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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.