, base-metal junctions usually develop a
much higher E.M.F. than the platinum metals, which enables stronger and
cheaper galvanometers to be used as indicators.
THERMAL JUNCTIONS USED IN PYROMETERS.
─────────────────────────────────────────────────┬────────────────
│ Upper limit to
│ which Junction
│ may be used.
Couple. ├────────┬───────
│ Deg. │ Deg.
│ Cent. │ Fahr.
─────────────────────────────────────────────────┼────────┼───────
Platinum and rhodioplatinum (10 per cent. Rh) │ 1400 │ 2550
2 Rhodioplatinum alloys of different composition │ 1600 │ 2900
Platinum and iridioplatinum (10 per cent. Ir) │ 1100 │ 2000
Nickel and constantan │ 900 │ 1650
Nickel and copper │ 800 │ 1475
Nickel and carbon │ 1000 │ 1850
Nickel and iron │ 1000 │ 1850
Iron and constantan │ 900 │ 1650
Copper and constantan │ 800 │ 1475
Silver and constantan │ 800 │ 1475
2 Nickel chrome alloys of different composition │ │
(Hoskin’s alloys) │ 1100 │ 2010
Nickel-chrome alloy and nickel-aluminium alloy │ 1100 │ 2010
2 Iron-nickel alloys of different composition │ 1000 │ 1850
─────────────────────────────────────────────────┴────────┴───────
The electromotive force developed by a junction of any given pair of
metals when heated to a given temperature varies according to the
origin of the metals. It is not unusual, for example, for two samples
of 10 per cent. rhodioplatinum, obtained from different sources, to
show a difference in this respect of 40 per cent. when coupled with
the same piece of platinum. Equal or greater divergences may be noted
with other metals; and hence the replacement of a junction can only be
effected, with accuracy, by wires from the same lengths of which the
junction formed a part. As showing how platinum itself is not uniform,
it may be mentioned that almost any two pieces of platinum wire, if
not from the same length, will cause a deflection on a sensitive
galvanometer when made into a junction and heated. It is therefore
customary for makers to obtain considerable quantities of wire of a
given kind, homogeneous as far as possible, in order that a number of
identical instruments may be made, and the junctions replaced, when
necessary, without alteration of the scale of the indicator.
The alloy known as “constantan,” which figures largely in the foregoing
table, is composed of nickel and copper, and is practically identical
with the alloy sold as “Eureka” or “Advance.” It has a high specific
resistance, and a very small temperature coefficient, and is much used
for winding resistances. Couples formed of constantan and other metals
furnish on heating an E.M.F. several times greater than that yielded
by couples of the platinum series, and show an equally steady rise
of E.M.F. with temperature. This alloy has proved of great service in
connection with the thermo-electric method of measuring temperatures.
Couples formed of nickel-chrome alloys, known as “Hoskin’s alloys,”
have been introduced into Britain by the Foster Instrument Company,
which may be used continuously to 1100° C., and for occasional readings
up to 1300° C. Another couple, much used in America, consists of an
alloy of 90 per cent. nickel and 10 per cent. chromium, and an alloy
of 98 per cent. nickel and 2 per cent. aluminium, which may be used up
to 1100° C. Other couples, formed of alloys of nickel, chromium, iron,
aluminium, etc., have been introduced by different makers, but have not
proved so satisfactory as those mentioned above.
=Changes in Thermal Junctions when constantly used.=—No metal
appears to be able to withstand a high temperature continuously without
undergoing some physical alteration; and for this reason the E.M.F.
developed by a given junction is liable to change after a period of
constant use. At temperatures above 1100° C., platinum, for example,
undergoes a notable change in a comparatively short period, but below
1000° C., the change is very slight, and if this range be not exceeded,
a platinum-rhodioplatinum or iridioplatinum junction may be used for
years without serious error arising from this cause. This liability
to change is one of the factors which restricts the range of thermal
junctions, which should never be used continuously beyond the
temperature at which the alteration commences to become large. A second
cause of discrepancy is the possible alteration in the composition
of an alloy, due to one of the constituents leaving in the form of
vapour, as is noted with iridioplatinum alloys, from which the iridium
volatilises in tangible quantities above 1100° C., causing a fall of
10 per cent. or more in the thermo-electric value of the junction of
these alloys with platinum. Constantan appears to be very stable in
its thermo-electric properties, and the various junctions in which
it plays a part show a high degree of stability if not overheated.
Rhodioplatinum alloys are very stable, and for temperatures exceeding
1100° C. a junction of two of these alloys, of different composition,
is more durable than one in which pure platinum is used. An extended
series of tests on base-metal junctions made in America by Kowalke
showed that continuous heating of couples as received from the makers
altered the E.M.F. considerably, the change in some cases representing
over 100° C. on the indicator. A stable condition, due to the relief of
strains or other change, was finally reached, and the conclusion drawn
that the materials should be thoroughly annealed before calibration. It
is desirable in all cases periodically to test the junctions at some
standard temperature, and if any conspicuous error be noted, to replace
the old junction by a new one.
In addition to the errors due to slow physical changes, a junction
may be altered considerably, if imperfectly protected, owing to the
chemical action of furnace gases, or of solids with which the junction
may come into contact. The vapours of metals such as lead or antimony
are very injurious; and platinum in particular is seriously affected
by vapours containing phosphorus, if in a reducing atmosphere. So
searching is the corrosive action of furnace gases that adequate
protection of the junction is essential if errors and damage are to be
avoided. When a wire has once been corroded, a junction made with it
will not develop the same E.M.F. as before.
=Electromotive Force developed by Typical Junctions.=—The
following table exhibits the E.M.F. generated by several junctions
for a range of 100° C., taken at the middle part of the working range
in each case. These figures are subject to considerable variation,
according to the origin of the metals.
──────────────────────────────────────────┬─────────────────────────
│ E.M.F. in millivolts for
Couple. │ a rise of 100° at middle
│ of working range.
──────────────────────────────────────────┼─────────────────────────
Platinum-rhodioplatinum (10 per cent. Rh) │ 1·1
Platinum-iridioplatinum (10 per cent. Ir) │ 1·2
Nickel-constantan │ 2·3
Copper-constantan │ 5·8
Nickel-copper │ 6·1
Iron-constantan │ 6·7
Hoskin’s alloys │ 7·4
──────────────────────────────────────────┴─────────────────────────
It will be noted that the base-metal junctions give much higher values
than the platinum series, and hence can be used with a less sensitive,
and therefore cheaper, indicator. Base-metal junctions are also, in
consequence of the greater E.M.F. furnished, capable of yielding more
sensitive readings over a selected range of temperature.
[Illustration: FIG. 4.—PRACTICAL FORM OF THERMO-ELECTRIC PYROMETER.]
=Practical Forms of Thermocouples.=—When expensive junctions are
employed, wires of the minimum thickness consistent with strength and
convenience of construction are used, a diameter of No. 25 standard
wire gauge being suitable. A common arrangement is shown in fig. 4, in
which J is the hot junction, the wires from which are passed through
thin fireclay tubes which serve as insulators (or through twin-bore
fireclay) to the reels R R, in the head of the pyrometer, upon which
a quantity of spare wire is wound to enable new junctions to be made
when required. Two brass strips, S, are screwed down on to the wires
at one end, and are furnished with screw terminals at the other
end, from which wires are taken to the galvanometer or indicator. A
protecting-tube, T, surrounds the wires and hot junction. The head,
H, may be constructed of wood, fibre, or porcelain, and should be an
insulator for electricity and heat. There are various modifications
in use, but the general method described is adopted by most makers.
In order to guard against errors arising from alterations in the
temperature of the cold junctions in the end of the pyrometer, some
firms construct the head so as to leave a hollow space, through which
cold water is constantly circulated (fig. 5), the arrangement being
known as a “water-cooled head.” In some forms the supply of spare wires
is made to take the form of two spiral springs in a hollow head, the
upper ends of the springs being taken to terminals.
[Illustration: FIG. 5.—PYROMETER WITH WATER-COOLED HEAD.]
The choice of a protecting-tube is a matter of considerable importance.
Obviously, such a tube should not soften at the highest temperature
attained, and when expensive metals are used to form the junction the
sheath should not be permeable to gases or vapours. It should also, if
possible, be a good conductor of heat, so that the junction may respond
quickly to a change of temperature in its surroundings, and should be
mechanically strong. It is difficult to secure all these properties
in any single material, and the choice of a sheath is decided by
the conditions under which the couple is to be used. The substances
employed, and their properties and special uses, may be enumerated as
follows:—
1. _Iron or Mild Steel._—For temperatures not exceeding 1100° C.
iron or mild steel covers are cheap and efficient from the standpoint
of conductivity, although liable to deteriorate owing to oxidation.
The tendency to oxidise is greatly diminished by “calorising” the
exterior by Ruder’s process, in which the iron is heated in a mixture
of metallic aluminium and oxide of aluminium, a surface alloy being
formed which resists oxidation. A result nearly as good may be obtained
by smearing the surface with fine aluminium powder, and bringing to a
white heat. This treatment greatly prolongs the life of an iron sheath.
Some makers employ an inner steel tube round the wires, and an outer
tube which comes into contact with the furnace gases, corrosion of the
latter being detected before the inner tube has given way and exposed
the junction. Some makers do not consider it safe to expose heated
platinum to an iron surface, with only air intervening, and hence use
an inner cover of silica or porcelain, which the outer iron or steel
tube protects from mechanical damage. For ordinary work seamless steam
or hydraulic steel tubing, with a welded end, is satisfactory; but for
dipping into molten lead or other metals the tube should be bored
from the solid. The great advantage of an iron or steel sheath is its
mechanical strength, which protects the couple from damage in case of
rough usage.
2. _Nichrom._—Certain alloys of nickel and chromium, and especially
that known as Nichrom II, may be kept at 1100° C. without oxidising
to any appreciable extent; and hence sheaths of this material may be
used up to the temperature named. In addition to being more durable
than iron, nichrom possesses the same advantages of strength and good
conductivity; on the other hand, it is more costly.
3. _Molybdenum._—This metal, which possesses a melting point of
about 2500° C., may be dipped in molten brass, bronze, copper, etc.,
without being attacked, and has been used to form the tip of a
protecting-tube designed to measure the temperature of molten alloys. A
junction covered only by a thin tube of molybdenum quickly attains the
temperature of its surroundings.
4. _Graphite and Graphite Compositions._—Carbon has the highest
melting point of all known substances, and in the form of artificial or
Acheson graphite may be easily machined to any desired shape. Graphite
sheaths are sometimes used for immersion in molten metals, but at 1000°
C. and higher Acheson graphite oxidises easily and becomes friable. It
is a good conductor of heat, but is easily broken. Compositions of
natural graphite and refractory earths, such as Morgan’s “Salamander,”
are inferior to pure graphite in conductivity, but are stronger and not
readily oxidised, and may be used to form sheaths for temperatures up
to 1400° C. or possibly higher, when penetration of furnace gases to
the junction is not of moment.
5. _Porcelain._—This material, in its best forms, may be used up to
1400° C., but must be efficiently glazed to prevent the ingress of
furnace gases to the junction. It is easily broken by a blow, and when
circumstances permit should be protected by an iron covering-sheath.
The variety known as “Marquardt” has been found very satisfactory for
high-reading thermal couples. Porcelain is not a good conductor of
heat, and a junction encased in it does not respond quickly to external
changes in temperature.
6. _Vitrified Silica._—This substance, which may be worked in the
oxy-hydrogen blowpipe, is largely used as a protecting-tube. It is not
advisable, however, to use it for continuous work above 1100° C., as
beyond this temperature devitrification occurs, and the tube becomes
porous. It is a fairly good conductor of heat, and withstands rapid
changes in temperature without cracking. It is very brittle, and for
this reason is generally encased in iron.
7. _Alundum._—This material is made from fused bauxite, and
has a melting point of 2050° C. A special form of alundum, used
for protecting-tubes, is non-porous up to 1300° C., and forms a
satisfactory covering. Alundum is a moderately good conductor of heat,
but is easily broken.
8. _Carborundum._—This is an electric furnace product, which may
be heated above 2000° C. without damage. For making into pyrometer
tubes, it is bonded with a suitable material, and baked after shaping.
Carborundum, and the amorphous variety known as “silfrax,” have proved
useful for protecting junctions at temperatures as high as 1600° C.
The thermal conductivity is relatively good, but the tubes are easily
broken.
9. _Magnesia._—Tubes of this material, which melts at a temperature
considerably above 2000° C., have been used for special work. Magnesia
is a poor conductor of heat, and has little mechanical strength.
10. _Zirconia._—This is a very refractory material, its melting point
exceeding 2500° C. It may be made into a vitreous variety, which is
non-porous and proof against sudden temperature changes. At present,
only a moulded form of pyrometer tube, made from zirconia powder, is
available, the material worked in this manner being termed “zirkite.”
Although zirconia is a bad conductor of heat, its other qualities
are such that it forms an excellent material for work at the highest
temperatures possible for thermal junctions; and when the vitreous
variety is available, may come into extended use.
[Illustration: FIG. 6. PYROMETER WITH SPECIAL COLD JUNCTION IN HEAD.]
It will be seen from the foregoing that the ideal protecting-tube has
yet to be found, and the user must choose the one which comes nearest
to his requirements. Special consideration must be given in cases when
chemical fumes are present, and a sheath selected which is not attacked
or penetrated by them.
Returning to the junction, it is advisable always to weld the wires,
and not to rely upon the contact resulting from twisting them together.
Platinum and the platinum alloys may be welded readily by placing the
junction in a coal-gas blowpipe fed with oxygen instead of air. For
work at lower temperatures the platinum metals may be soldered by means
of a small quantity of gold, in the flame of a Bunsen burner.
When cheap metals are used for the junction the construction may
be considerably modified, and often with advantage. In fig. 6, for
example, which represents a thermocouple made by A. Gallenkamp & Co.,
the metals used are copper and constantan, and the hot junction,
fastened by silver solder, is supplemented by a cold junction of the
same metals located in the head. The copper wire from the hot junction
passes directly to a copper terminal, from whence a copper wire lead
is carried to the galvanometer; and the same procedure is carried out
with the copper wire from the cold junction, thus realising the circuit
shown in fig. 2. The cold junction is kept in oil, the temperature
of which is registered by a short thermometer, thus enabling (as
will be explained later) the correct temperature of the hot junction
to be deduced under any circumstances. In this instrument twin-bore
fireclay is used to insulate the wires, and the protecting-tube is
of iron—which suffices for the upper limit (800° C.) to which the
junction may be used. Iron and constantan could be used in this manner
by employing iron leads to the galvanometer.
Another type of instrument, rendered practicable by the use of cheap
metals, and which may be termed the “heavy type,” is constructed of
thick pieces of the metals welded together instead of wires, thus
ensuring greater strength and longer life. Messrs Crompton & Co. were
the first to introduce thermocouples of this type, consisting of a
heavy steel tube, to one end of which a nickel rod is welded, the other
end being free, and the length of the rod suitably insulated from the
steel tube; leads for the rod and tube being taken to the galvanometer.
Fig. 7 shows a couple of this kind, made by Paul, consisting of an iron
tube down the middle of which a constantan rod is passed, insulated
from the tube by magnesia. At the tapered end the two metals are welded
together, and at the free end a special cap, fitted over the tube and
rod, the contact parts being insulated from one another, serves to
enable leads to be taken to the galvanometer. Similar thermocouples are
made by the Foster Instrument Company (fig. 8), and are simple, cheap,
and reliable up to 900° C. with an iron-constantan couple, and to 1100°
C. with nichrom couples. When worn out they may be replaced, at a
trifling cost, by others made from the same batch of metal.
[Illustration: FIG. 7.—HEAVY TYPE, CHEAP-METAL PYROMETER.]
The drawback to the use of carbon as one of the materials for a
junction is the difficulty experienced in securing a good contact with
the metal with which it is coupled. In nickel-carbon junctions the
contact is sometimes ensured by the aid of a spring, which presses
the two substances together. Such an arrangement is evidently not so
reliable as one in which the materials are welded, and a defective
contact, arising from any cause, would lead to serious error. A
preferable plan is to screw both the nickel and carbon rods into a
cross-piece of either element.
[Illustration: FIG. 8.—FOSTER’S CHEAP-METAL PYROMETER.]
When applying a thermal junction to the measurement of surface
temperatures, such as steam-pipes or the exterior of furnaces, the
wires may be passed through a thin disc of metal, about ¼ in. in
diameter, and soldered at the back. Suitable materials are copper and
constantan, soldered to a thin copper disc with silver solder, and
brought to a cold junction in the head of the instrument as shown in
fig. 6. The terminal piece of the insulation may be made of hard wood,
with the holes countersunk so as to cover the solder and enable the
wood to touch the disc, which, when pressed on the hot surface, will
then rapidly acquire the temperature. The author has found, by trials
under varying circumstances, that this method of measuring surface
temperatures gives reliable and concordant results. For very high
surface temperatures a platinum disc, with one of the usual platinum
metal couples soldered to the disc with pure silver, and a piece of
twin-bore fireclay brought to the back of the disc, will be found to
suffice for most cases arising in practice. A small blowpipe flame is
best for soldering the wires to the disc, borax being used as flux in
the first case; but no flux is necessary in soldering the platinum
metals with pure silver.
In deciding upon the length of a thermocouple it must be remembered
that the temperature recorded is that prevailing in the region of
the hot junction. When the temperature of a furnace is uniform it is
sufficient to allow the end of the thermocouple to protrude about 12
inches into the interior, but when following the change of temperature
undergone by objects in a furnace the end must be located near the
objects. If the distance from the exterior of the furnace to the
objects exceed 2 feet, the thermocouple should be inserted through the
roof so as to hang vertically, as if placed through the side it would
droop by its own weight at high temperatures. The distance between
the exterior of the furnace and the cold junctions should be at least
15 inches in all cases in which the heating of the cold junction
is not automatically compensated. After inserting the couple the
opening through the furnace wall should be closed by means of suitable
luting-clay.
In certain instances, such as flues, it is necessary to use a long
instrument in a horizontal position. A rail may then be placed across
the flue, at a suitable place, to serve as a support and so to prevent
drooping.
=Liquid Element Thermocouples.=—An investigation by the author
and A. W. Grace has shown that the continuity of the E.M.F. produced
by a rising temperature is not interrupted by fusion, except in the
cases of bismuth and antimony, which both show an abrupt change in
thermo-electric properties at the melting point. It would therefore
appear feasible to measure temperatures by constructing a thermocouple
so as to retain the circuit after fusion, the advantage gained being
that the range is restricted by the boiling point of the metals instead
of the melting point and higher readings are rendered possible. The
boiling points of some of the common metals are appended:—
───────────┬───────────────────
Metal. │ Boiling Point.
───────────┼─────────┬─────────
│ Deg. C. │ Deg. F.
├─────────┼─────────
Aluminium │ 1800 │ 3270
Silver │ 1955 │ 3550
Tin │ 2270 │ 4120
Copper │ 2310 │ 4190
Nickel │ 2330 │ 4225
Iron │ 2450 │ 4440
───────────┴─────────┴─────────
From inspection of these figures, it will be seen that if a suitable
couple could be obtained, common metals might be used to measure
temperatures equalling or even exceeding the limit of the range covered
by wire junctions of metals of the platinum series. Instead of using
two metals, graphite might form one member of the couple, provided that
no objection to its use existed on other grounds.
[Illustration: FIG. 9.—LIQUID-ELEMENT THERMOCOUPLE.]
The form of thermocouple designed by the author to permit of the use of
molten elements is shown in fig. 9. A rod of refractory material, R, is
perforated longitudinally by two holes, down which are passed rods of
the thermo-elements, A and B. The lower ends of A and B are inserted in
a graphite block G, which is jointed on its upper face to R; the whole
being surrounded by the refractory cover C. On either or both of the
elements melting, the circuit is maintained through G, which serves
also to prevent the mixing of A and B when molten, whilst not affecting
the E.M.F. developed. In order to allow for the expansion of the metals
on melting, A and B are made to fit loosely in R. When inserted in a
furnace to a depth represented by EF, only the portion of the metals
adjacent to the closed end will melt, the outer parts remaining solid.
At present it has not been found possible to procure the refractory
parts in a form suited to commercial use, but when this obstacle
is overcome this type of thermocouple should prove of service for
measuring temperatures beyond the scope of ordinary base-metal
junctions.
=Indicators for Thermo-electric Pyrometers.=—As the electromotive
force developed by a single junction when heated is small, a sensitive
galvanometer is required to indicate the minute current flowing
through the circuit. Delicate millivoltmeters, of the moving-coil
type, are universally employed, as they possess the advantage of
an evenly-divided scale combined with the requisite degree of
sensitiveness. The original d’Arsonval galvanometer, consisting of a
coil suspended by a metallic strip between the poles of a horse-shoe
magnet, was used by Le Chatelier, who, by its aid, was enabled to
lay the foundations of this branch of pyrometry. Three forms of this
instrument are now in use, viz. (_a_) the suspended coil “mirror” type;
(_b_) the suspended coil “pointer” type; and (_c_) the pivoted type.
Examples of each will now be described.
[Illustration: FIG. 10.—HOLDEN-D’ARSONVAL MIRROR GALVANOMETER.]
Fig. 10 represents a mirror galvanometer working on the d’Arsonval
principle, designed by Gen. Holden, F.R.S. The horse-shoe magnet
is laminated, and an iron core, supported by a pillar, is placed
between the poles. The coil, which moves in the space between the
core and the poles of the magnet, is suspended by a thin, flat strip
of phosphor-bronze, which carries a small circular mirror. A similar
phosphor-bronze strip is fastened to the lower part of the coil,
and is continued to an adjusting-screw in the base. The ends of the
suspension strips communicate with the terminals of the galvanometer,
and a current entering at one terminal passes through the metallic
suspensions and the coil to the other. The effect of passing a current
through the coil, which is located in a powerful magnetic field is to
produce an axial movement tending to twist the suspension strips, which
movement is greatly magnified by a spot of light reflected from the
mirror on to a distant scale. When the current ceases, the untwisting
of the strip restores the coil to its former position. Galvanometers of
this type are remarkably “dead-beat” in action, that is, the movement
and restoration of the coil are accomplished without vibration. A
semi-transparent scale, placed at 1 metre distance, and 50 centimetres
long, is suitable for use with this galvanometer. When used in
workshops, it is necessary to protect a mirror galvanometer from the
vibrations produced by machinery, which would cause the spot of light
to become unsteady. The best method of effecting this is shown in fig.
11, which represents the mode of suspension devised by W. J. Lambert
for use in the Royal Gun Factory, Woolwich Arsenal. The usual supports
of the galvanometer are abolished, and the instrument suspended from
the ring of a brass tripod, so as to keep three springs partly in
compression. When suspended in this manner, a mirror galvanometer
is quite suited to commercial use; in the quiet of the laboratory
the ordinary supports may be employed. The advantage gained by using
the mirror type is that a much longer scale is possible than with
instruments furnished with a pointer, and hence greater accuracy in
determining temperature readings may be secured.
[Illustration: FIG. 11.—LAMBERT’S ANTI-VIBRATION STAND FOR
GALVANOMETERS.]
[Illustration: FIG. 12.—SIEMENS’ THERMO-ELECTRIC INDICATOR.]
In suspended coil instruments furnished with a pointer, the
construction differs only in detail from the foregoing. In place of
the mirror, a light pointer is attached to the suspension so as to
rest on the coil and a scale is furnished over which the pointer
moves. Fig. 12 is an example of this type, made by Messrs Siemens, the
suspension being contained in the tube which rises from the body of the
instrument. The maximum length of scale moved over by the extremity
of the pointer is about 6 inches, as a longer and therefore heavier
pointer would reduce the sensitiveness below the point requisite for
thermo-electric work.
In the double-pivoted type, the suspension is eliminated, and pivots
are fastened to each end of the moving coil which rest in bearings.
The turning of the coil is made to compress a hair spring, made of
phosphor-bronze; and when the current ceases the unwinding of this
spring restores the coil to its former position. The coil carries
a pointer which moves over a scale. These instruments are not so
sensitive as those in which the coil is suspended, but can be made
sufficiently sensitive to work with any kind of junction in practical
use. The pivoted form is cheaper and stronger than the suspended type,
and is used whenever sufficiently sensitive.
The “Uni-pivot” galvanometer, made by R. W. Paul, is shown in figs. 13
and 17. The coil, which carries the pointer, is circular, and moves
round a spherical core of iron placed between the poles of the magnet.
A hole is drilled in the iron core, and the coil rests on a single
bearing at the bottom of this hole. A phosphor-bronze control-spring
serves to restore the coil to the zero position. The lessened friction
due to the use of a single pivot enables this instrument to be made
very sensitive when needed, so that a relatively small rise in the
temperature of a junction may cause the pointer to traverse the whole
length of the scale.
[Illustration: FIG. 13.—PRINCIPLE OF UNI-PIVOT GALVANOMETER.]
=Special Features of Indicators.=—All moving-coil instruments,
whether suspended or pivoted, are liable to alteration of the zero
point owing to what is termed “creep.” The suspension strip, when first
fixed in position, generally possesses a certain amount of initial
torsion, which comes into operation gradually and causes a slight
movement of the coil. Similarly, in a pivoted instrument, the strength
or shape of the control-spring undergoes a gradual alteration at first,
causing the pointer to move away from the zero position. For this
reason adjusting arrangements are fitted by means of which the spot of
light or pointer may be brought back to the zero. This creeping ceases
after a time—often requiring twelve months—and if not subjected to
any strain, error from this cause does not recur to any notable extent.
