Heat treating and inspection of metals Page: 10
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5 AIR CORPS
temperatures up to 2,0000 F., while the more expensive platinum and
rare metal combination may be used up to a temperature of 3,000.
Following are details of common thermocouple with data on their
Electropositive Electronegative Corresponding leads Useful tem-
1 2 1 2 range o F.
Chromel -__ Alumel .- . Copper__ Constantan_ 3000 to Must be kept
2,200. protected from
Iron - ------ Constantan _ Iron - Constantan_ 3000 to
Platinum- Platinum_... Copper _ Copper al- 00 to Must be pro-
rho d,iu m loy. 2,6500. tected by neu-
alloy. tral atmos-
Copper ____ Constantan_ Copper__ Constantan_ 3000 to
(b) Extension leads.--Alloy extension leads are made of flexible or
solid wire of the same material as the couple, thus, in effect the ther-
mocouple may be considered as extending from the furnace to the
meter. By the use of extension leads, the meter may be placed a con-
siderable distance from the furnace so that the cold ends will not be
subjected to sudden fluctuations in temperature. The leads are car-
ried in parallel and covered with a heat-resisting insulation.
(c) Meter.-Since a pyrometer does not indicate the actual tempera-
ture of the hot end of the thermocouple, but rather the difference in
temperature between the hot and cold ends, the temperature of the
cold end must be held constant and must be known. This is accom-
plished either by the use of a thermostat which is connected to a con-
trol spring in the instrument and makes the correction automatically,
or by means of a zero adjuster which is operated manually. If the
temperature of the cold junction of the thermocouple is constant, the
millivoltage (caused by the difference in temperature between the hot
and cold wire ends), will be proportional only to the temperature of
the cold junction. If, however, the temperature of the cold junction
is not kept constant or compensated for, the indications of the pyrom-
eter will not be correct and will vary with fluctuations of the tem-
perature surrounding the cold end. For example, if the temperature
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/12/: accessed July 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.