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.