With a mirror galvanometer it is better to move the scale, or turn the
galvanometer round on its axis to restore the correct zero, rather than
to twist the coil back; but with a fixed scale and pointer the only
remedy is to turn the coil bodily round. In a single-pivot indicator
constantly used in the author’s laboratory, the creep amounted to a
movement of the end of the pointer through an angle of 2 degrees in
the first few months, since when, after the lapse of several years, no
further alteration has occurred. It is advisable to test the zero point
of an indicator from time to time by breaking the circuit, and if an
error be discovered the pointer should be re-set, or an allowance made
in taking a reading.
The resistance of an indicator should be so high that the readings
should not be perceptibly altered by any fluctuations in the resistance
of the circuit which may arise in practice. If leads of considerable
length were used to connect the pyrometer with the indicator, and were
subject to fairly large alterations of temperature, the consequent
changes in the resistance of such leads would be noticeable on a
low-resistance indicator; and similarly, if a pyrometer were inserted
at different depths in a furnace at separate times, thus heating up
varying lengths of the junction wires, a discrepancy would arise for
the same reason. The resistance of an indicator, however, cannot be
raised beyond a certain point without reducing the sensitiveness below
the required limit. A mirror galvanometer of the type described may
have a resistance—partly in the coil and partly in an added series
resistance—of 1000 ohms or more, and still be sufficiently sensitive;
and in the latest types of instruments provided with pointers the
resistance may be made as high as 1000 ohms, although it is more
usually 400 to 500 ohms. Many indicators are in use, however, in which
the resistance is 100 ohms or less. As, from Ohm’s law, the current
varies inversely as the total resistance in the circuit, any alteration
in resistance should be small relatively to the total to render the
error negligible. This point is made clear in the following example:—
_Example._—A thermocouple and leads have a resistance of 5
ohms and are subject to alterations amounting to 1 ohm. To
find the errors resulting when indicators of resistances
800, 400, and 50 ohms respectively are used.
From Ohm’s law, C = E/R, the variation in C, with E
constant, will be 1 in 805, 1 in 405, and 1 in 55
respectively. As the indications are proportional to the
current, the alterations caused will be approximately
⅛ per cent., ¼ per cent., and 2 per cent. The first two
may be ignored; the last may be quite serious and lead to
the failure of an operation.
It will be seen from the foregoing that low-resistance indicators
should only be used for fixed thermocouples and short leads not subject
to temperature changes, or, in other words, in a circuit of fixed
resistance.
The resistance of an indicator, when unknown, may be found by the
following method, suggested by the author:—A resistance box is joined
at one end to one terminal of the indicator. To the other terminal a
fairly stout iron wire, 18 inches long, is connected, and a similar
length of constantan wire is coupled to the other end of the resistance
box. The free ends of the wires are twisted into a junction which is
dipped into boiling water. The deflection obtained with no resistance
in the box (D_{1}) is noted, and resistances (R) are then unplugged
until the deflection (D_{2}) is approximately one-half of D_{1}. The
resistance (G) of the indicator, ignoring that of the wires, is then
given by the formula
D_{2}R
G = ───────────────
(D_{1} - D_{2})
as may readily be proved from Ohm’s law, E being constant. This method
is extremely simple and reasonably accurate.
Reliable indicators are now procurable from many instrument-makers at
a comparatively small cost, progress in this direction having been
most marked in recent years, particularly in the case of pivoted
instruments. The most convenient form for workshop use is made with
an edgewise scale (fig. 14) and may be placed in a suitable position
fixed to a bracket. The flat-scale pattern is preferable for use on a
laboratory table, or for a portable pyrometer. The sector pattern is
also good for workshop use, the dial being visible from a distance.
[Illustration: FIG. 14.—INDICATOR WITH EDGEWISE SCALE.]
=Standardizing of Indicators to read Temperatures directly.=—The
temperature scale of an indicator, for use with a given thermal couple,
is always marked by the maker in the case of instruments furnished
with a pointer, and, generally speaking, is correct within reasonable
limits. It is customary and necessary to send with the instrument a
statement of the cold-junction temperature for which the markings
are correct; say 20° C. or 60° F. The user should then endeavour to
maintain the cold junction at this specified temperature when taking
a reading, or otherwise a considerable error may be introduced. It is
highly desirable, however, that the user should be able to perform the
standardizing himself, if only for checking purposes; and when using a
mirror galvanometer as indicator it is necessary to standardize on the
spot at which the instrument is fixed. Ability to prepare a temperature
scale is further useful, inasmuch as any good millivoltmeter, of range
0 to 20 millivolts, may be used for thermo-electric work of all kinds,
and may be calibrated for different junctions, a suitable series
resistance being added to enable E.M.F.’s higher than 20 millivolts to
be measured. Such an instrument may thus be made extremely useful, both
in the workshop and laboratory.
Standardization may be effected either by subjecting the hot
junction to several known temperatures, and noticing the deflections
corresponding thereto; or by measuring the electromotive force
developed by the junction, and calculating the corresponding
temperature from a formula which is known to hold for the range
comprehended by the instrument. The former method is simpler; and if
carefully conducted is quite accurate. The latter method possesses the
advantage that readings in millivolts may be translated directly into
temperatures when the constants of a given thermal couple are known.
It is now usual to mark indicators with a double scale, one reading
millivolts and the other temperatures.
=Standardization by Fixed Points.=—Taking any millivoltmeter
which, with a maximum of 20 millivolts at the terminals, will give a
full scale deflection, the first step is to arrange that the pointer
(or spot of light) shall just remain on the scale at the highest
temperature to be attained by the junction. This may be done by placing
the hot junction in boiling water and noting the deflection obtained,
either in millivolts or equal arbitrary divisions, and also the
temperature of the cold junction. The deflection observed is due to
a difference of temperature (100-_t_) deg. C, where _t_ is the
temperature of the cold junction. If the highest temperature to be
measured is 10 times (100-_t_), the deflection should be rather
less than 1/10 of the scale, and similarly for any other required
temperature limit. If the observed deflection exceed this proportion, a
series resistance should be added until the correct value is obtained.
This resistance is then permanently installed in the circuit for use
with the junction under trial.
Before proceeding further it is necessary to consider whether the
pyrometer is to possess a single cold junction of ascertainable
temperature (as in fig. 6), or whether it will be arranged with two
cold junctions in the head, as in fig. 4. In the former case it is
simpler to prepare a “difference” scale; that is, one which reads
differences of temperature between the hot and cold junctions, from
which the temperature of the hot end may be obtained by adding to the
difference that of the cold junction. In the latter case the cold end
should be kept by artificial means at the temperature likely to be
attained in practice—say 25° C.—a water-bath being suitable for this
purpose. It is advisable to remove the shield of the pyrometer when
standardizing, so as to expose the hot junction, as closer readings can
then be taken.
A number of materials—preferably cheap—of known boiling points or
melting points are then selected from a table of fixed points (page
16) so as to give about six points, distributed fairly evenly over the
scale. As an example, if it were desired to prepare a temperature scale
from 0° to 1000° C., the following might be chosen:—
════════════════════════════════════════════════╤═══════════════════
Substance and Condition. │ Temperature.
────────────────────────────────────────────────┼─────────┬─────────
Water at boiling point │ 100° C. │ 212° F.
Tin at melting point │ 232 │ 449
Zinc at melting point │ 419 │ 786
Antimony at melting point │ 631 │ 1167
Common salt at melting point │ 800 │ 1472
Copper at melting point (covered with graphite) │1084 │ 1983
════════════════════════════════════════════════╧═════════╧═════════
The hot junction is allowed to attain these temperatures successively,
and the corresponding deflection in each case is noted. It is then
possible to divide up the whole of the scale to read temperatures
directly.
The first reading is taken by placing the junction in a vessel of
boiling water, and for a locality near sea level it is not necessary
in ordinary work to take account of fluctuations in the boiling point
due to alterations of atmospheric pressure. To ensure that the other
readings are taken when the substances are exactly at the melting
point, the procedure is as follows: about 2-3 lb. of the substance are
melted in a salamander crucible, and a small fireclay tube, closed at
one end, is inserted in the molten mass. The hot junction is placed
in the fireclay tube, and the intervening space filled with asbestos
fibre. Great care must be taken not to let the junction touch the fused
substance. The crucible is now allowed to cool, and a reading of the
deflection taken every half-minute. When the substance is exactly at
its solidifying point—identical in general with the melting point—the
deflection remains stationary for several consecutive readings, owing
to the liberation of latent heat of fusion in sufficient quantity
to balance the loss by radiation. This stationary reading is noted
for each substance, and represents the deflection given when the hot
junction is at the temperature corresponding to the melting point, and
the cold junction or junctions at the temperature existing when the
observation is made. For melting the materials, a Davies furnace with
a large Teclu or Meker burner is convenient up to 850° C.; but to melt
the copper a blast lamp is requisite. The molten mass may be allowed to
cool in the furnace.
From these observations a calibration curve may be drawn either for
differences between hot and cold junctions, or for a steady temperature
of the cold junctions. Two sets of data are appended to illustrate the
procedure.
════════════╤═════════════════════════════════╤═════════════════════
Temperature │ Pyrometer 1. Iron-constantan. │ Pyrometer 2.
of Hot │ (Series resistance in │ Platinum-
Junction. │ galvanometer circuit.) │ iridioplatinum.
├───────────┬─────────┬───────────┼───────────┬─────────
│Deflection.│ Cold │Difference.│Deflection.│ Cold
│ │Junction.│ │ │Junction.
────────────┼───────────┼─────────┼───────────┼───────────┼─────────
100° C. │ 8·9 │ 15° C.│ 85° C. │ 5·5 │}
232 │ 21·8 │ 17 │ 215 │ 15·6 │}Constant
419 │ 40·6 │ 19 │ 400 │ 29·4 │} at
631 │ 63·8 │ 19 │ 612 │ 45·5 │} 25° C.
800 │ 83·0 │ 20 │ 780 │ 59·0 │}
1084 │ ... │ ... │ ... │ 82·0 │}
════════════╧═══════════╧═════════╧═══════════╧═══════════╧═════════
[Illustration: FIG. 15.—CALIBRATION CURVES FOR TWO THERMO-ELECTRIC
PYROMETERS.]
Fig. 15, A, is a calibration curve for thermocouple 1, connecting
deflections with corresponding differences between the temperatures
of the hot and cold junctions. In order to read from this curve the
temperature of the hot end, the reading corresponding to the observed
deflection is added to the existing temperature of the cold junction.
Thus if a deflection of 56 divisions were obtained with the cold
junction at 25°, the temperature of the hot junction would be (540 +
25) = 565° C. The advantage of this method of calibration is that it is
unnecessary to take precautions to keep the cold junction at a steady
temperature; and when a single cold junction is used, as in fig. 6,
this plan should always be followed. It will be noted that this curve
passes through zero, as no deflection represents no difference of
temperature.
Fig. 15, B, represents the calibration curve for pyrometer 2, and
is such that direct readings may be obtained corresponding to any
given deflection, for a cold junction temperature of 25°. This curve,
therefore, cuts the axis of zero deflection at 25°, as no deflection
corresponds to the condition when both hot and cold junctions are at
25°. This method of calibration may be used with advantage for couples
of the type shown in fig. 4, where two cold junctions exist in the
head, and the simple rule of adding the cold junction temperature
does not apply. Many suggestions have been made for correcting for
alterations in the temperature of the cold end of such a couple,
but none are accurate, and it is necessary to keep this part at the
temperature of standardization to secure correct readings. In both of
the above calibrations the galvanometer used possessed a scale divided
into 100 equal arbitrary divisions.
In making permanent temperature scales from these curves to attach
to the existing galvanometer scale, intervals of 100° may be taken
and marked opposite to the corresponding divisions on the existing
scale. Each 100° may then be equally subdivided into as many parts as
the length of scale permits, and numbered at suitable intervals. If
the junction used yield a calibration curve departing greatly from a
straight line, every 50° interval should be taken, or, if necessary,
every 25°. In the examples given both curves are nearly straight lines
in the working region, viz. 400° to 800° for the iron-constantan
junction, and 500° to 1100° for the platinum-iridioplatinum.
One precaution necessary in standardizing an indicator by this method
is to ensure that the metals used are pure, as impurities lower the
melting points. If ordered as “pure” from any dealer of repute, the
metals will generally be found satisfactory. The common salt used
should be the ordinary salt sold in blocks, and not a prepared table
salt. A second precaution, when observing melting points, is to guard
against a possible error due to the substance becoming “surfused” or
“overcooled”; in which case the temperature falls below the ordinary
freezing point before solidification commences. When freezing occurs,
however, the temperature rises to and remains at the true melting
point, and an increase of deflection following a gradual fall always
indicates overcooling. The higher deflection then attained is the true
freezing point. Antimony frequently overcools to 600° before freezing,
but on setting rises to the correct figure—631°. All metals and salts
are liable to overcooling occasionally.
=Standardization by Measurement of E.M.F.=—It has been found,
as the result of experiments, that the relation between the E.M.F.
developed by a junction and its temperature—under constant conditions
of the cold junction—may be expressed approximately by a formula as
under:—
log E = A log _t_ + B (Holman’s formula),
where E = electromotive force in microvolts, _t_ = temperature in
Centigrade degrees, and A and B are constants depending upon the
junction. With certain junctions this formula may be applied over
the working part of the scale with an error not exceeding 2° C., but
with others the discrepancy is greater. In order to determine the
constants A and B, it is necessary to measure the E.M.F. at two known
temperatures, which should be chosen as far apart as possible in the
working region. When these constants are known, a measurement of E
enables the temperature _t_ to be found by calculation.
_Example._—Le Chatelier found that a junction at the
temperature of melted aluminium (657° C.) gave 6200
microvolts; at the melting point of copper in air (1062° C.)
the figure was 10580. Applying in the above formula
log 6200 = A log 657 + B
and log 10580 = A log 1062 + B,
the value of A is 1·2196 and of B 0·302, as may be found
by taking logarithms and solving for A and B.
The values of the constants A and B vary for different junctions, and
also for different melts of what are reputed to be the same materials.
When once determined for a quantity of homogeneous wires, to which
the formula applies with sufficient accuracy, it is evident that an
indicator with a millivolt scale may be made to read temperatures
directly without any necessity for further experiment, although it is
always advisable to take one check reading at a fixed point in the
working range.
[Illustration: FIG. 16.—POTENTIOMETER METHOD OF MEASURING E.M.F.]
In order to determine the E.M.F. of a junction at different
temperatures, the potentiometer method is used, in which the E.M.F.
of the test-couple is balanced against the known E.M.F. furnished by
a constant cell. The circuit is shown in fig. 16, in which B is an
accumulator which sends a current through the resistances R_{1}, R_{2},
and the calibrated wire DE. The cold ends of the couple are attached
at P so as to be in opposition to B, and in this branch of the circuit
are included a sensitive galvanometer G and a portion of the wire DE. A
standard cadmium cell, S, is connected between R_{1} and R_{2} at one
end, and may be put in circuit with the galvanometer through the switch
A. In commencing, S is connected to the galvanometer and R_{1} adjusted
until no deflection is obtained on G. The switch A is now moved over
to the circuit of the couple, and the terminal F moved along the wire
until zero deflection is again obtained. The E.M.F. of the couple is
determined from the relation
E of junction Resistance of DF
────────────────── = ───────────────── .
E of standard cell R_{2}
By exposing the hot end of the junction to successive standard
temperatures, and maintaining the cold ends at a known constant
temperature, the necessary data for inclusion in a formula may be
obtained.
In fixing a permanent temperature scale, calculated from the formula,
to a millivoltmeter, it must be remembered that the values given by
the experiment are absolute, and independent of the resistance of the
circuit composed of the thermo-element and galvanometer. On the other
hand, a millivoltmeter is marked to read difference of potential at
its terminals; and if in series with a junction and leads of notable
resistance, its indications will not be the E.M.F. of the junction. An
example will make this point clear.
_Example._—A millivoltmeter has a resistance of 100
ohms, and is marked to read P.D. at its terminals. A
thermocouple and leads connected to the millivoltmeter
have a resistance of 5 ohms. To find the relation between
the true E.M.F. of the junction and the readings of the
indicator.
If E = the E.M.F. developed by the junction, and V, the
reading of the millivoltmeter, = P.D. at its terminals,
then the current in the circuit = E/105 = V/100; and V =
(100/105)E. That is, the readings are lower by 5 per cent.
than the true E.M.F. of the junction. In the same way a
low resistance voltmeter, if applied to a cell of high
resistance, shows a lower reading than the E.M.F. of the
cell.
This example indicates how a table connecting true E.M.F.’s with
reading in millivolts may be calculated when the resistances concerned
are known. It is presumed, in preparing a scale in this manner, that
the resistance of the couple will not be subject to such alterations as
to affect the reading.
The advantages of this method of calibration are manifest when a number
of junctions are being made from a given batch of wires, as it is only
necessary to divide the scale of the indicator so as to represent
millivolts—a simple operation—and then to attach a temperature scale.
This procedure is much more expeditious than standardizing each
indicator at several fixed points when a number are concerned, but for
a single junction the fixed point method is easier. The potentiometer
method of measuring E.M.F. may also be used to determine temperatures
in place of an indicator, and is of great service in cases where very
accurate readings are specially required, being far more delicate in
detecting small differences of temperature than an indicator. Special
potentiometers for thermo-electric work are made by the Cambridge
and Paul Instrument Company, Siemens, and others, and are useful in
conducting accurate research, but are too elaborate for workshop or
ordinary laboratory practice.
=Cold Junction Compensators.=—The necessity for paying attention
to the cold junction has led to various attempts to compensate
automatically for changes of temperature at this part of the pyrometer.
A thermometer located near the cold junction, as in fig. 6, is all that
is needed to correct a two-junction circuit; but when a three-junction
circuit is used a correct reading is not secured by adding the excess
temperature of the thermometer over the calibration temperature to
the reading on the indicator. In Bristol’s arrangement a mercury
thermometer, with a large bulb and wide stem, is stationed at the cold
junction, and participates in any temperature change. In the stem is
placed a loop of thin platinum wire, which forms part of the pyrometer
circuit. When the mercury is heated it expands up the stem and
short-circuits a portion of the loop, thereby diminishing the
resistance of the pyrometer circuit, and tending to increase the
deflection on the indicator. Simultaneously the cold junction will
be heated, tending to diminish the current, and so to cause a less
deflection. By adjustment these two tendencies may be counterbalanced,
so that the reading is unaffected, but such adjustment will only
apply to a given E.M.F., and therefore to one temperature of the hot
junction. Hence this method fails in general application.
Peake’s compensated leads are intended to remedy cold-junction errors
by transferring this junction, in effect, to the galvanometer. They
are used for pyrometers in which the platinum metals are employed, and
consist of wires of two different alloys of copper and nickel, which
connect the cold end to the indicator. These alloys are such that
the electromotive forces set up at the junctions in the head—Pt and
Cu-Ni 1, and Pt-Ir with Cu-Ni 2—are equal and opposite at all working
temperatures, and hence changes at the cold junctions do not affect the
reading. At the indicator, however, temperature changes would cause an
alteration in deflection; but as the indicator is generally placed well
away from the furnace, and is not liable to notable heating or cooling,
the possible errors are greatly reduced by the use of these leads. They
are obviously of no value for use with base-metal pyrometers, as the
wires used in such may be prolonged to the indicator, with an identical
result.
[Illustration: FIG. 17.—DARLING’S COMPENSATOR, FITTED TO GALVANOMETER.]
An automatic compensator for use with base-metal pyrometers has been
devised by the author, and is illustrated in figs. 17 and 18. A spiral
made of a compound strip of two metals is attached to the needle of the
indicator, and coils or uncoils when cooled or heated, thereby moving
the pointer over the scale. The length of the spiral is such that an
alteration of a given number of degrees in its temperature moves the
pointer by the same number of degrees on the scale—or, in other words,
the temperature scale of the pyrometer is identical with that of the
spiral. The metals forming the junction are continued, in the form of
wires, to the interior of the galvanometer, where a cold junction is
formed, which will always possess the same temperature as the spiral.
The scale is constructed to represent differences of temperature between
the hot and cold junctions, and before coupling up the pyrometer the
pointer indicates the temperature of the spiral; that is, of the cold
junction. On connecting the thermocouple the pointer is moved by the
coil of the indicator through an amount represented by the difference
in temperature between the two junctions, and therefore finally
indicates the temperature of the hot junction.
[Illustration: FIG. 18.—INDICATOR FITTED WITH DARLING’S COMPENSATOR.]
_Example._—If the cold junction were at 20°, the pointer,
before connecting the couple, would indicate 20° on the
scale. If the hot junction were 580° hotter than the cold,
then on completing the circuit the pointer would move 580
additional degrees along the scale, so that the figure
indicated would be (20 + 580) = 600°, the temperature of
the hot junction. If now the indicator were heated by 10°,
the spiral would tend to augment the deflection by 10°,
but simultaneously the deflection due to the junctions
would fall off by 10°, and the reading would still be 600°.
This method of compensation renders the readings independent of the
cold junction, and, in addition to its use for high temperatures,
enables ordinary and low temperatures to be read simply and correctly,
as will be shown later. The spiral is located in the tower rising from
the top of the indicator in fig. 18.
In Paul’s method of compensation the thermocouple and indicator
are placed across a Wheatstone bridge, two arms of which contain
resistances of copper, whilst the resistances in the other two arms are
of manganin. Any change in temperature at the cold junction is shared
by these four resistances, and, whilst affecting the resistance of the
copper parts, no change is caused in the manganin parts, as this alloy
has a negligible temperature coefficient. If, therefore, the bridge
were initially balanced at 20° C., and the temperature rose to 30°,
the increased resistance of the copper would destroy the balance, and
permit of a small current passing through the indicator. A fall to
10°, by diminishing the resistance of the copper, would cause an equal
current to pass through the indicator in the opposite direction. The
amount of this current is arranged so as to add the rise in temperature
of the cold junction to the reading of the indicator in the one case,
and to subtract the fall in the other, thus retaining true readings for
the cold-junction temperature at which the couple was standardized.
=Constant Temperature Cold Junctions.=—If the cold junction
can be kept at a steady temperature, compensators are unnecessary,
but no good practical means of achieving this end has yet been
devised. Water-cooled heads have already been referred to; but in many
situations the connecting-pipes entailed would be objectionable, and
hence this arrangement is not greatly used. An alternative method,
suggested by Prof. A. Zeleny, is to bury the cold junction in the
ground. Recent experiments, conducted at Cambridge by R. S. Whipple,
showed that a junction buried 10 feet deep did not vary in temperature
by more than 2° C. over a period of three years. This has led to the
adoption of buried junctions in special cases; but it is probable that
much greater variations would be experienced in the ground beneath
large furnaces, in which case the advantages of this procedure would
be lost. A common workshop method is to locate the cold junction in a
thermos flask filled with oil, when a temperature constant to 2° C. may
be secured, although the changes in the temperature of the surrounding
atmosphere may be as great as 150 C. For special work, ice may be
used in the thermos flask, thus securing absolute constancy; but this
procedure is not feasible in ordinary works practice.
=Special-Range Indicators.=—When the working range of a pyrometer
is from 600° C. upwards, it is evident that the part of the scale
occupied by the first 600° is useless, and that it would be an
advantage if the whole scale could be utilised for the special working
range, so as to secure more exact readings. This may be accomplished
by a “set-up” against the movement of the pointer caused by the
thermocouple, so as to prevent any motion over the scale until an
assigned temperature is reached. For example, a junction developing
12 millivolts at 1000° C. may be coupled to an indicator in which the
full-scale deflection of the pointer is produced by 6 millivolts. If an
E.M.F. of 6 millivolts be opposed to the junction, no deflection will
occur until the temperature at which the couple develops 6 millivolts
is reached—when the opposing E.M.F. will be overcome. This temperature
may be 500° C., so that the whole scale may be divided up between
500° and 1000°. The length of the indicator scale is thus effectively
doubled; and by using different values for the set-up, it is evident
that any desired range may be obtained within the limits of sensitivity
of the indicator. The method of procuring the opposing E.M.F. varies
with different makers. The Cambridge and Paul Instrument Company employ
a dry cell and a series resistance, connected so as to oppose the
thermocouple; and by adjusting the resistance any desired set-up may be
obtained, the value of which, in degrees, may be read off by connecting
the cell and resistance to the indicator, the couple having been
switched out of the circuit. Thus, to adjust for a range of 500°-1000°
on an indicator giving full-scale deflection for 500°, the resistance
is regulated so that the cell alone causes the pointer to move to the
end of the scale. The method adopted by Paul consists of suitable
resistances inserted in a Wheatstone bridge, which may be thrown off
the balance, and thus cause an opposing E.M.F. of the correct amount at
the terminals of the indicator.
A mechanical set-up has been introduced by the Cambridge and Paul
Instrument Company, the indicator in this case having a suspended
coil. By turning a milled-head a twist may be given to the suspending
strip, and by the turning of a second head the pointer may be brought
back to zero, retaining the initial twist, which is opposed to that
produced by the current due to the couple. Thus, if the imposed twist
were such as to move the pointer to the 400° mark on the scale, the
temperature indicated by the junction would be the observed reading
plus 400. By this method it is possible to obtain any desired range
within the limits of the indicator. The danger of producing errors due
to “creeping” is said to be negligible.
[Illustration: FIG. 19.—CIRCUIT OF NORTHRUP’S “PYROVOLTER.”]
=Potentiometer Indicators.=—The advantage of measuring E.M.F.
by the potentiometer method is that the result is independent of the
resistance of the circuit under test, whereas an indicator is affected
by changes in the resistance of the circuit in which it is inserted.
When long leads are used to connect a couple to its indicator, notable
errors may be caused by the varying resistance of the leads, due
to changing temperature; and, in addition, the resistance of the
couple-wires varies according to temperature and depth of insertion in
the furnace. Attempts have therefore been made to produce indicators
based on the potentiometer principle, suitable for workshop use, and
one form, known as Northrup’s “Pyrovolter,” is arranged as shown in
fig. 19, A. A cell D sends a current through a rheostat R, a copper
coil C, and a manganin coil S. The copper coil has the same resistance
as the copper winding of the indicator G. The couple is connected,
with G in circuit, across the manganin coil S, the resistance of this
material being unaffected by temperature. By adjusting R until no
deflection is shown on G, the drop of volts across S is made equal
to the E.M.F. of the couple. To measure this drop, a key is pressed,
altering the circuit as shown in B, the indicator being now in series
with S and the couple detached. The value of the current passing
through S is unchanged, as the indicator coil has the same resistance
as the copper coil C, which it now replaces. The deflection on G
indicates the value of this current, and, as the drop of volts across
S is proportional to the current, G may be marked off to read E.M.F.
and the corresponding temperature of the junction. The advantages
claimed are that the indicator may be used with any type of junction,
and is unaffected by temperature changes in the circuit. A similar
instrument is made by the Brown Company of Philadelphia. Up to the
present potentiometer indicators have not been adopted to any extent
in Britain, and the adjustments necessary to obtain a reading must be
accounted a distinct drawback from a workshop standpoint.
=Recorders for Thermo-electric Pyrometers.=—It is frequently of
importance to know not only the existing temperature of a furnace, but
also the fluctuations to which it is subject. Continuous observation of
a pyrometer would involve too much labour, and it is therefore evident
that an automatic recorder would possess many advantages in such cases.
A continuous record shows whether the attendant has maintained the
temperature between the prescribed limits, and furnishes a permanent
history of a given operation, which often serves as a guide to future
procedure.
The first successful recorder, suggested by Sir W. Roberts-Austen and
designed by Gen. Holden, F.R.S., was used in conjunction with a mirror
galvanometer. In its original form, the spot of light from the mirror
was made to fall on a sensitized plate, to which a gradual vertical
motion was conveyed by connecting the dark slide to a water-float by
means of a chain and pulley. The float was placed in a tank of water,
which was gradually emptied through a tap, causing the float to sink
and the plate to rise. If the deflection of the spot of light remained
steady, a vertical straight line was traced on the plate, fluctuations
producing a sinuous line. Trials at known temperatures enabled a
standard plate to be obtained, divided into degrees, which could be
superposed on a trial plate, and the temperatures thus determined. Much
valuable work was accomplished with this recorder by Roberts-Austen
for the Alloys Research Committee of the Institution of Mechanical
Engineers.
[Illustration: FIG. 20.—ROBERTS-AUSTEN RECORDER.]
In its modern form (fig. 20) the photographic plate is replaced by a
sheet of sensitized paper wound round a drum which rotates at a known
rate—say, once in 12 hours—by means of internal clockwork, shown to
the left of the figure. The galvanometer is placed at the opposite
end, and the mirror is illuminated by means of an electric lamp placed
externally, the rays from which are reflected from a prism in the
interior on to the mirror. The ray of light leaving the mirror is
broken into two portions, one of which passes through a narrow slit on
to the sensitized paper, whilst the other portion is reflected on to
a ground-glass scale on the lid, divided so as to read temperatures.
In this manner the arrangement serves not only as a recorder, but
also indicates the existing temperature without necessitating the
examination of the sensitized paper. The whole arrangement is made
impervious to light, so that it may be used in daylight. A dark room is
necessary for fixing the records. When desired, records of two or more
pyrometers may be taken on the same sheet, a clockwork device being
used to switch each instrument in turn on to the galvanometer for a
given period, an external dial indicating which pyrometer is for the
time being in circuit.
Whilst it is a drawback to the use of this recorder that the record is
not visible, the use of a mirror galvanometer confers a high degree of
sensitiveness to the instrument, not possessed by the recorders to be
described subsequently.
[Illustration: FIG. 21.—PRINCIPLE OF THREAD RECORDER.]
=The Thread Recorder.=—In this instrument an intermittent record
is secured in ink, possessing the advantages of visibility during
the period over which readings are taken, and of permanence without
subsequent treatment of the chart. The principle is shown in fig.
21, where A is a boom terminating in a V-shaped piece of ivory, and
attached to the galvanometer suspension B. By means of a cam E, rotated
by clockwork, a bar D is made to descend at stated intervals, pressing
the end of A on to an inked thread G, and causing the thread to touch
a paper wound round the drum C. This drum rotates on its axis once in
25 hours by the action of internal clockwork. The continued rotation
of the cam E alternately raises and depresses the boom A, leaving it
free for a sufficient time to enable it to attain the position it would
occupy if the mechanism were absent. The thread G is passed over
pulleys, and is wound round through an ink-well, so that the portion
opposite A is always moist. With the bar D descending every two
minutes, the successive dots form a nearly continuous line. The paper
on C is divided horizontally into temperatures, and vertically into
time units, so that the temperature existing at any given time may
readily be ascertained. The front of the bar D, or a separate strip
parallel to it, is divided so as to enable temperatures to be read
without reference to the chart. The actual instrument is shown in fig.
22. When several simultaneous records are required, the drum C is
extended, and other galvanometers introduced, to which the separate
pyrometers are connected. Several records can be taken on one chart by
introducing a clockwork mechanism to couple each pyrometer in turn to
the one galvanometer.
[Illustration: FIG. 22.—THREAD RECORDER.]
[Illustration: FIG. 23.—SIEMENS’ RECORDER.]
=The Siemens Recorder.=—In this instrument (fig. 23) the boom
from the galvanometer terminates in a knife-edge, and moves over a
thin horizontal rail, the top of which is rounded. Between the rail
and the boom are placed an inking ribbon and a paper chart, which is
moved forward by clockwork. A chopper-bar, also actuated by clockwork,
descends at about half-minute intervals, and depresses the end of the
galvanometer boom, thus producing a small dot on the chart. The paper
is 12 cms. wide and 40 yards long; it is divided into time and
temperature units, and moves forward at the rate of 2 cms. per hour.
Levelling screws are fixed to the base of the recorder.
[Illustration: FIG. 24.—FOSTER’S RECORDER.]
=Foster’s Recorder.=—Foster’s recorder (fig. 24) is designed
for use with base-metal couples of the nickel-chromium type, known as
Hoskin’s alloys, which yield an E.M.F. about five times as large as
a platinum-rhodioplatinum couple. The force available in this case
enables the coil of the galvanometer to be pivoted in a horizontal
position, the pointer being vertical, and yet to be sufficiently
sensitive. The chart is mounted on a vertical plate which rotates on
its axis, the time ordinates taking the form of concentric circles,
which are cut at an angle by the temperature ordinates. At the terminus
of the pointer is placed a small capillary tube, fitted with an inked
wick, which, when pressed upon the chart, makes a mark. The presser-bar
is curved to the same radius as the pointer, and carries a pad wetted
with ink, so that at each depression the supply of ink to the wick is
replenished by an amount equal to that imparted to the chart. This
recorder is sometimes fitted with special contacts, so that when the
correct temperature exists an electric lamp with a white bulb remains
lighted; whereas when too low or too high a green or red lamp is lit
up, and an alarm thus given. Such an addition involves the use of a
relay circuit, but is advisable in cases where expensive articles
might suffer if overheated. It can be modified to permit of several
simultaneous records being taken, and possesses the advantage that the
whole chart is visible at any time. On the other hand, the circular
coordinates may be accounted a drawback by some, as not being quite
so familiar to read as charts in which the lines are straight. Robust
construction is a feature of this recorder.
[Illustration: FIG. 25. PAUL’S RECORDER.]
=Paul’s Recorder.=—In the recorders previously described, the
motive power is furnished by clockwork. R. W. Paul has introduced an
instrument in which all the moving parts are actuated by a motor driven
with power from the mains. This recorder is shown in fig. 25. The
motor is furnished with a special type of governor to ensure constant
speed, and is connected by suitable gearing to the mechanisms moving
the chart, presser-bar, and inking ribbon, provision being made to vary
the speeds of these movements by changing the gear. The galvanometer
is of uni-pivot pattern, and the pointer is pressed at intervals on to
a typewriter ribbon which lies above the chart. Immediately beneath
the ribbon is placed a thin metal rod over which the paper passes, and
the result of the contact is to produce a small dot. As in the thread
recorder, the chart is divided into rectilinear coordinates, the ribbon
in this case serving the same purpose as the thread in the former
instrument. The lower part of the recorder is prolonged so as to
display a considerable length of the chart, which is in the form of
a roll, and is drawn forward by the mechanism. When two records are
taken simultaneously the ribbon consists of two strips, one moistened
with black ink, and the other with red; and it is arranged that each
strip in turn is over the thin rod on to which the pointer is pressed,
so that the records appear in separate colours. This recorder can also
be arranged for multiple records, or fitted with a scale-control.
With a view to workshop use, all the covers are fitted with faced
metal joints, which are much better for keeping out dust than wooden
ones. A further useful feature is that the various units in the
recorder—galvanometer, motor, feed and record mechanism, and reducing
gear—are all separate and interchangeable. By introducing a suitably
divided chart this recorder will also serve for a radiation pyrometer,
or, as will be shown later, for a resistance pyrometer.
[Illustration: FIG. 26.—LEEDS-NORTHRUP RECORDER.]
=The Leeds-Northrup Recorder.=—The Leeds and Northrup Company,
of Philadelphia, manufacture a recorder which is largely used in the
United States. As in Paul’s recorder, all the mechanism is motor
driven; but the other arrangements are entirely distinct. Instead of
measuring the deflection of the pointer, a zero deflection method is
used. The pyrometer forms part of a potentiometer circuit, and the
function of the mechanism is to oppose an E.M.F. equal to that of the
pyrometer, from which the temperature is known. This has the advantage
that the measurement is independent of the resistance of the leads, and
is capable of great accuracy. The manner in which the adjustment of the
opposing E.M.F. is controlled may be understood from fig. 26, in which
the galvanometer coil is shown at the top of the figure. The shaft from
the motor carries four cams, B, C, D, D, and at each revolution the cam
B raises the bar (5) so as to press it against an arm attached to the
galvanometer coil. At the same moment the cam C pushes against the bar
(3), and thereby releases a clutch (2) from the disc beneath. As shown,
the boom from the coil is to the right of the central position, and
is gripped between a bar (5) and the lever (4) when the former rises,
causing an angular movement of the clutch-arm (2). As the rotation
continues the cam C leaves the bar (3), which then springs back and
engages the clutch on the disc. The cam D then descends and presses
on the projection of the clutch-arm to the left, causing the disc to
rotate. The movement of the disc is conveyed to an arm which moves over
the slide wire of the potentiometer; and this movement continues until
the galvanometer boom is in the central or zero position, when neither
of the levers 4, 4 is gripped, and consequently the disc is not fed in
either direction. If the boom swing to the left, the movement of the
disc will evidently be in the converse direction to that described.
In this recorder considerable power is available to drive printing or
other mechanisms. The arm moving over the potentiometer wire carries a
pen which marks the moving chart, or, when several records are taken
simultaneously, a stamping machine is used which impresses the number
of the pyrometer on the chart. The same galvanometer mechanism serves
also for use with resistance pyrometers, as will be explained later.
=Control of Furnace Temperatures.=—Many attempts have been made
to secure the automatic regulation of furnace temperatures by means of
mechanisms controlled by an indicator or recorder. In the arrangement
employed by the Brown Company of Philadelphia, movable stops are
provided, which may be brought to any part of the scale, the mark
between the stops representing the temperature it is desired to
maintain. The indicator (or recorder) is provided with a presser-bar
which descends periodically; and if the temperature be too low the
depressed pointer completes a circuit through the inner stop, whilst
if too high the circuit is through the outer stop. Both circuits
contain a relay which brings a mechanism into operation, the result
being to increase the supply of electricity or gas if the temperature
be too low, or to diminish the supply when too high. When correct,
the depression of the pointer fails to complete either circuit, and
in this manner control between small limits may be ensured. In the
case of large furnaces the relay circuits are employed to light lamps
of different colours, the adjustment then being made by the man in
charge of the furnace. Arrangements of this kind effect a considerable
saving in fuel by preventing unnecessary heating, and are particularly
valuable in cases where overheating would be deleterious to the
articles in the furnace. The future will probably witness considerable
developments along these lines.
=Contact-Pen Recorders.=—The force with which the pointer of an
indicator is urged over the scale is relatively small, particularly in
the case of pyrometers in which the platinum series of metals are used,
as these furnish only a low E.M.F. If, therefore, the pointer terminate
in a pen which is in continuous contact with the record-paper, the
friction thus occasioned interferes considerably with the free movement
of the pointer. When cheap-metal pyrometers are used, which yield a
much higher E.M.F., the use of the pointer as contact-pen becomes
more feasible, and if uniform friction at all parts of the paper can
be ensured, records may be taken in this manner; and a recorder so
constructed is simpler and cheaper than those of the intermittent
type. Contact-pen recorders are used in America to some extent, being
made by Bristol, Brown, and others; but so far British makers have not
developed the manufacture of these instruments. At present, contact-pen
recorders must be considered less accurate and reliable than those in
which the contact is intermittent.
=Installations of Thermo-electric Pyrometers.=—When a number of
furnaces in the same establishment are to be controlled, considerable
economy may be effected by making one indicator serve for all the
couples, which in this case must necessarily be made up of wires
identical in thermo-electric value. Such an arrangement is shown in
fig. 27, in which H^1 and H^2 represent two couples, one wire from
each being connected to one of the terminals of the galvanometer G.
The other terminal of the galvanometer is connected to the arm D of
a switch, and the remaining thermocouple leads are connected to the
points 1 and 2 respectively on the circumference. As shown, H^1 is
connected to the galvanometer, and by turning the arm D to the point
2 the other couple would then be connected. Any number of junctions
may thus be arranged with a single indicator. When this arrangement
is adopted in a workshop, it is advisable to construct a small wooden
building at a spot convenient for most of the furnaces, in which the
indicator and switchboard are kept, and which could also contain a
recorder if necessary; a spot as free as possible from vibration being
preferable. Separate indicators are only necessary when a furnace is
used for special work.
[Illustration: FIG. 27.—CONNECTIONS FOR AN INSTALLATION OF
PYROMETERS.]
In some instances a second indicator is kept in the shop office, to
which all the pyrometers are wired, and which serves as a standard.
The scale of the office indicator is checked daily at one point;
and by connecting a given couple first with the shop indicator, and
immediately afterwards with the office standard, any errors can be
detected. It is also possible to ascertain the temperature of any given
furnace in the office at any time, and so to control the whole. In
fixing up such an arrangement it is necessary that each couple and its
leads, up to the indicator, should possess the same resistance, or
should not differ by an amount sufficient to affect the readings. The
general experience of a properly managed installation is that the cost
is saved in a few months in fuel alone; and, in addition, the work is
carried out to much better advantage owing to complete control from the
office.
=Management of Thermo-electric Pyrometers.=—Generally speaking,
thermo-electric pyrometers give little trouble in practice, but the
management should always be placed in skilled hands. It is advisable
to test each instrument periodically at a fixed point near the working
temperature, by the method explained on page 57; and if two or three
pounds of material be used, the protecting shield need not be removed.
A useful material for checking pyrometers near the critical range of
steel is an alloy of 60 per cent. of copper and 40 per cent. of tin,
which gives a well-defined freezing point at 738° C., and which may be
used indefinitely in a reducing atmosphere Any serious error is easily
detected by observing that the indications differ widely from those
generally obtained under the same working conditions. If an error of
20° C. or more is noted, it is advisable to form a new junction, as the
discrepancy will probably become greater, being due to a change at the
hot junction. A small error, of the nature of 5 or 10° C., may be due
to “creep” in the indicator, which may be adjusted accordingly, or
a numerical correction may be made when taking a reading. An iron
protecting sheath may be saved from rapid oxidation by black-leading
once per week, which greatly prolongs its useful life, but should be
replaced immediately it becomes dangerously thin in any part. Coating
with aluminium powder also greatly prolongs the life of an iron sheath.
When used in lead baths, the immersed part, if of iron or steel, should
be bored from the solid, and left thick at the portion opposite the
surface of the lead, where most corrosion occurs. A graphite tube,
or one made of a composition containing graphite, is often useful in
cases where iron is readily corroded, and can be used to much higher
temperatures.
When a number of instruments are in use, it is advisable to keep a
standard pyrometer for checking purposes, preferably one which has been
certified by the National Physical Laboratory. In conducting a test,
the couples, with protecting-tubes removed, may be placed in the tube
of an electric furnace of the type shown in fig. 29, in close proximity
with the standard junction. On raising the temperature gradually, the
readings of each working instrument may be compared with the standard,
and the necessary corrections discovered. Care must be taken to prevent
contact with the furnace tube, and this may be accomplished by passing
the wires through an asbestos stopper fitted into the end of the tube.
When recorders are used the attendant should make himself thoroughly
conversant with the details of the mechanism, so as to be able to
remedy any minor ailments, which are, as a rule, easily cured. On no
account should an unskilled workman be trusted with recorders; it is
better and safer to keep these in the office, where they will not be
likely to be damaged or tampered with. All records should be kept
for future reference, properly dated, and labelled according to the
operations represented.
=Laboratory Uses of Thermo-electric Pyrometers.=—Numerous
operations carried out in muffle furnaces at prescribed temperatures
require no special precautions beyond those previously given.
In determining the melting points of metals or alloys, however,
a porcelain or silica sheath is inadvisable, as they are easily
corroded. An iron sheath is proof against some metals, but not
against others, and it is always safer to fix a thin fireclay sleeve,
closed at the end, over the part immersed. A sheath of graphite or
graphite composition may be used for temperatures above 1100° C.; and
occasionally a sheath bored from a thick arc-lamp carbon, coupled to
an iron tube beyond the heated part, will be found useful at high
temperatures. Alundum is useful up to 1600° C, and for temperatures of
this order the higher refractories such as silfrax and zirkite may also
be used to advantage.
[Illustration: FIG. 28.—DIFFERENTIAL METHOD FOR DETERMINING CRITICAL
POINTS OF STEEL.]
The determination of the “critical” points of steel call for special
mention. In cooling down a mass of steel the fall of temperature is
arrested at one or more points, observations of which are frequently
of service in deciding the subsequent treatment of the steel. A
method commonly employed is known as the “differential method,” and
is indicated in fig. 28. The sample of steel, A, is placed side by
side with a piece of nickel, B, of equal dimensions, in the tube of
an electric furnace. A naked junction, C, is placed in a hole drilled
in A, and is connected to the galvanometer G, which is calibrated to
read temperatures. A two-junction circuit, formed of a junction D
placed in the hole in A, and another junction E located in the hole in
B, are connected to a delicate galvanometer H. The furnace is heated
until the galvanometer G indicates 900° C., when the arrangement is
allowed to cool. As A and B, under normal circumstances, cool at an
equal rate, the junctions D and E will be at the same temperature, and
no deflection will be observed on H. When, owing to recalescence, the
cooling of A is arrested, B, not being thus affected, will continue to
cool, thus producing a difference between the temperatures of D and E,
and consequently a deflection on H. The temperature of A at the time
this occurs is read off on G.
[Illustration: FIG. 29.—ELECTRIC TUBE-FURNACE.]
The furnace illustrated in fig. 29 is suited to this determination. It
consists of a silica tube 1 foot long, wound with a special resistance
wire and efficiently lagged, and may be heated in safety to 1000° C.
for long periods, and to 1200° for a short time. It may be placed
across the electric mains directly, and reaches 900° C. in less than
half an hour. It consumes 600 to 700 watts at the highest temperatures,
and the cost of re-winding is small. This furnace is useful as a
general laboratory appliance, and may be kept at a given steady
temperature by the use of an external resistance.
The wires in this experiment should be platinum and iridioplatinum or
rhodioplatinum, or a good pair of base metals, and the junctions in A
should be separated from each other and from the specimen by asbestos;
the same precaution being taken to prevent the junction E from touching
B. A thin layer of mica should be used beneath A and B, to avoid
contact with the furnace tube, which, when hot, allows of leakage of
current from the heating coil. Both A and B may be 1½ in. long, ¾ in.
diameter, with a hole ¼ in. diameter drilled to a depth of ¾ inch.
An alternative method is to insert a junction in a hole in the
specimen, and to take direct readings as the temperature slowly
rises or falls, when an arrest in the movement of the pointer of the
indicator shows that the change-point has been reached. Special sets
are made for this purpose.
=Measurement of Lower Temperatures by the Thermo-electric
Method.=—Many cases arise in practice in which a thermal junction
and a sensitive galvanometer are preferable to a mercury thermometer;
and below -39° C., at which temperature mercury freezes, a thermal
junction is frequently better to employ than an alcohol or pentane
thermometer. A number of practical examples of the use of thermal
junctions for ordinary and low temperatures will now be considered.
=Measurement of Surface Temperatures.=—A mercury thermometer,
when laid on a hot surface, only touches along a line, and does not
show the true surface temperature. The construction of a thermal
junction suitable for this purpose is described on page 41, and for
steam-pipe surfaces, hot plates, and the exterior of furnaces, a
specially calibrated millivoltmeter, giving a full-scale deflection
with 20 millivolts, may be used. In making the temperature scale,
boiling water (100°C.), boiling aniline (184° C.), and melting tin
(232° C.) are convenient standards. If the surface temperature be less
than 100° C. a mirror galvanometer should be used, and the junction
standardized in paraffin wax (freezing point usually about 50° C.,
but should previously be determined with an accurate thermometer),
absolute alcohol at boiling point (79° C.) and boiling water. The
author has found that this method yields excellent results in the case
of steam-pipes, the exterior of rotary cement kilns, and hot surfaces
generally.
=Measurement of Low Temperatures.=—Junctions of iron and
constantan, Hoskin’s alloys, copper and German silver, or copper and
constantan, are suited to these measurements. In a laboratory the
cold junction may be kept in ice in a Dewar vessel, the mechanically
protected form known as the “Thermos” flask being very useful for this
purpose. With a good mirror galvanometer precise readings may be
secured, 1/10 of a degree C. being easily detected. Calibration between
-40° and +40°C. may be effected by comparison with a standard mercury
thermometer, a water-bath being used above 0°, and alcohol surrounded
by a freezing mixture of ice and calcium chloride crystals below
zero. For very low temperatures (-200°C. or less) the junction maybe
calibrated in solid carbon dioxide (-78°C.) and liquid air (-184°C.).
Dewar has found that copper and German silver form a reliable junction
for very low temperatures, and the author has successfully used a
couple of Hoskin’s alloys for special work down to -200°C., a pivoted
indicator being employed. No couples tested show a linear relation
between E.M.F. and temperature at these low ranges.
Owing to the magnitude of the error caused by changes in the cold
junction, the thermo-electric method is not suited to the measurement
of atmospheric temperatures, or for explosive magazines or cold stores.
In such cases instruments of the resistance type, to be described
later, are used.
=Temperature of Steam, Exhaust Gases, etc.=—For measuring the
temperature of ordinary or superheated steam, the exhaust gases from
internal combustion engines, etc., iron-constantan junctions, with
suitable indicators, are satisfactory. When placed in a pipe the
junction should be as nearly as possible in the centre, so as to
avoid the cooling effect of the walls. Several junctions, situated in
different parts of the pipe, may be used with a single indicator and
suitable switchboard. The above remarks also apply to the hot-blast for
blast furnaces, and similar instances where the temperature does not
exceed 900° C.
=Measurement of Differences of Temperature.=—Cases frequently
arise in practice in which the difference in temperature between two
points is required, and if this difference be subject to rapid changes,
a mercury thermometer, from its large mass, would not respond with
sufficient rapidity to indicate these changes. In such cases a circuit
is made after the manner of fig. 2, one junction being located at
each point; thin wires of iron and constantan being used. For small
differences—1° C. or less—a mirror galvanometer should be used.
Calibration may be performed by placing one junction in hot water and
the other in cold, the water temperatures being read with an accurate
thermometer.
=Advantages of the Thermo-electric Method of Measuring
Temperatures.=—Compared with other methods, the thermo-electric
possesses the following points of superiority:—(1) Simplicity, no
special experiment being necessary to obtain a reading; (2) cheapness
of outfit; (3) adaptability to a variety of purposes; (4) ease of
repair in case of damage; (5) robustness, not being liable to get
out of order under workshop conditions; and (6) suitability to the
purpose of a centrally controlled installation. The drawbacks are:—(1)
Liability to error owing to fluctuations in the cold junction (which
may be avoided with care); and (2) lack of sensitiveness at very high
temperatures compared with the resistance method—a point seldom of
great practical importance, as the limit of accuracy is usually within
the amount by which an ordinary furnace fluctuates in temperature under
working conditions.
CHAPTER IV
RESISTANCE PYROMETERS
=General Principles.=—When a pure metal is heated, its resistance
to electricity increases progressively with the temperature. Certain
alloys, on the other hand, show a practically constant resistance at
all temperatures, examples of such alloys being constantan, manganin,
and platinoid. All the elementary metals, however, exhibit a tangible
rise in resistance when the temperature is augmented; and Sir W.
Siemens, in 1871, proposed to apply this principle to the measurement
of high temperatures by determining the resistance, and deducing the
corresponding temperature from a table prepared under known conditions.
The choice of a metal is in this case more greatly restricted than in
the selection of materials for a thermal junction. A certain amount
of external corrosion does not alter the E.M.F. of a junction; but an
alteration in size produces a marked difference in the resistance of
a wire, which varies directly as the length and inversely as the area
of cross-section. To the necessity for the absence of any internal
physical change affecting the resistance is therefore added the further
condition of permanence of external dimensions. For temperatures
above a red heat the only feasible metals to use are platinum or the
more expensive metals of the platinum series—and hence platinum is
universally employed for this purpose. The original Siemens pyrometer
consisted of 1 metre of platinum wire, 1 millimetre in diameter,
wrapped round a porcelain rod, and protected from furnace gases by an
iron sheath An elaborate method of measuring the resistance, involving
the electrolysis of acidulated water, was adopted for workshop use,
but was too involved to become popular. Later, Siemens employed the
differential galvanometer method, and finally the Wheatstone bridge,
to measure the resistance. Both methods are still in use in connection
with resistance pyrometers, and the principle of each will now be
explained.
=Measurement of Resistance by the Differential Galvanometer.=—A
differential galvanometer is one which possesses two windings, arranged
so that a current passing through the one tends to turn the pointer
in one direction, and through the other to cause a movement in the
opposite direction. If the currents in each winding simultaneously be
equal, the pointer remains at rest under the action of two equal and
opposite forces. The experimental attainment of the condition of rest
serves as a means of measuring resistance, the circuit being arranged
as in fig. 30. Current from a battery B passes through a divided
circuit, one branch containing the adjustable resistance R and one coil
of the galvanometer G; and the other the unknown resistance P and the
opposite coil. The resistance R is adjusted until on tapping the key K
no deflection on the galvanometer is noted, when the current in each
branch of the circuit will be the same. The resistances of each coil of
the galvanometer being equal, it follows from Ohm’s law that P is equal
to R when no deflection is obtained.
[Illustration: FIG. 30.—DIFFERENTIAL GALVANOMETER METHOD OF MEASURING
RESISTANCE.]
The accuracy of this method depends upon the sensitiveness of the
galvanometer, and also upon the extent to which the two coils may be
regarded as truly differential, as the measurement evidently assumes
complete equality in resistance and effect on the moving part. With
modern galvanometers of this pattern, it is possible to secure readings
of sufficient accuracy for the purposes of pyrometry. The method,
however, is less sensitive than the Wheatstone bridge, now to be
described.
[Illustration: FIG. 31.—PRINCIPLE OF WHEATSTONE BRIDGE.]
=Measurement of Resistance by the Wheatstone Bridge.=—The
principle of this method is shown in fig. 31, where _a_ and _b_ are
two fixed resistances of known value; _d_ is an adjustable resistance;
_x_ the resistance to be measured; B a battery; and G a sensitive
galvanometer. If, in this circuit, _d_ be adjusted until no deflection
is shown on the galvanometer, then _a_/_b_ = _x_/_d_; or _x_ = (_a_ ×
_d_)/_b_. Hence, if _a_ = _b_, then _x_ will be equal to _d_.
It is not difficult to construct a portable apparatus, suitable for
workshop use, by means of which the value of _x_ may be determined to
0·01 ohm; and in the laboratory, with a very delicate galvanometer,
0·001 ohm may readily be detected. The Wheatstone bridge method is the
best for the accurate measurement of resistance; but in resistance
pyrometers it is sometimes advisable to sacrifice extreme accuracy
in order to gain advantages in other directions, as will be shown
subsequently.
=Relation between the Resistance of Platinum and Temperature.=—As
platinum is the only feasible metal to use in the construction of
resistance pyrometers, it is essential that the effect of temperature
on the resistance of this metal should be known. Difficulties were
experienced, in the early days of resistance pyrometers, from the fact
that different samples of platinum wire, of varying degrees of purity,
gave widely differing results in this connection; and no certainty was
attained until 1886, when Professor Callendar thoroughly investigated
the subject, and evolved a formula from which the temperature of a
given kind of platinum could be deduced with great accuracy from the
resistance. In order to understand this formula and its application, it
will be necessary to consider the underlying principles upon which it
is founded.
If the resistance of a platinum wire be measured at a number of
standard gas-scale temperatures, and the results depicted graphically
by plotting resistances against corresponding temperatures, the curve
obtained is part of a parabola, exhibiting a decrease in the rate at
which the resistance increases at the higher temperatures. A second
platinum wire, of different origin and purity, and of the same initial
resistance as the foregoing, would furnish a curve which, although
parabolic, would not overlap that obtained with the first wire. The
advance made by Callendar was to deduce a formula from which the
temperature of any kind of platinum wire could be deduced from its
resistance, after three measurements at known gas-scale temperatures
had been determined. The calibration of a resistance pyrometer was
thereby reduced to three exact observations, instead of a large number
distributed over the scale; and, moreover, the formula in question
was found to give results of great accuracy over a wide range of
temperature for any kind of platinum wire.
Before dealing with Callendar’s formula, the term “degrees on the
platinum scale” will be explained. Such degrees are obtained by
assuming that the increase of resistance of platinum is uniform at
all temperatures; that is, that the temperature-resistance curve is a
straight line, and not a parabola. For example, a piece of platinum
wire of 2·6 ohms resistance at 0° C. will show an increase to 3·6 ohms
at 100° C.—an addition of 1 ohm for 100°. We now assume that a
further augmentation of 1 ohm, bringing the total to 4·6 ohms, will
represent an increase of 100°, or a temperature of 200°. Similarly,
a total resistance of 5·6 ohms would indicate 300°, and 12·6 ohms
1000°. The temperature scale obtained by this process of extrapolation
is called the “platinum scale,” and differs considerably from the
true or gas scale, the difference becoming greater as the temperature
rises. This is indicated in fig. 32, in which A represents the true
parabolic relation between resistance and temperature, and B the
assumed straight-line relation. Reading from curve A, the temperature
corresponding to 8 ohms resistance is 600° C.; but from B the same
resistance is seen to represent only 545° C., which is the “temperature
on the platinum scale” to which this resistance refers. An inspection
of fig. 32 shows that at all temperatures, except between 0° and 100°,
the platinum-scale readings for given resistances are less than those
indicated on the gas scale.
Callendar’s formula is expressed in terms of the difference between the
gas-scale and platinum-scale readings, and takes the form
_t_ - _p_ = δ{(_t_/100)^2 - (_t_/100)},
where _t_ = temperature on the gas scale,
_p_ = temperature on the platinum scale.
δ = a constant, depending upon the purity of
the wire.
[Illustration: FIG. 32.—CONNECTION BETWEEN RESISTANCE OF PLATINUM AND
TEMPERATURE: A, ON GAS SCALE; B, ON PLATINUM SCALE.]
In order to determine the value of δ, it is necessary to measure the
resistance of the wire at 0°, 100°, and a third temperature, which
should be considerably above 100°. The readings at 0° and 100° are
requisite to establish the platinum scale of temperatures; the third
reading is required to calculate the value of δ, as _p_ and _t_ are
equal at 0° and 100°, these points forming the basis of both scales. An
example is appended to make this matter clear.
_Example._—A platinum wire has a resistance in ice of 2·6
ohms; in steam, 3·6 ohms; in boiling sulphur, 6·815 ohms.
To find the value of δ, the boiling point of sulphur being
444·5 on the gas scale.
Since an increase of (3·6 - 2·6) = 1 ohm is produced by
100°, the increase observed in boiling sulphur,
(6·815 - 2·6) = 4·215 ohms, will represent a temperature,
on the platinum scale, of (4·215 × 100)/1 = 421·5° _p_.
Applying Callendar’s formula,
(444·5 - 421·5) = δ{(444·5/100)^2 - (444·5/100)}
the value of δ is found to be 1·5.
Callendar, in his experiments, employed the boiling point of sulphur
for the third point, and determined this temperature on the gas scale
with great accuracy, The necessity for extreme precision in applying
this formula is made clear by noting the effects on the value of δ
resulting from small differences in the figures chosen in the above
example. If, for instance, the boiling point of sulphur on the gas
scale were taken at 2° lower, or 442·5, the value of δ would work out
to 1·37; and the error at 1200° C. thus caused would amount to 17°. The
same discrepancy would be observed if the resistance in boiling sulphur
were taken as 6·835 ohms, an error of 0·02 ohm; and a still greater
error would result if the difference in resistance at 0° and 100° were
measured as 0·99 ohm instead of 1 ohm. From an extensive experience of
the difficulties attendant on correctly determining the value of δ,
the author has found that no reliable result can be obtained unless
measuring instruments of the highest precision are used, and elaborate
precautions taken to ensure the exact correction for alterations in
the boiling points of water and sulphur occasioned by changes in
atmospheric pressure. Unless the necessary facilities are at hand, an
operator would be well advised to standardize a resistance pyrometer by
taking several fixed points and drawing a calibration curve, after the
manner recommended for a thermo-electric pyrometer.
If a resistance pyrometer be calibrated so as to read in platinum-scale
degrees, and the value of δ be known for the wire, the correct
gas-scale temperatures may be calculated from Callendar’s formula. The
table on next page gives the results of a number of calculations made
in this manner.
=Changes in Resistance of Platinum when constantly Heated.=—The
resistance of platinum undergoes a gradual change when the wire is kept
continuously above a red heat; and if the temperature exceed 1000° C.
the change becomes very marked after a time, leading to serious errors
in temperature indications when used in a pyrometer. The alteration
under notice is due, as shown by Sir William Crookes, to the fact that
platinum is distinctly volatile above 1000° C., and hence the diameter
of the wire diminishes. This variation constitutes a serious drawback
to the use of resistance pyrometers for temperatures exceeding 1000° C.
COMPARISON OF GAS AND PLATINUM SCALES.
δ = 1·5.
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬
Platinum Thermometer│Air Thermometer Reading│ Difference
Reading (Pt.). │ _t_ (deg. C.). │(_t_ ▬ Pt.).
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬
-100 │ -97·1 │ +2·9
│ │
0 │ 0 │ 0
50 │ 49·6 │ -0·04
100 │ 100 │ 0
200 │ 203·1 │ 3·1
300 │ 309·8 │ 9·8
│ │
400 │ 420·2 │ 20·2
500 │ 534·9 │ 34·9
600 │ 654·4 │ 54·4
700 │ 779·4 │ 79·4
800 │ 910·7 │ 110·7
│ │
900 │ 1049·4 │ 149·4
1000 │ 1197·0 │ 197·0
1100 │ 1355·0 │ 255·0
1200 │ 1526·7 │ 326·7
1300 │ 1716·0 │ 416·0
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬
=Terms used in Resistance Pyrometry.=—Following on the researches
of Callendar and others, certain terms relating to resistance
pyrometers have come into use, and will now be defined.
(1) _The Fundamental Interval_ is the increase in resistance between
0° C. and 100° C, or R_{100}-R_{0}. It should be remembered that the
increases between 200° and 300°, or 800° and 900°, all temperatures
being taken on the gas scale, differ from the fundamental increase.
(2) _The Fundamental Coefficient_ is that fraction of the resistance
at 0° C. by which it increases per degree between 0° and 100°, on the
average, or
(R_{100} - R_{0})
───────────────── .
(R_{0} × 100)
This figure is in reality the average temperature coefficient between
0° and 100°. For pure platinum the value is or 1/260 or 0·003846.
(3) _The Fundamental Zero_ is the temperature, on the platinum scale,
at which the resistance would vanish; it is evidently the reciprocal of
(2), prefaced by a minus sign, or
(R_{0} × 100)
─ ───────────────── .
(R_{100} - R_{0})
For pure platinum this temperature would be -260_p_, since it is
assumed that the average increase or decrease per degree holds
throughout; that is, for every degree the metal is cooled the loss of
resistance is taken to be 1/260 the resistance at 0°. Hence at -260_p_
the resistance, on this assumption, would vanish.
(4) _The Difference Formula_ is the expression which gives the relation
between gas-scale and platinum-scale temperatures, or
_t_ - _p_ = δ{(_t_/100)^2 - (_t_/100)}
This formula has already been fully dealt with.
(5) _The Platinum Constant_ is δ in the above expression. The value
for pure platinum is about 1·5, but small quantities of impurities may
alter the figure considerably. The truth of the formula (4), however,
is unaffected by changes in δ, as _p_ would be correspondingly
altered.
[Illustration: FIG. 33.—PLATINUM RESISTANCE PYROMETER.]
=Practical Forms of Resistance Pyrometers.=—A typical form of
resistance pyrometer, made by the Cambridge and Paul Instrument
Company, is illustrated in fig. 33. The coil of platinum wire is
wound round the edges of a mica framework, made of two strips of mica
fastened at right angles so as to form a + in section. This method of
winding is due to Callendar, who discovered that mica was chemically
inert towards platinum, even at high temperatures. The leads, also of
platinum wire, pass from the coil through mica washers to terminals
fastened to the boxwood head. A second wire, not connected with the
coil, but identical in length and diameter with the ordinary leads,
is bent into two parallel branches, which are passed through the mica
washers side by side with the leads, and are brought to a second pair
of terminals in the head. The function of this wire is to compensate
for changes in the resistance of the leads when heated, by opposing
the compensating wire to the pyrometer in the measuring arrangement,
when the resistance of the leads and wire, being equal, will cancel,
the resistance actually measured being in consequence that of the
coil only. Fig. 34 shows the connections for a Wheatstone bridge when
this method of compensation is employed, _a_ and _b_ representing
two equal fixed resistances, P the pyrometer coil, _x_ the leads, L
the compensating wire, and _d_ the adjustable resistance. When no
deflection is observed on the galvanometer,
_a_ _x_ + P
──── = ──────── ,
_b_ L + _d_
and since _a_ = _b_ and _x_ = L, it follows that P = _d_.
The protecting tube used by the Cambridge and Paul Instrument Company
is made of porcelain, which is found to shield the platinum completely
from the furnace gases, but is extremely fragile, and for workshop use
should be protected by an outer iron sheath.
[Illustration: FIG. 34.—WHEATSTONE BRIDGE AS USED WITH A RESISTANCE
PYROMETER.]
Resistance pyrometers made by other firms differ in detail from the
foregoing. In the Siemens pyrometer the coil is wound on special
fireclay, and protected by an iron sheath, the space between the coil
and the sheath being filled with magnesia, which effectively prevents
the corrosion of the platinum; and compensation is effected by means
of a single wire passing down the centre and connected to one end of
the coil, a special form of Wheatstone bridge being used to take the
measurement. In the instruments made by R. W. Paul the coil is made
of flat strip rolled out from wire, wound on mica, and protected by
a silica tube and outer iron sheath. The Leeds-Northrup Company of
Philadelphia employ a rod of obsidian on which to wind the coil, and
also make a form in which the coil is wound so as to be
self-sustaining, thus dispensing with the support. In all cases the
coil is wound non-inductively, _i.e._ the wire is doubled before making
into a spiral.
The zero resistance of a given instrument depends upon the accuracy
of the measuring appliances used, and upon the degree of precision
it is desired to attain. If, for example, it is intended to read to
1°C., with appliances capable of measuring to 1/100 of an ohm, a
convenient zero resistance is 2·6 ohms; it being found that with pure
platinum the resistance rises from 2·6 ohms at 0° to 3·6 ohms at 100°
C., an increase of 1/100 of an ohm for 1° C. With coarser measuring
arrangements, for the same degree of precision, a correspondingly
higher zero resistance will be required; thus if 1/25 ohm be the least
amount detectable by the measuring device, a zero resistance of 10·4
ohms would enable 1° C. to be observed. It is evident that a suitable
zero resistance may be calculated similarly in all cases when the
limit of the measuring appliance is known, and the minimum temperature
interval specified.
For work above a red heat, the leads from the coil should always
be made of platinum. Copper leads, when heated, give off vapour in
sufficient quantity to attack the platinum; and the same applies to
a greater degree to all kinds of solder. For low temperature work,
however, copper leads may be used, thus reducing the cost of the
instrument. Mica, above 1000° C., tends to crumble; and most forms melt
at 1300° C. or lower; hence a mica-wound instrument should not be
used continuously above 1000° C. The fireclay winding used by Siemens
permits of occasional readings being taken up to 1400° C., and the same
applies to wires wound on obsidian (melting point = 1550° C.), or those
in which the coil is self-sustaining. As previously mentioned, however,
alterations in the platinum itself render continuous readings above
1000° C. inaccurate after a short time.
It has been pointed out that with accurate measuring devices, a
resistance corresponding to a change of 1° C. can be measured; and it
might appear at first sight that the resistance method is considerably
more accurate in practice than the thermo-electric. If a perfectly
constant temperature were to be measured, a resistance pyrometer
would undoubtedly give a closer indication; but constancy to 10°
C. is seldom possible with gas-fired or coal furnaces or other hot
spaces in which pyrometers are used. The accuracy of a pyrometer under
workshop conditions therefore depends upon the rapidity with which it
responds to temperature fluctuations, which condition will evidently
be influenced by the thermal conductivity of the sheath. As it is
necessary to protect a resistance pyrometer with a porcelain or silica
sheath, both of which are poor conductors of heat, this instrument is
in consequence not capable of following a rapidly changing temperature.
The same applies to the magnesia packing used in the Siemens form;
whereas a thermo-electric pyrometer is often sufficiently shielded
by an iron tube, which transmits heat with a fair degree of freedom.
The superior delicacy of the resistance method is therefore nullified
by the sluggishness of its indications; and for reading changing
temperatures the thermo-electric pyrometer is at least equally
accurate. If, however, a constant temperature can be obtained, as
in the determination of melting points, or when using experimental
furnaces capable of exact regulation, the steady temperature reading
may be secured with greater precision by using the resistance pyrometer.
=Indicators for Resistance Pyrometers.=—All existing indicators
for resistance pyrometers are in reality outfits for measuring
resistance, either by the Wheatstone bridge, differential galvanometer,
or other method, the resistance being translated on the dial into
corresponding temperatures. Typical examples will now be described.
[Illustration: FIG. 35.—SIEMENS’ DIAL INDICATOR.]
=Siemens’ Indicator.=—This instrument is based upon the
Wheatstone bridge principle, and is shown in fig. 35. The galvanometer
is mounted in the centre of the dial, round the edge of which is fixed
a ring on which the adjustable resistance is wound in spiral form.
Suitable terminals are provided, duly labelled, to which the battery,
pyrometer leads, and compensator are attached. A brass arm, movable
about the centre of the dial, terminates in a tapping-key which moves
over the adjustable resistance; the key being placed in the battery
circuit. The fixed known resistances are located in the interior of the
indicator. The adjustment consists in moving the key round the
circumference until, on tapping, no deflection is obtained on the
galvanometer. The pointed end of the movable arm then indicates
the temperature of the pyrometer on the dial, which is marked in
temperatures corresponding to the resistance opposed to the pyrometer
for different positions of the key. In taking a reading, the operator
is guided by the fact that when the temperature indicated is too high,
the movement of the galvanometer needle will be in one direction;
whereas if too low an opposite deflection will be given. The
intermediate position of no deflection must then be found by trial; and
the procedure should not occupy more than two minutes if the observer
possess an approximate notion of the temperature to be measured.
[Illustration: FIG. 36—WHIPPLE’S INDICATOR.]
=Whipple’s Indicator.=—This instrument (fig. 36) is employed
by the Cambridge and Paul Instrument Company, and is also a form of
Wheatstone bridge. The pyrometer leads and compensator are connected to
properly labelled terminals T, and the battery to other terminals
at the opposite side of the box. The pointer of the galvanometer is
visible through the small window B, and a battery of two dry cells is
placed at the side of the box. The fixed resistances are contained in
the interior, and the adjustable resistance consists of a continuous
wire wound on a drum, which may be rotated by the handle H. The shaft
connecting H with the drum is screwed, and works in a nut, so that the
turning of H produces a spiral movement of the drum. The adjustment
consists in rotating H until, on tapping the key F, no deflection of
the galvanometer pointer is observed. The temperature of the pyrometer
is then read off directly from a paper scale wound round the drum and
rotating with it, visible through the window A, the reading being
indicated by a fixed pointer. This arrangement forms a compact and
convenient indicator.
[Illustration: FIG. 37.—THE HARRIS INDICATOR.]
=The Harris Indicator.=—In the Siemens and Whipple indicators it
is necessary, before a reading can be taken, to adjust a resistance
until the galvanometer shows no deflection—an operation which takes
up time and requires a fair amount of skill. This is obviated in the
Harris indicator, made by R. W. Paul, and shown in fig. 37. This
instrument is a special form of ohmmeter, which automatically indicates
the resistance of the pyrometer by the movement of the pointer; the
scale, however, being divided so as to read corresponding temperatures.
In this indicator the scale may be made to notify an excess
temperature—say 100°—above a given fixed number, and hence is capable
of yielding an exact reading over the working range for which it is
used. It may also be connected so that the whole scale represents the
complete range—say 0° to 1000° C.—or other specified interval. The
advantage possessed by this instrument is that the manipulation is much
simpler than in the indicators previously described.
=The Leeds-Northrup Indicator.=—In this apparatus the Wheatstone
bridge principle is employed, but the galvanometer is provided with a
scale divided or temperatures. Coils are provided which correspond to
an increase of resistance due to a rise of 100° C. on the part of the
pyrometer, and by inserting these coils in the circuit the temperature
is obtained to the nearest 100°. If the temperature were exactly at an
even hundred—say 700°—the pointer of the galvanometer would be at
zero on its scale; but if now the temperature rose, the system would
no longer be balanced, and the galvanometer pointer would move over
its scale by an amount depending upon the potential difference at its
terminals. A very sensitive galvanometer would give a movement to the
end of its scale with a slight alteration from the correct balance of
the system; but by using a coarser instrument the pointer would remain
within bounds; and the greater the increase of resistance, the larger
would be the deflection. It is possible, in such a case, to divide
the galvanometer scale to read temperatures corresponding to a given
increase above that of the coils placed in the circuit. In one form
of the Leeds-Northrup indicator, the whole scale is thus divided to
read 100°, and the reading is obtained by adding the figure shown on
the galvanometer to the hundreds represented by the coils inserted.
In another form the galvanometer has a central zero, and its scale is
divided both right and left, one side giving the number of degrees
above, and the other below, the nearest hundred. The observations are
thus much simpler than in the case where adjustment to the condition of
no deflection is requisite.
=Siemens’ Differential Indicator.=—This form of indicator is
still in use, and consists of a differential galvanometer and box of
resistance coils, connected as shown in fig. 30. By adjusting the
coils until no deflection is produced, the resistance of the pyrometer
is obtained, and the corresponding temperature read off from tables
provided. This form of indicator is preferred by some users, but it is
less sensitive than the more recent Wheatstone bridge indicator made by
this firm (fig. 35), and equally difficult to manipulate.
=Recorders for Resistance Pyrometers.=—The value of records in
high-temperature work has led to the invention of recording mechanisms
for use with resistance pyrometers. The form in common use in Britain
is that devised by Callendar, shown in fig. 38, and consists of a
mechanism for restoring automatically the balance of the resistances
in a Wheatstone bridge circuit, in such a manner as to indicate the
existing resistance on a chart. To this end the moving coil of the
galvanometer carries a boom, or contact-arm, which, on swinging to the
right or left, completes one of two electric circuits. The closing of
either circuit brings into action a clockwork mechanism, which causes
a slider carrying a pen to move over the bridge wire until the balance
is restored, and incidentally to produce a mark in ink on a paper wound
on a drum, which rotates at a known speed. When the resistance of the
pyrometer is balanced, the galvanometer boom will be in a central
position, and the slider at rest; whereas a rise in temperature causing
an increase in the resistance of the pyrometer, will result in the
boom swinging over and completing the circuit, which introduces more
resistance in opposition to the pyrometer. A fall in temperature will
similarly result in the liberation of the second mechanism, owing to
the boom swinging in the opposite direction, with the result that the
slider moves so as to oppose a less resistance to the pyrometer. If the
chart be divided horizontally into equal spaces, representing equal
increments or decrements of resistance, they may be marked to represent
degrees on the platinum scale, which may be translated into ordinary
degrees by reference to a conversion table. In careful and skilled
hands this recorder gives excellent results, and the value of the
records obtained is clearly shown by an inspection of the example shown
in fig. 39, which represents the fluctuations of an annealing furnace
during a period of nine hours. It will be noted that during the period
covered by workman A the furnace has received constant and careful
attention; but workman B has evidently neglected his duty conspicuously
at two separate times.
[Illustration: FIG. 38.—CALLENDAR’S RECORDER.]
[Illustration: FIG. 39.—RECORD OBTAINED WITH CALLENDAR’S RECORDER.]
=The Leeds-Northrup Recorder.=—In the Callendar recorder the
boom which completes the electric circuits is pressed against the
contact-surface merely by the small force due to the axial twist of the
galvanometer coil, which necessitates the use of delicate mechanism
if certainty of action is to be secured. A surer contact is secured
in the instrument made by the Leeds-Northrup Company of Philadelphia,
by means of an intermittent action which will be understood from the
annexed drawing (fig. 40). The boom from the galvanometer terminates in
a platinum tip, P, which moves between two blocks, the upper of which
consists of two pieces of silver, A and B, separated by a strip of
ivory, I, whilst the lower block, C, is another piece of silver, which
is moved periodically up and down by an electro-magnetic contrivance
not shown in the drawing. When the galvanometer is at the position of
balance, the tip of the boom is beneath the ivory piece I; and when C
ascends the tip P is then squeezed on to the ivory, and no current will
then pass from the battery through either of the circuits E or F. If,
however, the point of the boom be beneath A, owing to an alteration in
the temperature of the pyrometer, then on C rising the circuit through
E will be completed; and, similarly, if beneath B the circuit through F
will be established. The result in either case is to bring into action
a mechanism which moves a slider, carrying a pen, over a resistance
wire opposed to the pyrometer in such a manner as to restore the
balance. Certainty of contact is thus secured, which enables all the
parts to be strongly made. The actual recorder is shown in fig. 41, in
which it will be seen that the slider carries an ordinary stylographic
pen in contact with the chart. This recorder is worked on the
differential galvanometer method; and the adjusting resistance, over
which the slider moves, consists of a manganin wire wound on a tapered
core, such that horizontal movements represent equal changes of
temperature, and not of resistance, thus obviating the necessity of
translating platinum-scale readings into ordinary degrees. Concordant
and accurate results, coupled with robust construction, are claimed for
this instrument by the makers. The other type of recorder made by this
firm (fig. 26) may also be used in conjunction with a resistance
pyrometer. In this case the movements described introduce or cut out
resistance opposed to the pyrometer in a Wheatstone bridge circuit,
until the balance is restored.
[Illustration: FIG. 40.—PRINCIPLE OF LEEDS-NORTHRUP RECORDER.]
[Illustration: FIG. 41.—LEEDS-NORTHRUP RECORDER.]
=Paul’s Recorder.=—This instrument, as used for thermo-electric
pyrometers, has already been described. By replacing the galvanometer
by a Harris indicator, and using a suitable chart, the same mechanism
serves to record the indications of a resistance pyrometer.
=Installations of Resistance Pyrometers.=—The resistance method
cannot be so readily applied to the purpose of a centrally controlled
installation as the thermo-electric, owing to the difficulty of
producing a set of pyrometers exactly equal in resistance. The
introduction of the ohmmeter method of measuring resistances, as in
the Harris indicator (page 122), has, however, rendered this project
feasible, as it is possible in this arrangement to bring a set of
pyrometers to a common resistance by adding the requisite amount in
the form of a wire of negligible temperature coefficient. Several
instruments, brought thus to a zero resistance of 3 ohms, for example,
may then be wired up to a Harris recorder, and will give closely
identical results. For various reasons, however, a thermo-electric
installation is preferable.
=Management of Resistance Pyrometers.=—It is not advisable to use
resistance pyrometers continuously above 900° C. (1650° F.), although
an occasional reading may be taken up to 1200° C. (2190° F.). Great care
must be taken that metallic vapours or furnace gases do not find access
to the interior, and for this reason a cracked or defective sheath
should immediately be replaced. As the resistance gradually changes,
even when 900° C. is not exceeded, a reading should be checked at
a fixed point in the neighbourhood of the working temperature, and
allowance made for the observed error. Another method of correction
recommended by some makers is to measure the resistance in ice, and
to note how much this differs from the zero resistance noted when the
indicator was marked, and to correct by simple proportion. Thus, if the
observed resistance in ice were 10·2 ohms, the original having been
10·0 the reading on the indicator would be multiplied by 10·0/10·2 =
0·98, a correction which assumes a linear relation between resistance
and temperature, and is therefore only approximate. Generally speaking,
any serious defect entails the sending of the instrument to the maker,
as a special degree of skill is required to execute the necessary
repairs.
As the indicators are usually not automatic in action, care should be
taken in the manipulation not to damage any part, particularly the
galvanometer; and it is advisable not to trust the instruments to
unskilled observers. The remarks applying to recorders and protecting
sheaths in relation to thermo-electric pyrometers (page 92) apply
equally in this case.
=Special Uses of Resistance Pyrometers.=—In all cases in which
an exact reading is required, and a steady temperature can be secured,
the resistance pyrometer can be used to advantage. Thus for accurate
determinations of melting points and boiling points, or for exact
readings of temperatures in experimental furnaces, a resistance
pyrometer is superior to appliances of other kinds. On the other hand,
it is not capable of responding to changes with the same rapidity as a
thermal junction, and is therefore inferior for such purposes as the
determination of recalescence points, or the temperature of exhaust
gases from an internal combustion engine. The resistance method may
be applied to atmospheric and very low temperatures (liquefied gases,
etc.), to measure steady conditions with accuracy, nickel wire being
sometimes used instead of platinum below 400° C. Many cold stores
are fitted with resistance thermometers, the temperature being read
directly on the galvanometer, which is placed across a Wheatstone
bridge, and shows a deflection which depends upon the amount by which
the bridge is thrown out of balance. Changes in the temperature of the
resistance element may thus be read accurately. Whether the resistance
method is suitable to a given purpose must be decided by the three
factors: (1) temperature to be measured, which must not exceed 1000°
C. continuously; (2) degree of accuracy required (a thermo-electric
pyrometer giving results to 10° C.); (3) stability of the temperature
measured, rapid changes not being readily shown by resistance
pyrometers.
One advantage of resistance pyrometers is that the readings are
independent of the resistance of the wires used to connect the
pyrometer with the indicator, as such wires are duplicated and opposed
to each other in the measuring device, their resistance being thereby
cancelled. Hence the same reading is obtained at any distance, and,
in addition, the head of the pyrometer may vary in temperature to any
extent without altering the reading. These are points of superiority
over the thermo-electric method; but, on the other hand, resistance
pyrometers and indicators are more costly, more fragile, more difficult
to repair, require more skilled attention, and are more liable to get
out of order when used for industrial purposes. These drawbacks have
resulted in restricting the use of resistance pyrometers to special
purposes, the general run of observations being conducted by means of
thermo-electric pyrometers.
CHAPTER V
RADIATION PYROMETERS
=General Principles.=—It is a common experience that the heat
radiated by a substance increases as its temperature rises; and it
would obviously be an advantage if the temperature of a hot body could
be deduced from the intensity of its radiations, as the measurement
could then be made from a distance, without the necessity of placing a
pyrometer in contact with the heated substance. At temperatures above
1000° C., when difficulties are experienced either with the metals
or protecting sheaths of thermo-electric or resistance pyrometers,
the advantage gained would become more conspicuous as the temperature
increased. A brief survey of our knowledge of the relations between
radiant energy and temperature will indicate how this desired end may
be achieved.
Any substance at a temperature above absolute zero (-273° C.) radiates
energy to its surroundings by means of ether waves. Below 400° C. these
waves produce no impression on the retina of the eye, and the radiating
body is therefore invisible in a dark room. Above 400° C., however, a
proportion of visible waves are emitted; and as the temperature rises
the effect on the retina is enhanced, and the body increases in
brightness. The difference between the non-luminous and luminous waves
is merely one of wave-length, the shorter wave-lengths being visible
to the eye; and both represent radiant energy. In addition to giving
out radiant energy, a substance receives waves from its surroundings,
which it absorbs in greater or less degree, and which when absorbed
tend to raise the temperature of the receiving substance. A number of
objects in a room, all at the same temperature, are therefore radiating
energy to one another, and equality of temperature is established when
each object receives from its surroundings an amount of energy equal
to that which it radiates. A hot substance radiates more energy than
a cold one; thus if a hot iron ball be hung in a room it will radiate
more energy to its surroundings than it receives from them, and will
therefore cool until the outgoing energy is balanced by the incoming,
when its temperature will be equal to that of the other objects in the
room.
The rate at which a substance emits or takes up radiant energy depends
upon the nature of its surface. A rough, black surface, such as may
be obtained by holding an object in the smoke from burning camphor,
radiates and absorbs heat with greater freedom than any other; whilst
a polished, metallic surface, which acts as a reflector, is worst of
all in these respects. Even a surface of finely divided soot, however,
does not completely absorb all the radiations which fall upon it, but
exhibits a small degree of reflection. An “absolute black surface,” if
such could be found, would be totally devoid of reflecting power, and
would absorb all the radiant energy incident upon it; and conversely
would radiate all energy reaching it from its under side, without
reflecting any back, or allowing any to pass through in the manner that
light waves are transmitted through a transparent substance. No such
perfect surface is known; but, as Kirchoff showed, it is possible to
make a radiating arrangement which will give the same numerical result
for the energy radiated as would be obtained by a perfect surface at
the same temperature. Such an arrangement is termed a “black body,” and
radiations from it are designated “black-body radiations.”
[Illustration: FIG. 42.—BLACK-BODY RADIATIONS.]
Any enclosure, if opaque to radiant energy, and kept at a constant
temperature, constitutes a black body, and radiations received from the
interior through a small opening in the side are black-body radiations.
Fig. 42 represents such an enclosure; in which, to show the application
to pyrometry, a body A is indicated opposite to an opening in the side,
through which radiations escape from the surface of A. If this surface
were “perfect,” all the waves falling upon it would be completely
absorbed and completely radiated; but to prevent change of temperature
the energy radiated must balance the energy received. If, on the other
hand, the surface of A were a polished metal, the waves falling upon
it from the sides of the enclosure would in the main be reflected; but
here again the energy leaving the surface must equal the amount
received if the temperature be constant. It follows, therefore, that if
no alteration in temperature occur, the energy leaving the surface of A
is independent of the nature of that surface; and the amount escaping
through the opening will therefore be the same, whatever be the
character of the surface opposite the opening. With a good radiating
surface the rays from the enclosure will first be absorbed and then
radiated through the opening; in the case of a poor radiating surface,
the rays will be directly reflected through the opening; the total
energy escaping being the same in either case. It will be seen later
that radiation pyrometers are based upon black-body radiations; and it
is important to note that the arrangement under discussion is realised
in a furnace at a constant temperature, in which A might represent
an object such as a block of steel. It happens, therefore, that the
condition of perfect radiation is attained by the appliances in
everyday use; and, moreover, black-body radiations can always be
secured by placing a tube, closed at one end, in the heated space,
and receiving the radiations through the open end; for this again
represents an enclosure at a constant temperature. Similarly,
radiations from a solid in the interior of the tube of the electric
furnace shown in fig. 29 will be of the same description, and we can
therefore apply with accuracy any instrument based upon black-body
radiations, knowing that the same may be readily realised in practice.
The law connecting the energy radiated by a substance, under given
conditions, with its temperature, was variously stated by different
observers until Stefan, in 1879, deduced the true relation from certain
experimental data obtained by Tyndall. Stefan concluded that the
figures given by Tyndall indicated that the energy radiated by a given
solid varied as the fourth power of its absolute temperature. Numerous
experiments, under different conditions, showed that the fourth-power
law did not apply to all kinds of surfaces or circumstances; but a
strong confirmation of its truth when applied to black-body radiations
was forthcoming in 1884, when Boltzmann showed, from thermodynamic
considerations, that the quantity of energy radiated in a given time
from a perfect radiator must vary as the fourth power of its absolute
thermodynamic temperature. Certain assumptions made by Boltzmann in
this investigation were subsequently justified by experiment; and
numerous tests under black-body conditions have since amply verified
the law. It is upon the Stefan-Boltzmann law that radiation pyrometers
are based; the energy received by radiation from the heated substance,
under black-body conditions, being measured by the instrument, and
translated into corresponding temperatures on its scale.
Expressed in symbols, the fourth-power law takes the form—
E = K(T_{1}^4 - T_{2}^4),
where E is the total energy radiated; T_{1} the absolute temperature
of the black body; T_{2} the absolute temperature of the receiving
substance, and K a constant depending upon the units chosen. If E be
expressed as watts per square centimetre, the value of K is 5·6 ×
10^{-12}; if in calories per square centimetre per second, the value is
1·34 × 10^{-12}. The introduction of the temperature of the receiving
substance, T_{2}, is rendered necessary by the fact, previously cited,
that energy will be radiated back to the hot body, and the net loss
of energy will evidently be the difference between that which leaves
it and that which returns to it from the receiving substance. If
T_{2} were absolute zero, the energy leaving the black body would be
KT_{1}^4; whereas if T_{2} were equal to T_{1}, the loss of energy
would be nil, as a substance cannot cool by radiation to a lower
temperature than its surroundings. The temperatures T_{1} and T_{2}
refer to the thermodynamic scale (page 9), but as the gas scale is
practically identical, Centigrade degrees may be used, measured from
absolute zero, or -273°. An example is appended to illustrate the
application of the law:—
_Example._—To compare the energy radiated through an opening
in the side of a furnace at temperatures of 527°, 727°,
and 927° C. respectively, to surroundings at 27° C.
The quantities will be as
K(800^4 - 300^4) : K(1000^4 - 300^4) : K(1200^4 - 300^4).
since 273 must be added to each temperature to convert
into absolute degrees. Dividing each by K, and expanding
in each case, the ratio becomes
(4096 - 81) × 10^8 : (10000 - 81) × 10^8 : (20736 - 81) × 10^8.
Dividing each by 10^8 and subtracting, the result is
4015 : 9919 : 20655, or 1 : 2·47 : 5·12.
It will be noted in the above example that the effect of the
surrounding temperature, taken as 27° C., is small in quantity,
and becomes proportionately less as the temperature of the furnace
increases. If T_{2} had been ignored in the calculation, the amounts of
energy radiated would have appeared as
1 : 2·44 : 5·06.
It will be seen later, that in calculating the temperature scale of a
radiation pyrometer, the temperature of the surroundings is for this
reason not taken into account. Fig. 43 is a graphic illustration of the
fourth-power law.
[Illustration: FIG. 43.—ENERGY RADIATED BY A BLACK BODY AT
DIFFERENT TEMPERATURES.]
When the relation between temperature and quantity of energy radiated
is known, any instrument which will indicate the amount of the
radiations it receives may be used to measure temperatures. The ray,
for example, may be focused on a thermal junction, which will be heated
in proportion to the amount of energy incident upon it, and when
connected to a millivoltmeter will cause deflections proportional to
the energy it receives. A thin strip of metal might be used in place
of a junction, and by measuring its resistance the heating effect
of the radiations, and hence the amount thereof, may be deduced. A
third method would be to focus the rays on to a compound strip of two
metals, which by altering in shape could be made to furnish a clue to
the quantity of energy received by it. In theory, it is only necessary
to allow the radiations to fall on the working part of any instrument
for measuring low temperatures, when the rise in temperature produced
may be taken as proportional to the energy received, and the thermal
condition of the radiating body deduced from the fourth-power law.
In practice, however, it is desirable that the receiving thermometer
should be small in size; of low thermal capacity, so as to respond
rapidly; and capable of giving a sensitive indication—hence an
ordinary mercury thermometer would be unsuitable for this purpose. A
thermopile, placed at a fixed distance, would fail owing to the cold
junctions gradually warming up by conduction through the pile. The
part receiving the radiations should be coated with lamp-black, so
that practically all the waves impinging upon it, whether luminous or
non-luminous, may be absorbed, and the energy they represent utilised
in producing a rise in temperature.
=Practical Forms of Radiation Pyrometers: Féry’s Instruments.=—In
the year 1902 Féry introduced a pyrometer in which the rays were
focused by the aid of a lens upon a small, blackened thermal
junction, in the same way that the rays of the sun may be focused
by a burning-lens. The junction was connected to a special form of
d’Arsonval galvanometer, which recorded the E.M.F. developed. By taking
the readings of the galvanometer as proportional to the temperature of
the junction—that is, to the radiant energy impinging upon it—the
temperature of the source could be calculated from the fourth-power
law. The drawback to the use of this instrument was the fact that a
proportion of the rays was absorbed by the glass, this proportion,
moreover, varying at different temperatures, so that the fourth-power
law could not be applied with accuracy. By using a fluorspar lens in
place of glass, this error was overcome, but the cost of a good lens
of this material being high, its use in ordinary workshop practice
was rendered prohibitive on account of the price. A number of these
pyrometers, furnished with glass lenses, and calibrated by comparison
with a standard possessing a fluorspar lens, were placed on the market,
but were superseded in 1904, when Féry hit upon the plan of focusing
the rays by means of a concave mirror, thus overcoming the error due to
absorption by the glass lens. This plan, which serves admirably, has
since been adopted in most radiation pyrometers.
=Féry’s Mirror Pyrometer.=—This instrument is shown in
longitudinal and also in cross section in fig. 44. A concave mirror, M,
which has a gilt reflecting surface, is placed at one end of a metal
tube, and is fastened to a rack which engages in a pinion moved by
the milled-head, P, so that on turning P, a longitudinal movement is
imparted to the mirror. A small, blackened thermal junction, shown at
the centre of the cross section, and consisting of a copper disc to
which wires of copper or iron and constantan are fastened, receives the
rays after reflection, and may be brought into focus by suitably moving
the mirror.
[Illustration: FIG. 44.—FÉRY’S MIRROR PYROMETER. SECTION.]
[Illustration: FIG. 45.—FÉRY’S MIRROR PYROMETER. END VIEW.]
The wires pass to terminals _b_ and _b´_ on the outside of the tube,
from which leads are taken to the indicator. In order to discover
when the junction is in the focus of the mirror, an eye-piece, O, is
fitted in the end of the tube, which enables the junction to be seen,
magnified, through a hole in the centre of M. By means of an optical
device placed near the junction, the image of the sighted object,
produced by M, is reflected in two portions to the eye-piece O. When
the junction is exactly in the focus of M, a circular image is seen
round the junction; when out of focus, the appearance presented is
that of two semi-circles not coinciding laterally. The adjustment
consists in moving the mirror until the separate semi-circles produce a
continuous circle; a method at once simple and definite. The front end
of the pyrometer is shown in fig. 45, in which it will be seen that the
entrance may be partially closed by a diaphragm, or left entirely open,
as required. The diaphragm is used to cut off a definite proportion of
the radiations, and is used for very high temperatures, at which, with
full aperture, the indicator needle would be urged beyond the limits
of the scale. On the indicator two separate temperature scales are
provided, one referring to full, and the other to partial aperture.
The same end might be achieved by inserting a suitable resistance in
series with the indicator: but in this case the junction might be
unduly heated, and possibly damaged thereby. The proportions of the
pyrometer are such that at the highest temperatures measured the heat
incident on the junction never raises it above 110° C. Although the
intensity of radiations diminishes as the square of the distance, the
quantity impinging on the junction is, within limits, independent of
the distance: This arises from the property of concave mirrors with
respect to the relation between the size of an image and the distance
of the object producing it. If _r_ = the radius of the mirror, _u_
the distance of the object, and _v_ the distance of the image, both
measured from the centre of the mirror, the relation 1/_u_ + 1/_v_ =
2/_r_ holds for a concave mirror, and when two of these are known the
third may be calculated. Further, if _d_ be the linear dimension of
an object, and _d_{1}_ that of its image, the relation _d_/_d_{1}_
= _u_/_v_ also holds, and from these two expressions all the points
arising in connection with the Féry pyrometer may be determined, as
will best be made clear by examples.
_Example I._—To find the position of the image of an object
formed by a mirror of 6 inches radius, with object at
distance (_a_) 10 feet, (b) 20 feet.
Reducing to inches, and applying in the formula
1 1 2 1 1 1
────── + ────── = ─────── , ──── + ─────── = ───
_u_ _v_ _r_ 120 _v_ 3
and
1 1 1
─── + ───── = ───
240 _v_ 3
from which the values of _v_ are 3-1/13 inches and 3-1/26
inches respectively, a difference of only 1/26 of an inch.
If _u_ were 6 inches, _v_ would also be 6 inches; if u
were infinity, _v_ would be 3 inches. The movement of
the image, when an object is brought towards it from a
great distance, would in the mirror under notice be from
3 inches away to 6 inches away, and at distances of 10
feet and upwards would only differ in position by small
fractions of an inch.
_Example II._—To find the area of the image of a circular
opening, 1 foot in diameter, formed by a mirror of 6
inches radius distant from the opening (_a_) 10 feet;
(_b_) 20 feet.
Since _d_ _u_
──────── = ───── ; then, from the results of
_d_{1}_ _v_
Example 12 120
──────── = ──────── at 10 feet distance,
_d_{1}_ (3-1/13)
and 12 240
─────── = ──────── at 20 feet.
_d_{1}_ (3-1/26)
Hence the linear dimensions, _i.e._ the diameters of
the circular images, will be 0·308 and 0·152 inch
respectively; and the areas 0·074 and 0·0182 square inch.
These areas are to each other practically as 4 : 1.
That is, the area of the image decreases in size directly
as the _square_ of the distance of the object; the squares
of the distances being 100 and 400, or as 1 : 4; whereas
the areas of the images are as 4 : 1.
_Example III._—To find, for a 6-inch mirror, and a junction
of 1/10th of an inch in diameter, the greatest distance
at which the mirror may be placed from an opening 1 foot
in diameter, so as to give an image not less than the
junction.
From Example I it is evident that at any distance
exceeding 20 feet the position of the image will only be
a minute and negligible fraction over 3 inches; hence _v_
may be taken as 3.
Applying values in the formula _d_ _u_
─────── = ───── and
_d_{1}_ _v_
taking _d_{1}_ as equal to the diameter of the junction,
= 0·1 inch,
12 _u_
─── = ──── , and _u_ = 360 inches, or 30 feet.
0·1 3
Beyond this distance the image would be less than the junction. The
conclusions to be drawn from the foregoing examples are: (1) that the
amount of energy received by the junction does not vary, provided the
image overlaps it; and (2) that the limiting distance at which a
correct reading can be secured is that at which the size of the image
is equal to that of the junction. Thus, taking distances of 10 and 20
feet, as in Example II; at the former distance the energy striking
the mirror is four times as great as with the latter; but, on the
other hand, the area of the image at 10 feet distance is four times
as great as that obtained at 20 feet. Hence, at the greater distance,
the proportion of the image impinging on the junction is four times as
great, and the fact that only ¼ the amount of energy strikes the mirror
is thus counterbalanced. All the reflected rays which fail to strike
the junction are ineffective, and pass out through the entrance of the
tube.
[Illustration: FIG. 46.—FÉRY’S SPIRAL.]
The two-scale form of instrument described above is extremely useful
for general purposes, but when all the temperatures to be controlled
fall within the limit of one of the scales, it is simpler and cheaper
to dispense with the diaphragm, and to use an indicator furnished with
one scale only. The single-scale mirror pyrometer is for this reason
more generally employed for industrial purposes; and the Cambridge and
Paul Instrument Company now make a pivoted indicator for use with full
aperture, which is less liable to damage than one which possesses a
suspended coil.
[Illustration: FIG. 47.—FÉRY’S SPIRAL PYROMETER. SECTION.]
=Féry’s “Spiral” Radiation Pyrometer.=—This instrument differs
from the preceding merely in the fact that the rays are focused on a
small spiral, formed of a compound strip of two metals, fixed at one
end and furnished with a pointer at the free-moving end (fig. 46). The
effect of alterations of temperature on this spiral are to cause it to
coil up or uncoil, according to whether the temperature rises or falls.
This movement is magnified by the pointer, the end of which moves over
a dial graduated to read temperatures directly. This arrangement is
shown in section in fig. 47, where C is the mirror, E the eye-piece, S
the spiral, P the pointer, and D the dial, viewed through the window W.
The appearance of the apparatus when viewed from the front is shown
in fig. 48. The advantage gained by the use of the spiral is that the
instrument is self-contained, no galvanometer being necessary; but, on
the other hand, the indications are not so exact, an error of 20° C.
being probable at temperatures over 1000° C. In using this pyrometer,
it is observed that after focusing the hot substance, the pointer moves
rapidly for a time and then pauses, after which it again commences to
creep along the scale. The temperature indicated at the moment the
pause occurs is generally taken as the reading, but this is not always
correct.
[Illustration: FIG. 48.—FÉRY’S SPIRAL PYROMETER. FRONT VIEW.]
The creeping movement is probably due to the whole instrument, and
the air in the interior, becoming heated by the entering rays, and by
proximity to the hot source. In a number of trials made by the author,
it was noticed that when the instrument was allowed to stand near the
furnace for some time before using, thereby attaining the temperature
existing in the vicinity, the “creep” almost entirely vanished. All
things considered, the spiral form of Féry’s pyrometer must be regarded
as more portable but less accurate than that in which the rays are
received on a thermal junction.
=Foster’s Fixed-Focus Radiation Pyrometer.=—The necessity for
focusing, common to all Féry’s radiation pyrometers, is obviated in
Foster’s pyrometer, which, however, cannot be used from so great a
distance. The principle involved in the fixed-focus pyrometer is that
the amount of energy received by a concave mirror and focused on a
thermal junction will not vary so long as the area of the surface
sending rays to the mirror, through a fixed opening, increases as the
square of the distance. This will be understood from fig. 49, in which
C is the mirror, D a thermal junction fixed so as to be in the focus of
the opening E F, and A B the heated surface. The lines joining E and
F to the edge of the mirror intersect in a point G, and provided the
lines G E and G F, if produced, fall within the heated surface A B,
the quantity of energy falling on D will always be the same. A cross
section of the cone G A B is a circle; and if A B be twice as far
away from G as E F, the areas of the circles of which A B and E F are
diameters will be in the ratio 4: 1. But as A B is twice as far from
G as E F, the intensity of its radiations will be as 1: 4; and hence
loss of radiating power is exactly balanced by increase in area.
[Illustration: FIG. 49.—PRINCIPLE OF FOSTER’S FIXED-FOCUS PYROMETER.]
In the actual instrument the tube in which the mirror is placed is
blackened internally, so that no rays reach the mirror by reflection
from it. The diameters of the opening E F and the mirror C are such
that the perpendicular from G on to A B is ten times the length of A
B. Hence, if the heated object be 6 inches in diameter, the limiting
distance of G is 10 × 6 = 60 inches. The position of the point G
is indicated by a ring on the outside of the tube, and in taking a
measurement the tube is brought well within the distance prescribed,
which is in all cases ten times the diameter of the heated object.
Temperatures are read from a galvanometer connected to the thermal
junction, the whole arrangement being portable, as shown in figs. 50
and 51, which represent the instrument in use.
[Illustration: FIG. 50.—FOSTER’S PYROMETER, MOUNTED ON STAND.]
The advantages derived from the use of a fixed focus instrument are
simplicity and cheapness; but, as many occasions arise in practice
in which focusing on an object is a necessity, Foster’s pyrometer
must be regarded as a simplified apparatus not capable of the wider
applications of Féry’s instruments, but of great service in many cases.
Whipple has recently adapted the Féry spiral pyrometer to produce
an instrument with a fixed focus, by fastening the instrument to a
fireclay tube, on the closed end of which the pyrometer is permanently
focused. This form is specially useful for determining the temperature
of molten metals, into which the end of the fireclay tube is plunged,
thus giving true black-body conditions.
[Illustration: FIG. 51.—FOSTER’S PYROMETER, IN USE.]
=Paul’s Radiation Pyrometer.=—Thwing, in America, has introduced
a radiation pyrometer in which the rays from the furnace enter the wide
end of a cone, and by internal reflection are brought to the apex,
at which a thermal junction is located. Paul, in this country, has
marketed a similar instrument, the action of which is shown in fig. 52,
where E is a tube containing a polished cone, C, at the apex of which
is fixed a thermal junction, T. Rays from the hot source A A´ enter the
tube at D, and pass into the cone, being finally reflected on to T,
which is connected to the indicator. So long as the lines joining the
outside of the cone with the extremities of the entrance D, crossing at
O, fall within the hot source, A A´, the reading will be the same at
all distances. Fig. 53 shows the actual pyrometer, mounted on a tripod.
[Illustration: FIG. 52.—PRINCIPLE OF PAUL’S RADIATION PYROMETER.]
=Indicators for Radiation Pyrometers.=—When the radiations are
focused on a thermal junction, the temperature of which is raised
in consequence, the E.M.F. developed is in accordance with the laws
discussed in Chapter II, and any thermo-electric indicator, if
sufficiently sensitive, will serve for the purposes of a radiation
pyrometer. The effect on the galvanometer is influenced by: (1) the
nature of the junction; (2) the size of the mirror or cone; and (3)
the highest temperature attained by the junction. The indicators used
in connection with radiation pyrometers are of the pivoted type, which
can now be made sufficiently sensitive to give full-scale deflection
for a rise of 100° C. in the temperature of the junction. For the
junction itself, Heil’s alloy (zinc and antimony in atomic proportions)
partnered with constantan has been used, owing to the high E.M.F.
developed; but cases of deterioration of this alloy have been noted,
causing it to be replaced by some makers by iron. Two iron or copper
constantan junctions in series give an E.M.F. for a rise of 100° C.,
sufficient to work a pivoted indicator, and are preferable to Heil’s
couple for a radiation pyrometer.
[Illustration: FIG. 53.—PAUL’S RADIATION PYROMETER.]
=Calibration of Indicators for Radiation Pyrometers.=—The
deflections on the indicators are due to the E.M.F. generated, which is
proportional to the difference in temperature between the hot and cold
junctions. If both these are at the same temperature—say, 20° C.—the
deflection is zero; and on allowing the radiations to fall on the hot
junction its temperature is raised by an amount depending upon the
intensity of the radiations—say, to 90° C. The deflection produced is
then due to a difference of (90-20) = 70°, the radiations having raised
the temperature of the hot junction 70° above its surroundings. If the
surroundings (including the cold junction or junctions) had been at
15° to commence with, the hot junction under the same conditions would
have risen to 85°, giving again a difference of 70°, and thus causing
the same deflection as before. Provided both hot and cold junctions are
located so as to attain the same atmospheric temperature in the absence
of radiations, a given quantity of energy impinging on the hot junction
will always produce in it the same _excess_ temperature, and will
therefore give rise to the same deflection at all ordinary atmospheric
temperatures. As the junctions are so arranged in radiation pyrometers
as to fulfill this condition, no correction for fluctuations in the
cold junctions is necessary. The deflections, therefore, correspond to
excess temperatures of the hot junction, which in turn are directly
proportional to the energy received by the junction. Readings in
millivolts on the indicator thus represent directly the proportions
of energy received by the hot junction, 4 millivolts corresponding to
twice the energy, which produces 2 millivolts, and so on; and hence the
millivolt scale becomes an energy scale.
In order to translate energy into corresponding temperatures, the
fourth-power law must be applied. If E_{1} correspond to an absolute
temperature T_{1} on the part of the black body from which radiations
are received, and E_{2} correspond to another temperature T_{2}, the
following relations will hold good:
E_{1} = K(T_{1}^4 - _x^4_), and E_{2} = K(T_{2}^4 - _x^4_),
where _x_ is the temperature of the surroundings receiving the
radiations. As previously pointed out (see Example on page 140), the
term _x^4_ may be ignored for the range of high temperatures measured
by a radiation pyrometer, hence E_{1} = KT_{1}^4, and E_{2} = KT_{2}^4;
and therefore E_{1}/E_{2} = T_{1}^4/T_{2}^4. But, as shown above,
readings in millivolts on the indicator are directly proportional to
the energy received, and if R_{1} and R_{2} = millivolts due to E_{1}
and E_{2}, the relation R_{1}/R_{2} = T_{1}^4/T_{2}^4 is then obtained.
In order to prepare a temperature scale from this relation, it is
necessary to take one correct reading at a known temperature, after
which the remainder of the scale may be marked by calculation, as shown
in the example appended:—
_Example._—A tube closed at one end is at 927° C. (1200°
abs.), and gives a deflection corresponding to 2
millivolts on the indicator. To find the temperatures
which would yield deflections due to 1, 3, 4, and 5
millivolts.
Taking the case of 1 millivolt and applying in the formula
R_{1} T_{1}^4 2 1200^4
───── = ───────, ─── = ─────── ;
R_{2} T_{2}^4 1 T_{2}^4
1200^4
from which T_{2}^4 = ────── and T_{2} = 1009° abs.
2
= 736° C. Similarly, 3 millivolts represent 1055° C.;
4 millivolts = 1154° C.; and 5 millivolts = 1236° C. These
values are readily obtained by the use of four-figure
logarithms.
Having calculated the temperature corresponding to each whole
millivolt, a curve may be plotted to represent millivolts against
corresponding temperatures, and intermediate values deduced from it.
Evidently, the standard reading must be taken with great accuracy,
as the whole scale hinges upon it; and for this purpose an accurate
resistance or thermo-electric pyrometer may be used, placed inside
the tube of an electric furnace, and the radiation pyrometer sighted
on a thin sheet of iron placed just in front of the naked junction. A
check at the higher readings of the scale is necessary, as an exact
realisation of the fourth-power law is seldom obtained in practice.
This may be taken in the same manner, as thermocouples may now be
calibrated directly against the gas scale up to 1550° C., thus enabling
the gas-scale reading to be transferred to the radiation pyrometer.
For delicate readings over a given range, the scale of a mirror
galvanometer may be calibrated in this manner, sufficient resistance
having first been added in series to ensure that at the highest
temperature employed the spot of light will remain on the scale.
[Illustration: FIG. 54.—RECORD OBTAINED WITH RADIATION PYROMETER.]
=Recorders for Radiation Pyrometers.=—Any of the thermo-electric
recorders described in Chapter II may be applied to radiation
pyrometers, the chart being suitably divided according to the
fourth-power law. When taking a record, the pyrometer is fixed on a
stand or bracket and focused on the desired spot. Fig. 54 is an example
of a record taken with a Thread recorder and Féry pyrometer, in which
the division of the temperature scale according to the fourth-power law
will be noticed. It is possible to arrange that the working temperature
shall lie on the open part of the scale, by adjusting the sensitiveness
of the galvanometer accordingly before calibrating.
=Management of Radiation Pyrometers.=—It is not advisable to
place a radiation pyrometer in the hands of an unskilled observer, as
intelligent oversight is required if good results are to be secured.
Care must be taken to adjust the galvanometer needle to zero before
taking a reading, and the needle should always be locked during
transit. When focusing on an object in a furnace it is necessary to
make certain that the red image seen is actually that of the object,
which may be done by moving the pyrometer until the side of the
object, or some special feature, is visible in the eye-piece, when
the pyrometer may be moved until the image surrounds the junction.
Occasions may arise, as in taking the temperatures of various zones
of a rotary cement-kiln or other furnace, in which it is required to
focus the mirror for a specified distance; in which case the author
has adopted the plan of placing a fixed pointer opposite the milled
head which controls the mirror (P, fig. 44) and focusing the bars of
a window at measured distances, marking the same on the milled head
opposite the pointer; and it would be a convenience if all radiation
pyrometers were thus marked initially. A good check to correct
focusing in the case of a heated object is to alter the focus in both
directions, and finally to adjust to the maximum reading, which should
correspond to the true focus.
Great care should be taken not to damage the mirror. If, in a workshop,
the surface become covered with dirt, this should be removed by gentle
brushing with a camel-hair brush or by blowing air over the mirror. The
focusing device should never be strained beyond its working limits;
when these are reached, the pyrometer should be moved bodily until the
object can be correctly sighted within the ordinary limits of the
movement of the milled head. If metallic fumes or dense smoke intervene
between the furnace and the pyrometer, the radiations will be impeded
and the temperature recorded will be too low; and in such cases the
pyrometer should be placed at the open end of a tube and sighted upon
the closed end, which should terminate at the spot under observation.
In all cases it must be borne in mind that the indications only apply
to black-body conditions. If a block of steel be sighted inside a
furnace, and then be removed to the exterior and again sighted,
the external reading will be much less than the internal, owing to
the inferior radiating power of the surface, which now derives no
assistance from the furnace. All readings should therefore be taken
whilst the object is still in the furnace, or (as in taking the
temperature of molten metal in a ladle) a fireclay tube with a closed
end inserted in the mass may be used, and readings taken through the
open end. Statements are sometimes made that the difference between
external readings and black-body readings is constant for a given
surface, and that the one may be translated into the other; but this is
true only for unchanging surfaces, such as platinum, and seldom applies
to ordinary working surfaces. As black-body conditions are so easy to
ensure, it is simpler and safer always to arrange to take observations
under such conditions, rather than to trust a relation seldom constant
in practice.
When using a radiation pyrometer for a number of furnaces, fireclay
tubes, closed at one end, may be inserted in each, so that the closed
end terminates at the working spot, the open end being left flush
with the exterior of the furnace. The diameter of such tubes will
depend upon the length and also upon the make of the pyrometer; in all
cases the image of the closed end must be large enough to overlap the
receiving junction or spiral. Information on this point can always be
obtained from the makers, or can be discovered by trial with openings
of known diameter. When using the pyrometer to obtain temperatures
in the interior of the tube of an electric furnace, such as that
illustrated in fig. 29, a solid object, such as a short fireclay
cylinder, or a piece of graphite, should be placed in the middle of the
tube, and focused on the junction.
=Special Uses of Radiation Pyrometers.=—For regular use at
temperatures above 1000° C. or 1850° F. the radiation pyrometer will
be found to be more useful than instruments of the thermo-electric
or resistance type as the latter undergo deterioration owing to the
continuous action of the furnace gases, which becomes more marked
as the temperature increases. Examples of industrial processes in
which 1000° C. is considerably exceeded are the manufacture of glass,
pottery, and cement, the treatment of special steels, and the casting
of metals and alloys. Even for temperatures between 750° and 1000° C.
a radiation pyrometer may be used, but is not so convenient for this
range as a thermo-electric instrument. There is no upper limit to the
instrument, which may be calibrated by the fourth-power law to the
highest temperature attainable, that of the electric arc, which has
been found to be 3720° C. by the use of a Féry radiation pyrometer.
Measurements may therefore be made beyond the limits of thermal
junctions, such as the temperature of electric furnaces and of thermit
in the mould, and of molten steel before pouring, thus opening out
the possibility of accurate control at extremely high temperatures.
There is always a danger, however, of the cold junction becoming
unduly heated when near to large masses at very high temperatures, and
serious errors may arise from this cause. Two examples may be cited
to illustrate the usefulness of the radiation pyrometer in practice:
(1) the hardening of steel projectiles; and (2) the determination of
the temperature of the clinkering zone in a rotary cement kiln. In
(1) the projectile is brought to a given spot near the brink of the
furnace, where it is in the focus of a radiation pyrometer, and when
at the specified temperature is raked out of the furnace and drops
into an oil-trough. It has been found that a difference of 10° C. from
the standard temperature at which the projectiles should be quenched
may cause a serious lowering of the penetrative power of the finished
projectile; and hence a radiation pyrometer, which may readily be
sighted on each individual shell, is the best to use for this purpose.
In (2) the hottest spot may be found by focusing the pyrometer to
different distances up the kiln, and, by taking a record, any fall in
temperature due to defect of coal or air supplies, or to excessive
feed of raw material, may be detected, thus furnishing information
from which the process may be regulated to the best advantage. At the
temperatures prevailing in such kilns—1300° to 1450° C., or 2370°
to 2640° F., according to the nature of the kiln—a Féry radiation
pyrometer is quite sensitive to changes of 10° C. or 18° F., and the
author has found it to be entirely satisfactory in this connection.
The adaptability of radiation pyrometers to all temperatures above a
red heat, combined with the absence of deterioration, renders these
instruments of great value, and the possibility of obtaining records is
a further recommendation. The radiation method, however, is not suited
to the purposes of an installation, as even if mirrors and junctions
could be constructed so as to be identical, the arrangement would be
very costly. A cheap adaptation of the radiation principle, by means
of which a number of furnaces, such as a set of cement-kilns, could be
controlled from a centre, would be of great advantage, and would add
further to the general utility of this class of pyrometer.
CHAPTER VI
OPTICAL PYROMETERS
=General Principles.=—When a solid is heated to 450° C., it
commences to send out luminous radiations and appears a dull-red colour
in a darkened room. As the temperature rises, the luminous radiations
become more intense; the colour changes to a lighter red, then to
orange, yellow, white, and finally to a dazzling white. Attempts have
been made to assign temperatures to specified colours, and Pouillet, in
1836, introduced a table which purported to give the relation between
colour and temperature. The following table, published by Howe in 1900,
differs considerably from that of Pouillet, who had no accurate means
of measuring the temperatures he assigned to the colours:—
HOWE’S TABLE.
───────────────────────────────┬───────────────┬──────────────
Description. │ Temp. Deg. C. │ Temp. Deg. F.
───────────────────────────────┼───────────────┼──────────────
Lowest red visible in darkness │ 470 │ 878
” ” ” daylight │ 475 │ 887
Dull red │ 550 to 625 │ 1022 to 1157
Full cherry │ 700 │ 1292
Light red │ 850 │ 1562
Full yellow │ 950 to 1000 │ 1742 to 1832
Light yellow │ 1050 │ 1922
White │ 1150 │ 2108
───────────────────────────────┴───────────────┴──────────────
If it were possible for all observers to detect exactly the colours to
which these temperatures refer, the table would be of great utility;
but in practice any two persons might differ in judgment to the extent
of 50° C. below a yellow; and when the white is reached, and becomes
dazzling, accurate discrimination is impossible. At the same time, a
trained workman, used to quenching steel at a fixed temperature, say
850° C., acquires a high degree of judgment with constant practice, and
may not vary by more than 20° C. at temperatures below a light yellow.
The personal equation, however, is too great for colour judgment by the
unaided eye to be taken as an accurate guide to temperature. A fairly
close approximation, however, may be obtained by matching the colours
against prepared standards, as will be referred to later.
The determination of the intrinsic brightness of the heated substance
by a photometric method naturally suggests itself as a possible means
of ascertaining temperatures by optical means, and it will be found
that all the optical pyrometers used for industrial purposes are based
on this procedure. The law connecting the intensity of the whole
of the light waves emitted with temperature, for a given solid, is
approximately given by Rasch’s formula:—
┌ ┐_x_
I_{1} │ T_{1} │
───── = │ ───── │
I_{2} │ T_{2} │
└ ┘
where I_{1} and I_{2} are the intensities corresponding to absolute
temperatures T_{1} and T_{2}; and the exponent
25000
_x_ = ───── .
T_{1}
Hence at 1250° abs. the brightness increases as the 20th power, and at
2500° abs. as the 10th power of the temperature. This rapid increase
in brightness for a small rise in temperature enables small increments
to be readily observed; but a difficulty arises in practice owing to
vast differences in brightness displayed by different substances at the
same temperature. For example, the light emitted by an incandescent
gas-mantle, which consists of thorium oxide, is vastly greater than
that given out by a metal, such as platinum, at the same temperature;
and it is therefore evident that the luminosity of a substance depends
not merely upon its temperature, but also upon its nature. It is
possible, however, to obtain indications for any substance in terms
of a black body; thus if a heated solid possessed the same intrinsic
brightness as a black body at a temperature of T, the “apparent” or
“black-body” temperature of the solid would also be called T. All that
this would signify would be that the condition of the solid was such
that the light radiated was equal in intensity to that emitted by a
black body at temperature T; and to obtain the true temperature of the
solid, T must be multiplied by a factor which expresses the ratio of
its emissive power to that of a black body.
In all photometric methods a standard light is employed, which should
not vary in brightness, and with which the light from the source is
compared. In optical pyrometers no attempt is made to measure the
illumination in terms of candle-power; all that is necessary is to
bring the standard and the source to the same degree of brightness
by suitable adjustments. Amongst the standards employed are
carbon-filament electric lamps, amyl-acetate lamps, and for higher
temperatures the centre of an acetylene gas-flame; each of which is
capable of producing a fixed degree of brightness when used under
specified conditions. A black body, at known temperatures, is compared
with the standard used, thus furnishing a scale of “black-body”
temperatures to which the indications of a given source may be
referred, as explained in the previous paragraph. Above 1000° C.,
however, the light becomes too dazzling to enable a proper comparison
of the standard and source to be made, and absorbing glasses must
then be used to reduce the brightness. Any coloured glass, taken at
random, might not reduce the standard and source equally; but if a
monochromatic glass be used—that is, a glass which transmits light of
one wave-length only—a well-defined relation is found to exist between
the intensity of the transmitted light and the temperature of the
source. As optical pyrometers are used for temperatures above 1000° C.
in most cases, involving the use of such glass, it will be necessary
briefly to consider the relations between the wave-lengths of light and
the temperature of the radiating substance, which in all cases will be
assumed to be a black body.
=Wien’s Law.=—When the temperature of a substance increases,
the enhanced brightness which results is shared by all parts of its
spectrum; and if the substance were viewed through a glass prism, it
would be noticed that every portion was brighter than before. Taking a
ray of wave-length λ, the relation between its intensity and the
temperature of the (black-body) source is given by Wien’s formula:—
J = _c_{1}_λ^{-5} × _e_^{-_c_{2}_/λT} (1)
where J = energy corresponding to wave-length λ; _e_ = the base of the
natural system of logarithms; T = absolute (thermodynamic) temperature
of the black-body source, and _c_{1}_ and _c_{2}_ are constants, the
values of which may be found by measuring J at two known temperatures
for light of a known wave-length. Experiment has shown that this formula
is correct for wave-lengths which lie in the visible spectrum, but does
not hold for longer waves; and modifications of Wien’s equation have
been given by Planck and others which are of more extended application.
For the purposes of optical pyrometry, however, using red light of
wave-length about 65 millionths of a centimetre, Wien’s law may be
applied with great accuracy; and a calibration based upon this law
agrees closely with the values obtained by other pyrometric methods.
Wien’s formula may be written in the form—
log_{10}J = K_{1} + K_{2}(1/T) (2)
where K_{1} = log _c_{1}_-5 log λ and K_{2} = _c_{2}_(log _e_/λ).
This simplified expression shows a linear relation between log J and
1/T; and hence if the temperatures corresponding to two intensities
be known, the results may be plotted on squared paper in the form of
a straight line connecting T and J, from which line intermediate or
extraneous readings of temperatures may be obtained for any given
intensity. Another useful form of Wien’s equation, referring to the
ratio of two intensities J_{1} and J_{2}, is as under:—
┌ ┐
J_{1} _c_{2} log _e_ │ 1 1 │
log ───── = ─────────────── │───── - ───── │ (3)
J_{2} λ │ T_{2} T_{1}│
└ ┘
where T_{2} and T_{1} are the absolute temperatures corresponding to
J_{2} and J_{1} The value of _c_{2}_ is 1450000, when λ is expressed in
millionths of a centimetre. Evidently, if the ratio J_{1}/J_{2} and the
value of _c_{2}_, λ, and T_{2} be known, T_{1} may be calculated. When
λ is not known, as in the case of a piece of red glass for which its
value has not been determined, two readings at known temperatures will
establish the value of (_c_{2}_ log _e_)/λ, and all other results may
then be calculated. Examples illustrating the application of the formula
will now be given.
_Example I._—A black body at an absolute temperature
T_{1} is found to give twice the intensity observed at
1200° abs., the comparison being made with red glass
transmitting wave-length 65 × 10^{-6} cms. To find the
value of T_{1}.
Applying values to formula (3)
┌ ┐
1450000 │ 1 1 │
log 2 = ─────── log 2·7183 × │ ──── - ──── │
65 │ 1200 T_{1 │
└ ┘
and ┌ ┐
1450000 × 0·4343 │ 1 T_{1} │
0·3010 = ──────────────── × │ ───── - ────── │
65 │ 1200 1 │
└ ┘
from which T_{1} = 1237° abs.
_Example II._—The intensity of the radiations from a black
body at 2000° abs. are found to be equal to those from a
given standard, taken as unity. To find the intensity at
3000 abs., compared with the same standard. λ = 65 × 10^{-6} cms.
Applying in (3) as before,
┌ ┐
J_{1} 1450000 × 0·43435 │ 1 1 │
log ──── = ───────────────── × │ ───── - ──── │
1 65 │ 2000 3000 │
└ ┘
from which log J_{1} = 1·615, and J_{1} = 14·5.
In applying Wien’s law to the calibration of an instrument in which
the intensity of a source may be measured photometrically against
that of a standard, an electric furnace (fig. 29) may be used, with a
piece of iron in the centre, coated with oxide, which gives black-body
radiations. A thermo-electric pyrometer in contact with the oxide may
be used to measure the standard temperatures, and brightnesses may then
be compared with that of an amyl-acetate or other lamp giving a flame
of constant luminosity. Temperatures corresponding to other intensities
may then be deduced by calculation, as previously shown.
=Practical Forms of Optical Pyrometers.=—The instruments used in
practice fall under the following heads:—
1. The standard light is constant, and the intensity of the light from
the source varied in the instrument until equal to the standard. (Féry,
Le Chatelier, Wanner, and Cambridge.)
2. The standard is varied until equal to that of the source, which
may be reduced in intensity if this exceed that of the standard.
(Holborn-Kurlbaum, made in commercial form by Siemens.)
3. The colour of the source is matched against a standard colour, made
to agree with that obtained in a given operation (Lovibond); or the
source may be made to produce a standard colour by a polarising device
(Mesuré and Nouel); or the colour of the source is extinguished by
suitable absorbents (various forms).
Examples of each type will now be described.
[Illustration: FIG. 55.—FÉRY’S OPTICAL PYROMETER. SECTION.]
[Illustration: FIG. 56.—FÉRY’S OPTICAL PYROMETER. EXTERNAL VIEW.]
=Féry’s Optical Pyrometer.=—This instrument (shown in figs. 55
and 56) consists of a telescope furnished with a side-branch, in which
a standard lamp E is placed. Light from E is focused upon a piece of
transparent glass F, inclined at an angle of 45° to the axis of the
telescope, from whence it is reflected into the eye-piece. To render
the light received from the lamp monochromatic, a piece of red glass
is interposed between E and the mirror. The telescope is sighted on
the hot substance, rays from which pass through a piece of red glass
D, and thence through two wedges of darkened glass, which diminish
the intensity to a greater or less degree according to the thickness
of absorbent glass interposed, which is reduced by sliding the wedges
apart, and increased by the contrary movement. After passing through
the wedges, the light proceeds through the inclined mirror to the
eye-piece; consequently, the appearance presented to the eye is that of
a field illuminated one-half by the standard lamp, and the other by the
hot source. The adjustment consists in sliding the wedges, by a screw
movement, until both portions of the field are equally illuminated. A
temperature scale is provided on the moving piece which actuates the
wedges, and is derived by Wien’s equation from the thickness of the
wedges interposed when equality is obtained. Calibration is effected
by noting the thickness of the wedges corresponding to two known
temperatures, from which a straight line connecting thickness with
the reciprocal of the absolute temperatures may be drawn, and a table
formed giving values of T in terms of the thickness of the wedges. The
calibration may be extended indefinitely, the accuracy of the readings
depending upon the truth of Wien’s law. Féry’s optical pyrometer is a
convenient instrument for occasional readings of high temperatures,
combining simplicity with portability.
=Le Chatelier’s Optical Pyrometer.=—This pyrometer was the
original form of instrument in which the temperature of a luminous
source was deduced by photometric comparison with a standard light; and
Féry’s apparatus, described above, is merely a convenient modification
of the original. Instead of the absorbent glass wedges, Le Chatelier
employed an iris diaphragm to reduce the quantity of light entering the
telescope; the adjustment being carried out by altering the size of
the opening in the diaphragm until the brightness of the source agreed
with that of the standard. The intensity of the light received in the
telescope will vary as the square of the diameter of the opening;
and calibration at two known temperatures with a given monochromatic
glass enables a temperature scale corresponding to diameter of opening
to be computed by Wien’s law. Le Chatelier’s pyrometer is a valuable
implement for research work in the laboratory, but is not so convenient
for workshop purposes as Féry’s modification.
[Illustration: FIG. 57.—WANNER’S PYROMETER. SECTION.]
=Wanner’s Pyrometer.=—The principle of this pyrometer is the
comparison of the brightness of a red ray from the standard with
that of the ray of some wave-length obtained from the source, both
rays being produced spectroscopically and therefore being truly
monochromatic. The brightness is compared by the aid of a polarising
device, resulting in a somewhat complicated optical arrangement, which
is shown in fig. 57. Light from a standard electric lamp passes through
the slit S_{1}, and from the hot source through S_{2}. Both beams are
rendered parallel by means of an achromatic lens O_{1}, which is placed
at a distance equal to its focal length from the slits. The parallel
beams are dispersed by the direct-vision spectroscope P; and then pass
through the polarising prism R, which separates each beam into two
beams, polarised in planes at right angles. A biprism, B, placed in
contact with a second achromatic lens, O_{2}, is made of such an angle
that two fields of red light, polarised in planes at right angles, one
from the source and the other from the standard, are focused on the
slit D. These fields are viewed through an analyser A, and are brought
to equal brightness by rotating the analyser, to which a graduated
scale is attached, the temperature being deduced from the angle through
which the analyser is turned. The calibration is effected by Wien’s
law (equation (3) page 172), the intensities of standard and source
being related to the angle of rotation as indicated by the equation.
J_{2}/J_{1} = tan^2 Θ where J_{2} and J_{1} represent the intensities
of source and standard respectively, and Θ = angle of rotation.
Introducing this value into Wien’s equation (page 172), the relation
between Θ and T may be shown to take the form log tan Θ = _a_ + _b_/T,
where _a_ and _b_ are constants. Hence, if log tan Θ be plotted against
1/T a straight line is obtained, and hence by a few observations
at known temperatures a calibration curve may be drawn from which
intermediate and extraneous readings may be obtained. Messrs Hadfield
have introduced a special chart, divided so that actual readings in
degrees C. may be taken directly by observing the angle Θ. As sent out
for use, the temperature scale is prepared beforehand, so that direct
readings may be taken.
As the standard electric lamp will vary in brightness with repeated
use, means must be provided to restore it to its proper value. This can
be done by placing a rheostat in the circuit of the lamp, and adjusting
the current until the brightness, as viewed through the pyrometer,
exactly agrees with that of a ground-glass surface illuminated by a
standard amyl-acetate lamp. The flame of this lamp really constitutes
the standard; but as it would be blown about by air-currents when used
in a workshop, the electric lamp, lighted by a portable battery, is
brought to equality and used for general measurements.
=Cambridge Optical Pyrometer.=—During the recent war the
manufacture of pyrometers of this type was taken up by the Cambridge
and Paul Instrument Company. The external form of the Cambridge optical
pyrometer is shown in fig. 58, in which an observer is shown using
the instrument, the accessories consisting of a 4-volt accumulator,
an ammeter, and an adjustable resistance for regulating the current
through the electric lamp used for comparison; and a standard
amyl-acetate lamp for adjusting the electric lamp to the correct
brightness. The scale is marked on a circular disc, and direct readings
are obtained from the position of a pointer which rotates with the
analyser. By interposing a monochromatic glass to dim the source, the
range of the pyrometer can be modified; and instruments are provided
in four ranges: 700°-1400° C.; 900°-2000° C.; 1200°-2500° C., and
1400°-4000° C.
The Cambridge optical pyrometer has proved a useful instrument in
skilled hands, and has been found of great service in the steel, glass,
and pottery industries. Trained observers have found it possible to
detect a difference of 10° C. at the region of 1900° C. The adjustment
of the two fields to equality, however, involves a judgment which
varies with different observers, and in practice it is advisable for
one individual to be entrusted to take all readings.
[Illustration: FIG. 58.—CAMBRIDGE OPTICAL PYROMETER.]
=Holborn-Kurlbaum Pyrometer.=—In the optical pyrometers
previously described a constant standard is used, and the brightness of
the light from the source varied until equality is obtained. The idea
of varying the brightness of the filament of an electric lamp until its
colour matched that of the source, and deducing the temperature from
the current taken by the lamp, was due to Morse, who used a filament
in the form of a flat spiral, heated by a battery of E.M.F. 40 volts.
This spiral was placed in a metal tube and interposed between the
eye and the heated object. The Holborn-Kurlbaum pyrometer, as made
by Siemens, is a refinement of that of Morse, and capable of reading
over a more extended range. In fig. 59, L is a small electric lamp
with a hairpin filament, as shown at A. This lamp is placed in a
telescope, so that the filament is in the focus of the eye-piece and is
lighted by a 4-volt accumulator, in series with which is a rheostat,
R, and a milliammeter, M. The heated source is focused by moving the
object-glass of the telescope, and both lamp and source are viewed
through red glass placed in front of the eye-piece, D. The rheostat,
R, is then adjusted until the tip of the filament is indistinguishable
from the background, which is illuminated by the source. If the lamp be
too bright, the filament will appear as a bright line; if duller than
the source, as a dark line; and when equal to the source it will merge
into the background. When equality is obtained, the milliammeter is
read, and the temperature deduced from the current taken by the lamp.
[Illustration: FIG. 59.—HOLBORN-KURLBAUM PYROMETER. SECTION.]
The relation between current and the temperature of the filament varies
with each lamp, but is in all cases represented by a formula of the type
C = _a_ + _bt_ + _ct^2_
where C = current, _t_ = temperature in degrees C., and _a_, _b_,
and _c_ are constants depending upon the lamp used, and which can be
determined by making a number of observations at known temperatures.
The instrument is calibrated in this manner by the makers, and a scale
affixed from which temperatures may be read corresponding to observed
currents.
When the temperature of the source exceeds that of the standard at
maximum current, an absorbing device, E, consisting of two prisms of
darkened glass, with their reflecting faces parallel, is placed over
the end of the telescope, so as to reduce the intensity of the source
below that of the lamp. A separate calibration is performed with the
absorber in position, and a second temperature scale provided, from
which readings are taken when the absorbing device is used. Fig. 60
represents the instrument as made by Messrs Siemens, for use in a fixed
position, the telescope, milliammeter, and rheostat being mounted on an
upright supported by a tripod, and the current obtained from a portable
accumulator. A second form (fig. 61) is designed for use in cases when
observations at a number of different places are required, the rheostat
being mounted on the telescope, and the milliammeter contained in a
leather case provided with shoulder-straps.
[Illustration: FIG. 60.—SIEMENS’ OPTICAL PYROMETER, ON STAND.]
[Illustration: FIG. 61.—SIEMENS’ OPTICAL PYROMETER, PORTABLE FORM.]
The adjustment in this pyrometer is simple, and the condition of
equality sharply defined. Whereas, in matching the colours of two
contiguous fields, separate observers may disagree to an extent
representing 40° C. or more, a divergence of 10° C. is seldom exceeded
when different operators adjust the tip of the filament to extinction.
In a special test to decide this point, the author compared the
observations of five persons, some trained and others untrained, with
the result that all agreed to within 10° at a steady temperature in the
vicinity of 1200° C.; and in this respect the Holborn-Kurlbaum
pyrometer is superior to other forms of optical pyrometer. The
continuous accuracy of the readings depends upon the permanence of
the standard lamp, which is ensured by over-burning for 20 hours,
after which the lamp may be used at its proper voltage for a long
period without further change. As used for occasional readings in the
workshop, such a lamp will last for a year or more without varying in
brightness by an amount representing 10° C. at a temperature of 1800°
C. When a new lamp is used, a fresh calibration is necessary; the
makers, however, in such case send out a new temperature scale with the
lamp.
=Lovibond’s Pyrometer.=—It is possible, by the use of coloured
glasses superposed, to match closely any given colour; and Lovibond,
whose tintometer for this purpose is well known, has applied this
method to temperature measurement. Taking the case of a block of steel
in a furnace, it is possible to arrange combinations of glasses which,
when illuminated by a standard light, will give the same tint as the
steel at any specified temperature. If it be desired to work the steel
at 850° C., for example, glasses are provided which, when viewed by the
light transmitted from a 4-volt glow-lamp, using a constant current,
represent the tint of steel at 840°, 850°, and 860° respectively. The
image of the steel is reflected by a mirror through one hole in a brass
plate, which forms the end of a wooden box, at the opposite end of
which an eye-piece is placed. A second hole in the brass plate receives
light from the standard lamp, after passing through the glasses; and
the appearances of the two lights are then compared. A skilled eye can
readily detect a disagreement in the two fields corresponding to 10°
C.; and by introducing the glasses in turn it can be observed whether
the steel is within 10° C. of the temperature required. This instrument
is cheap and simple, but is obviously only useful in deciding a
pre-arranged temperature, as to take a measurement at an undefined
temperature would involve an unwieldy number of glasses, and absorb a
considerable time. The correct glasses to use for a given operation are
decided under working conditions at temperatures measured by a standard
pyrometer; after which any number of instruments may be made from
glasses of the same colour and absorptive power as those used in the
calibration. Correct matching is difficult below 700° C.
=Mesuré and Nouel’s Pyrometer.=—This instrument, shown in fig. 62,
consists of two Nicol prisms, between which is placed a piece of quartz
cut perpendicularly to its axis. Light from the source, in passing
through the first Nicol prism, is all polarised in the same plane; but
on passing through the quartz is polarised in various planes, according
to the wave-length. The colour seen after passing through the second
prism, used as analyser, will depend upon the angle between this and
the first or polarising prism. The analyser is connected to a rotating
disc, divided into angular degrees; and on viewing the heated source
the colour will appear red if the analyser be turned in one direction,
and green if rotated in the opposite. The intermediate colour is a
lemon-yellow; and the adjustment consists in rotating the analyser
until this tint is obtained. The angular reading is then taken, and
the temperature read off from a table prepared by making observations
at known temperatures. Observers may disagree by as much as 100° C. in
using this pyrometer, owing to differences in eyesight and judgment
of the lemon-yellow tint; but a given operator, who has trained
himself to the use of the instrument, may obtain much closer results
with practice. The chief use of this device is to enable a judgment
to be formed as to whether a furnace is above or below an assigned
temperature, within limits of 25° C. on either side at the best; and
hence it is convenient for a foreman or metallurgist to carry about for
this purpose when other pyrometers are not in use. A great advantage is
that the instrument is always ready for use, and has no accessories.
[Illustration: FIG. 62.—MESURÉ AND NOUEL’S PYROMETER.]
=Colour-extinction Pyrometers.=—Various attempts have been made
to produce superposed glasses, or cells of coloured fluids, which
will have the effect of extinguishing the colour of a heated source.
As an example, three cells containing various dyes in solution may be
prepared which, when looked through, will extinguish the colour at
840°, 850°, and 860° C. respectively. If it be desired to work at 850°,
a difference of 10° on either side may be detected by a trained eye;
but to follow a changing temperature a large number of cells would
evidently be necessary. Heathcote’s extinction pyrometer, in its early
form, consisted of an eye-shade in front of which two pairs of cells
containing coloured fluid were mounted. In bringing a furnace to an
assigned temperature, observation was made from time to time until a
faint red image was perceived through one pair of cells, when the heat
supply was regulated so as to maintain the existing temperature. When
viewed through the second pair of cells, which contained a slightly
darker fluid, no red image was to be seen at the correct temperature.
With training, a workman could control a furnace to a fair degree of
accuracy by this means, but the operation was tedious, and useful only
for the attainment of a single temperature. In a later instrument,
known as the “Pyromike” (fig. 63), Heathcote employs a single cell
with flexible walls, so that by turning the screw-end, the length of
the column of fluid interposed between the eye and the furnace can be
altered. In taking a reading, the furnace is sighted and the screw
turned so as to increase the length of the column of coloured
fluid, until the image is no longer visible. A direct reading of
the temperature is then obtained on a spiral scale marked on the
cylindrical body of the instrument, over which the screwed portion
rotates. This forms a simple and convenient temperature gauge for
workshop use.
[Illustration: FIG. 63.—HEATHCOTE’S EXTINCTION PYROMETER OR “PYROMIKE.”]
[Illustration: FIG. 64.—“WEDGE” PYROMETER.]
The “Wedge” Pyrometer, designed by Alder and Cochrane (fig. 64),
consists of a small telescope through which a prism of darkened glass
may be moved, and which is focused on the heated object. By turning a
head the wedge may be moved so as to interpose a thicker layer of dark
glass between the eye and the furnace, and the same operation causes a
temperature scale to pass in front of a fixed pointer. When the image
of the hot source is just extinguished, the temperature is read from
the mark opposite the fixed point. Training is needed to enable an
observer to judge the exact point of extinction, when it becomes
possible to obtain results of 20° C. in the region of 1300° C. On
the other hand, when used by one unaccustomed to the instrument, the
reading may be wrong by 50° C. or more. As an aid to the judgment near
the extinction point, the hand may be interposed between the telescope
and furnace, when, if extinction be complete, no alteration should
be observed in the field of view. The simple construction of this
pyrometer is an advantage, no accessories being needed; and when used
with the precautions stated above, readings sufficiently close for many
processes can easily be obtained.
=Management of Optical Pyrometers.=—Careful usage is essential
with optical pyrometers, which are liable to get out of adjustment
with rough handling; and for this reason a trained observer should be
in charge of such instruments. Skilled attention is equally requisite
in taking readings, as the matching of tints correctly is an operation
which demands a high degree of judgment. Careful attention must be paid
to the standard lights; if flames, regulation to the standard height is
essential; if electric lamps, care must be taken not to use them for
a longer period than necessary, in order to increase the useful life.
Accumulators should be recharged regularly—say once in two weeks—to
keep in good order. Separate parts, such as absorption glasses,
should be kept in a place of safety, as their destruction may involve
a new calibration. It should be kept in mind that the temperatures
indicated by optical pyrometers are “black” temperatures; that is, they
correspond to the readings that would be given by a black-body of the
same degree of brightness. In consequence, readings should always be
taken under black-body conditions, the precautions in this respect
being identical with those necessary for total-radiation pyrometers,
given on page 163. In some special cases the connection between the
apparent and true temperatures has been worked out for a given type of
pyrometer, but, owing to the different emissive powers of different
substances, no general relation can be given.
=Special Uses of Optical Pyrometers.=—The advantageous use of
optical pyrometers is restricted to observations at temperatures beyond
the scope of instruments which have the working part in the furnace; or
to cases in which occasional readings of temperature suffice. To follow
a changing temperature continuous adjustment is necessary, involving
labour, and therefore costly. Amongst workshop uses may be mentioned:
(1) ascertaining the temperature of pottery kilns and glass and steel
furnaces; (2) in the treatment of steels at very high temperatures,
to which end the pyrometer may be set to a given reading, and the
process carried out when the steel is observed to attain such assigned
temperature; (3) to take casual readings when a number of furnaces
are in use, or when a number of sighting-holes are provided, as in
large brickmaking furnaces; and (4) for occasional observations of the
firing end of rotary cement kilns. As an instrument of research in the
laboratory, a good form of optical pyrometer is very useful, as, for
example, in investigating the working temperatures of electric lamps,
and taking observations in electric furnaces. It is a great drawback
that records cannot be taken by optical pyrometers, as much valuable
information can be gathered from an accurate knowledge of temperature
fluctuations in most operations. This disadvantage must always militate
against the general use of these instruments.
CHAPTER VII
CALORIMETRIC PYROMETERS
=General Principles.=—If a piece of hot metal, of known weight
and specific heat, be dropped into a known weight of water at a
temperature _t_{1}_, which rises to _t_{2}_ in consequence, the
temperature of the hot metal, _t_{0}_, can be obtained by calculation,
as shown by the following example:—
_Example._—A piece of metal weighing 100 grams, and of
specific heat 0·1, is heated in a furnace and dropped
into 475 grams of water, contained in a vessel which
has a capacity for heat equal to 25 grams of water. The
temperature of the water rises from 5° to 25° C. To find
the temperature of the furnace.
The heat lost by the metal is equal to that gained by the
water and vessel. Equating these,
100 × 0·1 × (_t_{0}_ - 25) = (475 + 25) × (25 - 5)
from which _t_{0}_ = 1025° C.
The above calculation, which applies generally to this method, depends
for its accuracy upon a correct knowledge of the specific heat of the
metal used. This value is far from constant, increasing as the
temperature rises, and the result will only be correct when the average
value over a given range is known.
The metal used in the experiment should not oxidise readily, and should
possess a high melting point. Platinum is most suitable, but the cost
of a piece sufficiently large would considerably exceed that of a
thermo-electric or other outfit. Nickel is next best in these respects,
and is now generally used for the calorimetric method, up to 1000° C.
The specific heat varies to some extent in different specimens, but
can be determined for the ranges involved in practical use. This may
be done by heating a given weight to known temperatures and plunging
into water, the result being obtained as in the foregoing example,
_t_{0}_ in this case being known and the specific heat calculated. From
a series of such determinations, a curve may be plotted connecting
specific heat and temperature range, from which intermediate values may
be read off.
[Illustration: FIG. 65.—SPECIFIC HEAT OF NICKEL OVER RANGES FROM 0° C.]
Regnault, who first suggested the calorimetric method for high
temperature measurement, attempted to measure the specific heat of iron
over different ranges, with a view to using this metal in the process.
Owing to the absence of reliable means of determining the experimental
temperatures, however, Regnault’s values were considerably in error.
For the range 0 to 1000° C. he gave the average specific heat of iron
as 0·126, a figure much below the truth. Thus, if a piece of iron
be heated to 970° C., as measured by the thermo-electric method,
and dropped into water, the temperature calculated from an assumed
specific heat of 0·126 will be found to be 1210°, or 240° too high. The
values now employed are obtained by experiments with a thermo-electric
pyrometer, so that temperatures deduced by the calorimetric method
agree, within the limits of manipulative error, with those of the
standard scale. The accompanying curve, fig. 65, shows the average
specific heat of nickel over all ranges between 0° and 1000° C., and
from this curve the correct figure to use in the calculation for any
range may be determined. Thus for a furnace between 800° and 900° C.
the specific heat would be taken as 0·136; and although the choice
of the value to be taken involves a knowledge of the temperature
within 100°, no difficulty arises in practice, as it is easy to judge
this limit by experience at temperatures below 1000° C In the most
approved forms of calorimetric pyrometers for industrial purposes
the temperature of the hot metal may be read directly from a scale,
prepared in accordance with the value applying to the specific heat at
various ranges.
Copper and iron are still used to a limited extent in these pyrometers,
but lose continuously in weight by oxidation, the scales of oxide
falling off when quenched, necessitating weighing before each test
to ensure accuracy. Nickel oxidises very little below 1000 C., and
as the thin film of oxide which forms does not readily peel off, the
weight may increase slightly. Quartz would probably be more suitable
than metals, not being altered by heating and quenching, but does not
appear to have been tried for this purpose. Another possible material
is nichrom, which resists oxidation below 1000° C. The weight of the
solid should be at least 1/20 of that of the water, in order to ensure
a tangible rise in temperature, and the thermometer should be capable
of detecting 1/20 of a degree C. The rise in temperature should not be
so great as to cause the water to exceed the atmosphere in temperature
by more than 4° or 5° C., as otherwise radiation losses would have a
marked effect. The limits of accuracy of the method will be shown by
reference to examples.
_Example I._—A piece of nickel, weighing 100 grams, is
placed in a furnace, and after heating dropped into 2000
grams of water at 10° C., contained in a vessel of water
equivalent 50 grams. The temperature rises to 16·25° C.
The specific heat of nickel for the range is 0·137. To
find the temperature of the furnace and the limits of
accuracy, the thermometer being readable to 1/20° C.
Equating heat lost by the nickel to that gained by the
water and vessel:—
100 × 0·137 × (_x_ - 16·25) = 2050 × (16·25 - 10·0)
from which _x_ = 952° C.
If the error in each thermometer reading amounted to
1/40° the maximum difference in the above calculation is
obtained by introducing the altered values as under:—
100 × 0·137 × (_x_ - 16·225) = 2050 × (16·225 - 10·025)
when _x_ = 944° C.
The maximum error due to a possible incorrect reading of
1/40° is therefore less than 1 per cent.
_Example II._—The loss of heat by radiation in transferring
100 grams of nickel at 927° C., possessing a surface of
30 square centimetres, and with radiating power 0·7 of a
black body, may be shown by the fourth-power law to be 50
calories per second (see page 139). If two seconds were
occupied in the transfer, the error from this cause would
be 1 in 130; and adding this to the thermometric error,
the total is less than 2 per cent.
=Practical Forms of Calorimetric Pyrometers.=—When required
to estimate the temperature of a muffle furnace or other laboratory
appliance, a sheet-copper vessel of about 1500 c.c. capacity may be
used. This should rest on wooden supports in a second similar vessel,
about 2 inches wider, which acts as a shield against radiation. A
cylinder of nickel about 1½ inches long, and 1¼ inches in diameter,
with a hole of ½-inch diameter in the centre, is suitable for test
purposes. This may conveniently be heated in a nickel crucible; and
when transferring to the water the crucible may be grasped with a pair
of tongs, and tilted so as to allow the cylinder to drop into the
water. When used in a tube furnace, a length of thin nickel wire may
be attached to the cylinder to enable withdrawal to be accomplished
rapidly, allowance being made for the weight of the heated wire. The
transfer should be accomplished as speedily as possible, to avoid
radiation errors. The figure to be used to represent the specific heat
of nickel may be obtained from the curve (fig. 65), when the range
to be measured is approximately known. The water equivalent of the
vessel and thermometer should be determined as follows:—Place in the
vessel one-half the quantity of cold water used in the experiment—say
750 c.c.—and note the temperature (_t_{1}_) after stirring with the
thermometer. Then add an equal quantity of water at a temperature
(_t_{2}_) about 10° higher than _t_{1}_ Mix thoroughly with the
thermometer, and note the temperature of the mixture (_t_{3}_). Check
results may be obtained by varying the proportions of cold and warm
water, the total quantity always being equal to that used for quenching
the hot nickel. If W_{1} = the weight of cold water, and W_{2} that of
the warm, the water equivalent (_x_) is obtained from the equation
W_{2} (_t_{2}_ - _t_{3}_) - W_{1}(_t_{3}_ - _t_{1}_)
_x_ = ─────────────────────────────────────────────────────
_t_{3}_ - _t_{1}_.
This figure represents the weight of water equal in thermal capacity to
the vessel, and in a pyrometric measurement is added to the weight of
water taken.
In industrial practice, it is desirable to dispense, if possible, with
the necessity for calculations, so that a reading may be taken by
an unskilled observer. The earliest form of calorimetric pyrometer,
patented by Byström in 1862, consisted of a lagged zinc vessel into
which a piece of platinum was dropped, and a table was provided from
which the temperature of the furnace could be read by noting the rise
in temperature of the water. The modern industrial form, made by Messrs
Siemens, will now be described.
[Illustration: FIG. 66.—SIEMENS’ CALORIMETRIC OR “WATER” PYROMETER.]
=Siemens’ Calorimetric or “Water” Pyrometer.=—Fig. 66 shows
this instrument in longitudinal and transverse section. It consists
of a double copper vessel, the inner containing water, and the outer
provided with a handle. The space between is lagged with felt, to
prevent escape of heat from the water. The thermometer, _b_, is
protected by a perforated brass tube from damage that might be caused
on dropping in the hollow nickel cylinder, _d_. Opposite the stem
of the thermometer is placed a sliding-piece _c_, on which a
temperature scale is marked. In using the instrument, the specified
quantity of water is placed in the inner vessel, and the pointer on _c_
brought opposite to the top of the mercury column in the thermometer.
The nickel cylinder, which has been heated in a crucible or muffle
in the furnace, is then dropped in, and the vessel shaken to secure
an equal temperature throughout the water. When the thermometer is
stationary, the mark on _c_ opposite the top of the mercury gives the
temperature of the furnace, the scale on _c_ having previously been
marked from calculations made for each 50 degrees. The correctness of
the reading evidently depends upon the accuracy with which _c_ has
been calibrated, an operation which involves taking into account the
water equivalent of the vessel and the variation of the specific heat
of nickel at different temperatures. Allowing for the sources of error
attaching to the method, results by this pyrometer cannot be guaranteed
to better than 2 or 3 per cent, at 900° or 1000° C., but in cases
where this degree of inaccuracy is not of importance, the instrument
may be used with advantage. As no calculation is necessary, the
determination may be made in the workshop by any workman who exercises
care in conducting the operation. Copper and iron cylinders are
sometimes supplied instead of nickel, but are not to be recommended,
as they decrease in weight with each test, and necessitate the use
of a multiplying factor to convert the reading on _c_ into the true
temperature.
=Special Uses of Calorimetric Pyrometers.=—The great drawback
to the calorimetric method is that each observation necessitates a
separate experiment, involving time and labour. The accuracy, moreover,
is not comparable with that obtainable by the use of a thermo-electric
or resistance pyrometer; and practically the only recommendation
is the low initial cost of the outfit. When an occasional reading
of temperature, true to 3 per cent., suffices, the calorimetric
pyrometer may be used; and in special laboratory determinations the
method will frequently be found of value. Considering the low cost of
thermo-electric pyrometers at the present time, it is probable that
the calorimetric method will be entirely superseded in industrial
practice, as the former method gives a continuous, automatic reading,
and is capable of furnishing records. Many firms have already replaced
their “water” pyrometers by the more accurate and useful appliances now
available.
CHAPTER VIII
FUSION PYROMETERS
=General Principles.=—If a number of solids, possessing
progressive melting points, be placed in a furnace and afterwards
withdrawn, some may be observed to have undergone fusion whilst others
would be unaffected. The temperature of the furnace would then be known
to be higher than that of the melting point of the last solid melted,
and lower than that of the first which remained intact. Taking, for
example, a series of salts, the following might be used:-
══════════════════════════════════════╤═════════════════════
│ Melting Point.
Salt. ├──────────┬──────────
│Deg. Cent.│Deg. Fahr.
──────────────────────────────────────┼──────────┼──────────
1 molecule common salt + │ 650 │ 1202
1 molecule potassium chloride │ │
Common salt │ 800 │ 1472
Anhydrous sodium carbonate │ 850 │ 1562
” ” sulphate │ 900 │ 1652
Sodium plumbate │ 1000 │ 1832
Anhydrous potassium sulphate │ 1070 │ 1958
” magnesium sulphate │ 1150 │ 2102
══════════════════════════════════════╧══════════╧══════════
If, on inspection, it were found that the sodium sulphate had melted,
whilst the sodium plumbate had survived, the temperature of the furnace
would be known to lie between 900° C. and 1000° C. If a number of salts
or other solids could be found with melting points ranging between 900°
and 1000°, it would be possible to obtain a reading within narrower
limits. The accuracy of the method in all cases is decided by the
interval between the melting points of successive test materials.
Wedgwood, the famous potter, appears to have been the first to apply
this method of determining the condition of a furnace, his test-pieces
consisting of special clay compositions. The effect of the furnace
on these was noted, and the suitability of the temperature for the
work in hand deduced from the observations. Wedgwood in this manner
investigated the variations in temperature at different levels in his
firing-kilns, and was thus enabled to place the various wares at the
positions best suited for their successful firing. Modern potters still
use such test-pieces, as the information gained is not merely the
degree of heat, but the effect of such heat on the articles undergoing
firing. The fusion method, however, is now used to determine the
temperature of all kinds of furnaces, and the chief modifications will
now be described.
[Illustration: FIG. 67.—SEGER PYRAMIDS OR “CONES.”]
=Seger Pyramids or “Cones.”=—Seger, of Berlin, published in
1886 an investigation dealing with the production of silicates of
progressive melting points. By varying the composition, he was able to
produce a series of materials with melting points ranging from 1890°
C. to 590° C., the interval between successive compositions being 20°
between 1890° and 950°, and 30° from the latter temperature to 590°.
The highest member of the series has the composition Al_{2}O_{3},
SiO_{2}; and the lowest member 2SiO_{2}, B_{2}O_{3}. For convenience
in use the materials are made in the form of triangular pyramids, 5
cms. in height, and each side of the base 1·5 cms. long. Each pyramid
is stamped with a distinguishing number, and altogether 60 are made to
cover the range 1890° to 590°. When conducting a test, several pyramids
are selected with melting points known to be near the temperature of
the furnace, as discovered by previous trials. These are inserted in
the furnace standing on a slab of refractory material, as in fig. 67,
and may be watched through a sight-hole or withdrawn from the furnace
for examination after attaining the existing temperature. If the right
pyramids have been chosen, the appearance presented will be as in fig.
67, in which D is seen to have collapsed completely, C has bent over, B
has been rounded at the top, whilst A is intact. The temperature of the
furnace is then taken to correspond to the melting point of C, which is
found by reference to a table in which the melting points corresponding
to the different distinguishing numbers are given. The pyramids are
extremely cheap, and only those with melting points near to the working
temperature need be purchased. In cases where it is desired to increase
the heat to a specified point, and then to allow the furnace to cool,
these pyramids fulfil all requirements; an examination through a
sight-hole closed with darkened glass enabling the furnace attendant
to discover when the requisite temperature has been attained. The
procedure is more difficult when it is desired to maintain a steady
temperature, as this involves frequent renewal of pyramids already
melted. These appliances are sold under the name of Seger “cones,” the
latter word being evidently a misnomer.
=Watkin’s Heat Recorder.=—This arrangement consists of a small
block of fireclay, having a number of cylindrical holes in its upper
face. Pellets of materials of progressive melting points are placed
in the holes, in which they fit loosely. The block is placed in the
furnace, and afterwards withdrawn and examined, when those which have
completely melted will be seen to have sunk, and to possess a concave
surface; others which have been superficially fused, will show rounded
edges, whilst others will be intact. The melting point of the highest
member of the series which is observed to have rounded edges is taken
as the temperature of the furnace. The materials used in the
manufacture of the pellets are approximately the same as those employed
by Seger, being the same in number (60), and differing progressively
by similar intervals. It is not evident that the method of observation
is superior to the use of pyramids, although some workers may prefer
it, and the arrangement is merely an alternative plan of using the
Seger compositions. Watkin has also introduced a modification in which
straight bars of clay compositions are supported at the edges, the
temperature being deduced by observing which numbers melt, droop, or
remain intact.
=“Sentinel” Pyrometers.=—Under this name, Brearley, of Sheffield,
has introduced a number of compositions, chiefly of salts, which
possess definite melting points. These are made in the shape of
cylinders, about 1 inch long and ¾ inch in diameter, which collapse
completely when the melting point is attained. Compositions have
been found which melt at certain temperatures known to give the best
results in the treatment of different kinds of steel, and a cylinder
of correct melting point, placed in the furnace on a small dish near
to the steel, furnishes a simple and correct clue to the attainment of
the desired temperature. The existing condition of a furnace may be
discovered by taking a number of cylinders, having progressive melting
points, and making observations after the manner described under the
heading of Seger pyramids. A few “Sentinel” cylinders are frequently of
use in the workshop or laboratory for other purposes, such as a rapid
check of a given temperature in confirmation of the reading of an
indicating pyrometer, or in discovering whether a certain temperature
has been exceeded in a given case. “Sentinel” cylinders have been used
in such a manner as to give audible warning of the attainment of a
given temperature by means of a metal rod, which is made to rest on the
cylinder, and which, when the cylinder melts, falls and completes the
circuit of an electric bell. The upper range attainable by the use of
ordinary metallic salts is not so great as in the case of silicates,
but up to 1100° C. metallic sulphates, chlorides, etc., or mixtures of
these, give results quite as good as those obtained with Seger pyramids.
=Stone’s Pyrometer.=—This instrument is intended to indicate the
correct temperature at which a metal or alloy should be poured, and
consists of a silica tube at the bottom of which is placed an alloy
melting at the temperature at which the material operated on should be
poured. A silica rod rests on this alloy, and is connected at its upper
end to an iron extension, the extremity of which engages a pointer
moving over a scale. When the alloy in the silica tube melts, the rod
falls through the molten mass and moves the pointer over the scale,
thus giving a certain indication that the desired temperature has been
attained. Arrangements exist for adjusting the pointer to zero at the
commencement of a test.
=Fusible Metals.=—Instead of clays or salts, a number of metals
and alloys are sometimes used. These are placed in the form of short
rods in numbered holes in a piece of firebrick and inserted in the
furnace, and on withdrawal those which have undergone fusion will be
seen to have taken the form of the holes in which they were placed. The
temperature of the furnace is considered to lie between the melting
points of the last of the series to undergo fusion and the first which
remains unchanged. A series of metals of this description is more
costly than clays or salts, but is more rapid in action, owing to the
superior conductivity of metals.
=Fusible Pastes.=—These consist of salts incorporated with
vaseline or other suitable fat, and are used to detect the attainment
of a specified temperature by a piece of metal. If, for example, it
were desired to heat a piece of steel to 800° C. for a given purpose,
a paste containing common salt might be smeared on its surface before
placing in the furnace. On heating, the vaseline burns away, leaving a
white mark due to the salt, and this white mark will be visible till
the salt fuses. The disappearance of the white mark therefore indicates
that the required temperature has been reached; and the method is
simple and useful in cases where a number of articles are to be worked
at a uniform temperature.
CHAPTER IX
MISCELLANEOUS APPLIANCES
=Expansion and Contraction Pyrometers.=—Most substances, on
heating, exhibit an increase in size, and on cooling return to the
original dimensions. If, however, a chemical alteration occur during
the heating, the resultant material may be permanently altered in size,
so that on cooling the substance may be of less or greater dimensions
than before. Both these phenomena have been applied to the measurement
of high temperatures; the permanent shrinkage undergone by clay being
utilised by Wedgwood in the instrument which was the first practical
pyrometer; the expansion of a solid by Daniell, and of liquid by
Northrup. Both forms are still in use to a limited extent, and will now
be described.
[Illustration: FIG. 68.—WEDGWOOD’S PYROMETER.]
=Wedgwood’s Pyrometer.=—In 1782 Wedgwood introduced a method of
determining the condition of a furnace by observing the contraction
shown by cylinders prepared from a special clay. The measuring device
took the form of a tapered groove (fig. 68) made in two parts, each
6 inches long, and one a continuation of the other. Each inch of the
groove was divided into 20 equal parts, making 240 divisions in all, and
each division was called 1 degree. The width of the groove opposite the
zero mark was 0·5 inch, and opposite 240, 0·3 inch. Before firing, the
cylinders entered the groove until the lower end was opposite or near
the zero mark; and after being inserted in the furnace and allowed to
cool on removal, the cylinders were pushed as far as possible down the
groove, when the mark opposite the lower end indicated the condition of
the furnace in terms of Wedgwood’s scale. The degrees were, of course,
arbitrary; but with cylinders of uniform make a given position in the
groove after heating always represented the same furnace temperature,
and thus furnished an indication more reliable than the judgment of a
workman’s eye. Wedgwood attempted to express the divisions on his scale
in terms of Fahrenheit degrees, and by extrapolation of results obtained
at the highest limits of the mercury thermometer, where 1 degree of
contraction was caused by a rise of 130° F., arrived at figures which
now appear ludicrous, but which were accepted for forty years. As
examples, the melting point of silver was given as 4717° F.; of cast
iron, 17977° F.; and of wrought iron, 21637° F.—the last figure being
nearly 19000° higher than the present accepted value of 2770° F. The
error arose from the assumption of uniform contraction with increase
of temperature, and furnishes a striking example of the danger of
indefinite extrapolation from meagre data. But although the expression
of the result in Fahrenheit degrees was so erroneous, the observed
contraction always corresponded to a given condition of the furnace,
and the firing was continued until that known to be the best for the
work in hand was attained.
The permanent shrinkage referred to is caused by dehydration of the
clay, and it therefore follows that this method can only give uniform
results when exactly the same kind of clay is used for the test-pieces.
A given manufacturer might secure consistent indications by making
a quantity of clay, to be kept specially for this purpose; but the
same contraction at a given temperature would not be obtained by a
second observer who also had prepared a quantity of clay, as slight
differences in composition cause large variations in the observed
contraction. In practice, therefore, pyrometers of this type are not
interchangeable, and each user must standardize for his own special
conditions. Wedgwood’s pyrometer is still used to a small extent; its
replacement, however, by the more convenient and accurate instruments
now available is only a question of time.
=Daniell’s Pyrometer.=—In 1822 Daniell published an account
of a pyrometer based on the expansion of a platinum rod enclosed in
a plumbago tube. One end of the rod pressed against the end of the
tube, whilst the other end was free to move, and was connected to
a multiplying device which magnified the expansion, the increased
movement being indicated by a pointer, moving over a dial. The scale on
the dial was divided evenly into a suitable number of parts, it being
assumed that the difference between the expansions of graphite and
platinum was uniform at all temperatures. The scale was calibrated as
far as possible by comparison with a mercury thermometer, the remainder
being extrapolated. With this pyrometer Daniell obtained a value of
2233° F. for the melting point of silver, and 3479° F. for that of cast
iron—results considerably higher than those now accepted, but much
nearer than those obtained by Wedgwood. Daniell’s pyrometer was widely
used, and its modern representatives are fairly common. Platinum, owing
to its cost, is no longer used in these instruments, which are now
generally made with a graphite rod encased in an iron tube, on the end
of which the graduated dial is placed, as shown in fig. 69. Another
form, commonly used in baker’s ovens, is constructed with an iron rod
surrounded by a porcelain or fireclay tube.
[Illustration: FIG. 69.—EXPANSION PYROMETER.]
The defect of pyrometers of this type is that the coefficient of
expansion of the materials alters with prolonged heating, causing the
readings to become erroneous. Re-adjustment in boiling water or other
substance does not compensate properly for this alteration, as both
materials are not equally affected. Again, the readings will be too
low unless the whole of the expanding parts are in the interior
of the furnace, in which respect this pyrometer is inferior to a
thermo-electric instrument, which may be inserted at any convenient
depth, and may therefore be used for a greater variety of purposes.
The chief recommendation is cheapness; but an expansion pyrometer
should never be used for work of precision. A graphite rod in an iron
enclosure gives more consistent results than other materials.
=Northrup’s Molten Tin Pyrometer.=—Tin melts at 232° C., and
boils at 2270° C. It does not give off vapour sensibly up to 1700°
C., and expands with great uniformity. It is therefore suitable for
measuring high temperatures on the same principle as an ordinary
thermometer, and Dufour, in 1900, attempted to make a high-reading
thermometer by enclosing tin in a silica bulb. Northrup has constructed
an instrument in which the bulb and stem are of graphite, and the
height of the molten tin is determined by lowering a nickel wire
through a gland until it touches the tin, thereby completing an
electric circuit and causing a bell to ring or producing a deflection
on a galvanometer. The upper end of the nickel wire moves over a
scale, which may be marked at two suitable fixed points, and the scale
divided up as in the case of an ordinary thermometer. The durability of
the graphite cover will determine the utility of this pyrometer, and
protection by some good refractory will be essential to prevent
oxidation. Such a pyrometer will not respond quickly to changes in
temperature, but may prove useful in reading temperatures at ranges
beyond the scope of present thermo-electric pyrometers. Northrup
anticipates that this instrument may be used up to 1800° C.
=Vapour-Pressure Pyrometers.=—In these instruments mercury is
placed in a stout steel tube, to which a pressure-gauge is attached,
which registers the vapour-pressure of the mercury. Readings of
pressure may be translated into temperatures by calibration with a
standard pyrometer. The range of these instruments is limited—600° or
700° C.—and they are seldom used at present, having been superseded by
more modern types.
=Water-Jet Pyrometers.=—In these instruments water is passed
through a pipe placed in the furnace or hot space at a definite rate,
and from the rise in temperature produced in the water that of the
furnace may be obtained. An outfit of this kind entails the provision
of a steady source of water-pressure, and the indications can only
remain accurate so long as the bore of the pipe remains uniform. The
calibration is made by comparison with a standard pyrometer. The
drawbacks to the method are its inconvenience, and the necessity
for continuous skilled supervision; and in consequence of these the
arrangement is seldom used.
=Pneumatic Pyrometers.=—Attempts have been made to deduce furnace
temperatures by blowing air at uniform pressure through a pipe located
in the hot space, and noticing the increase in the temperature of the
air. In the Uehling pyrometer, air from the hot space is drawn through
an opening of fixed size by means of a steam-jet, which acts as an
aspirator. The opening is placed at one end of a chamber, and the
steam-jet aspirator at the other end; and a diaphragm with a central
hole divides the chamber into two parts. The pressures existing in
the two portions of the chamber vary according to the temperature of
the air drawn in, and are measured by water-gauges, the readings of
which may be translated into temperatures by calibration against a
thermo-electric or other pyrometer. The method is ingenious, but is
elaborate and costly; and is therefore little used.
=Conduction Pyrometers.=—If one end of a rod of metal be
inserted in a furnace, heat will be conducted along it to the portion
external to the furnace, and a steady condition will be obtained when
the heat escaping from the external part of the rod, by convection
and radiation, is equal to the quantity conducted along the rod. The
hotter the portion in the furnace, the higher will be the temperature
of all parts of the external length. A series of thermometers placed
at intervals in the exterior portion would show a progressive fall in
temperature along the rod; and the hotter the furnace the higher would
be the reading on each thermometer. In applying this principle to the
measurement of high temperatures, a bar of copper or iron is passed
through the wall of the furnace, so that a length of 2 feet or more
protrudes on the outside. Near the end of the external portion a hole
is drilled to a sufficient depth to cover the bulb of a thermometer,
which is inserted in the hole, into which a quantity of mercury is
poured to make a metallic contact between the bulb and the bar. The
reading of the thermometer furnishes an approximate clue to the
temperature of the furnace, rising or falling with corresponding
changes in the hot space. A calibration might be effected by comparison
with a standard; but the method is only applied to the production
of a prescribed condition, known by experience to be attained when
the thermometer reading has a certain value—say 120° C. Changes in
atmospheric temperature, or currents of air, seriously affect the
readings, and the method at best is only approximate.
=Gas Pyrometers.=—Wiborgh, Bristol, and others have constructed
pyrometers in which the pressure of an enclosed gas is recorded by a
Bourdon pressure-gauge, the scale of which is calibrated so as to read
temperatures. A porcelain bulb, terminating in a capillary tube which
is connected to the gauge, is used to contain the air or other gas; but
at temperatures above a red heat the readings become uncertain, owing
either to leakages or the distortion of the bulb. The most suitable
material for the bulb (alloy of platinum, 80 per cent., and rhodium,
20 per cent.) is too costly to use industrially, and would deteriorate
under the influence of furnace gases. In the Bristol recording
instrument the moving index of the pressure-gauge terminates in a pen,
which touches a chart-paper revolving by clockwork. Good results are
obtained up to 400° C., but beyond this the indications are uncertain,
and the instrument is more correctly described as a recording
thermometer.
=Wiborgh’s Thermophones.=—These consist of infusible clay
cylinders, 2·5 cms. long and 2 cms. in diameter, which contain an
explosive. When placed in a hot space, the explosion occurs after
a definite time, the interval being less at high temperatures than
at lower, as the rate at which heat is conducted through a solid
varies directly as the difference between the external and internal
temperatures. The interval elapsing between placing in the furnace and
the subsequent explosion is noted on a stop-watch to the nearest 1/5
second, and from the observed time the temperature is obtained from a
table, drawn up from the results of experiments under known conditions.
If the cylinders be kept dry, an observer experienced in the use of
thermophones may secure a reading to within 40° C.
=Joly’s Meldometer.=—This device, due to Dr Joly, is intended
for laboratory determinations of melting points. It consists of a
strip of platinum, heated by electricity, upon which a tiny fragment
of the material is placed, which is viewed through a microscope. The
temperature of the platinum is regulated by means of a rheostat in the
circuit, and in making a test the temperature is gradually raised until
the material is observed to become globular, or to flow over the
platinum strip. The temperature at which this occurs is deduced from
the linear expansion of the platinum strip, which is measured by a
micrometer attached to the instrument. When carefully used, very
accurate determinations may be made by the meldometer, the results,
moreover, being obtained rapidly, and with the use of the minimum of
material.
=Brearley’s Curve Tracer.=—This apparatus made by the Cambridge
and Paul Instrument Company, is designed to take a large-scale
record of an operation which only occupies a short period of time.
It consists of a drum, round which the record paper is wound, and
capable of rotating on its axis once in ten or thirty minutes by the
aid of clockwork. Attached to the arm of the pen is a pointer, which
moves along the scale of a sensitive mirror galvanometer to which a
thermocouple is connected. The operator, by turning a handle, moves
the drum longitudinally so as to keep the pointer opposite the centre
of the spot of light, and this movement is traced on the chart,
combined with the rotary movement, by the pen. In this manner the
large change in deflection, due to a few degrees increase or decrease
in temperature, can be recorded in ink. This instrument is of special
service in recording the critical points of steel, or any operation
which involves delicate readings over a limited range of temperature.
Index
Air pyrometers, 219.
Anti-vibration stand for galvanometers, 47.
Atmospheric temperatures, measurement of, 96, 132.
Automatic compensators, 66.
_Barrett, Sir W._, discovery of recalescence, 4.
_Becquerel, Ed._, optical pyrometer, 5.
Black-body radiations, 136.
_Brearley_, sentinel pyrometers, 208.
— curve-tracer, 221.
_Bristol_, air-recording pyrometer, 219.
— compensator, 66.
_Byström_, calorimetric pyrometer, 201.
Calorimetric or “water” pyrometers, 195.
— — — special uses of, 203.
Centigrade scale of temperatures, 6.
Clay-contraction pyrometers, 211.
Colour-extinction pyrometers, 189.
Colour, in relation to temperature, 167.
Comparison of gas and platinum scales, 111.
Compensators for cold junctions, 66.
Conduction pyrometers, 218.
Constant temperature cold junction, 71.
Contact-pen recorders, 88.
“Critical” points of steel, 94.
_Daniell_, expansion pyrometers, 2, 214.
_Darling_, automatic compensator, 68.
— and _Grace_, liquid element thermocouples, 43.
_Day_, extension of gas scale, 15.
Electromotive force (E.M.F.) developed by junctions, 31.
— — measurement of, 62.
Expansion pyrometers, 214.
_Fahrenheit_, temperature scale, 7.
_Féry_, lens pyrometer, 142.
— mirror pyrometer, 143.
— optical pyrometer, 174.
— spiral pyrometer, 150.
Fixed points for calibration of pyrometers, 16, 17.
_Foster_, base-metal pyrometer, 39.
— fixed-focus pyrometer, 152.
— recorder, 82.
Furnace, electric tube, 95.
— temperatures, control of, by pyrometers, 87.
Fusible metals, 209.
— pastes, 210.
Fusion pyrometers, 204.
Gas pyrometers, 219.
— scale of temperatures, 11.
— thermometer, constant volume, 11.
_Hadfield_, effect of temperature on hardness of steel, 4.
_Harris_, indicator, 121.
_Holborn-Kurlbaum_, optical pyrometer, 181.
_Holden-d’Arsonval_, galvanometer, 45.
_Holman_, formula for thermal junctions, 62.
_Howe_, colour-temperature table, 167.
Indicators, for radiation pyrometers, 156.
— for resistance pyrometers, 118-124.
— special range, 71.
— standardizing of, 54, 108, 157.
— for thermo-electric pyrometers, 45-53.
Installations of resistance pyrometers, 130.
— of thermo-electric pyrometers, 89.
_Joly_, meldometer, 220.
_Kelvin_, thermodynamic scale of temperature, 9.
_Kowalke_, base-metal couples, 30.
_Lambert_, anti-vibration stand for galvanometers, 47.
_Le Chatelier_, optical pyrometer, 177.
— thermo-electric pyrometer, 5, 22.
_Leeds-Northrup_, indicator, 122.
— recorders, 85, 127.
— resistance pyrometers, 115.
Liquid element thermocouples, 43.
_Lovibond_, optical pyrometer, 186.
Low temperatures, measurement of, 97, 132.
Meldometer, 220.
Mercury thermometer, limits of, 1.
_Mesuré and Nouel_, optical pyrometer, 187.
_Morse_, optical pyrometer, 182.
National Physical Laboratory, scale of temperatures, 17.
_Newton_, researches on high temperatures, 2.
_Northrup_, molten tin pyrometer, 216.
— pyrovolter, 74.
Optical pyrometers, 167.
— — management of, 192.
— — special uses of, 193.
_Paul_, base-metal pyrometer, 39.
— compensator, 70.
— radiation pyrometer, 155.
— recorder, 83.
— uni-pivot indicator, 49.
_Peake_, compensated leads, 67.
Pivoted galvanometers, 49.
_Planck_, modifications of Wien’s formula, 171.
Platinum scale of temperatures, 106.
Pneumatic pyrometers, 217.
Potentiometer indicators, 73.
— method for measurement of E.M.F., 63.
_Prinsep_, gas pyrometer, 3.
Protecting sheaths for pyrometers, 34.
Pyrometer, definition of, 1.
Pyromike, 189.
Radiation pyrometers, 134.
— — calibration of, 157.
— — indicators for, 156.
— — management of, 161.
— — recorders for, 161.
— — special uses of, 164.
_Rasch_, luminosity formula, 168.
Recalescence of steel, 4, 94.
Recorders for radiation pyrometers, 161.
— — resistance pyrometers, 124.
— — thermo-electric pyrometers, 75.
Resistance, measurement of, 102.
— of platinum, 105.
Resistance pyrometers, 101.
— — indicators for, 118.
— — management of, 130.
— — recorders for, 124.
— — special uses of, 132.
— pyrometry, terms used in, 111.
_Roberts-Austen_, recorder, 76.
Salts, melting points of, 204.
_Seebeck_, discovery of thermo-electricity, 3, 20.
_Seger_, pyramids or “cones,” 205.
_Siemens_, calorimetric or “water” pyrometer, 201.
— indicator for resistance pyrometer, 118.
— — — thermo-electric pyrometer, 48.
— optical pyrometer, 182.
— recorder, 81.
— resistance pyrometer, 114.
Specific heat of nickel, 197.
Standardizing of indicators, 54, 108, 157.
Standards of temperature, 9.
Steam, measurement of temperature of, 98.
_Stefan-Boltzmann_, fourth-power law, 139.
_Stone_, pyrometer, 209.
Surface temperatures, measurement of, 97.
Suspended-coil galvanometers, 45-48.
Temperature differences, measurement of, 99.
— scales, 7-9.
— fixed points of, 16-17.
Thermal junctions, changes in, 29.
— — choice of metals for, 21.
— — E.M.F., developed by, 31.
— — methods of joining, 26.
— — used in pyrometers, 27.
Thermo-electric circuits, 22.
— pyrometers, 20.
— — calibration curves for, 59.
— — for surface temperatures, 97.
Thermo-electric circuits, indicators for, 45-53.
— — management of, 91.
— — practical forms of, 32.
— — recorders for, 75-87.
— — standardization of, 54.
Thermodynamic scale of temperatures, 9.
Thermometer, constant volume gas, 11.
— mercury, 1.
Thermophones, 220.
Thread recorder, 78.
_Uehling_, pyrometer, 218.
Uni-pivot galvanometer, 49.
Vapour-pressure pyrometers, 217.
_Wanner_, optical pyrometer, 178.
Water-cooled cold junction, 33.
Water equivalent of calorimeter, 200.
“Water” pyrometer, 201.
Water-jet pyrometer, 217.
_Watkin_, heat recorder, 207.
Wedge pyrometer, 190.
_Wedgwood_, pyrometer, 211.
— test-pieces, 205.
_Wheatstone_ bridge for measuring resistance, 104-114.
_Whipple_, indicator, 120.
_Whipple-Féry_, pyrometer, 154.
_Wiborgh_, gas pyrometer, 219.
— thermophones, 220.
_Wien_, luminosity law, 171.
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