SM0486
(10) A pyrometer is usually associated with the measurement of extremely
high temperatures. When properly designed, a pyrometer can be used for relatively
low temperature measurements. Some of the principle types of pyrometers in use are
as follows:
Optical pyrometers.
Seger cone pyrometers.
Thermoelectric pyrometers.
Direct-radiation pyrometers.
Resistance pyrometers.
Our discussion is limited to the principles involved in the operation of the
thermoelectric and optical pyrometers.
(a) Thermoelectric pyrometers.
You should remember how a thermocouple
is designed to measure the difference in temperature between a cold and a heated
junction.
A millivoltmeter equipped with a scale which is graduated to read in
degrees of temperature is often used with the thermocouple.
This thermocouple-
millivoltmeter combination is sometimes called a millivoltmeter pyrometer and is a
typical thermoelectric pyrometer.
(b) Optical pyrometers.
Temperature measurement with an optical
pyrometer consists of comparing the monochromatic illumination from the source
being measured with the illumination from the filament of a standard lamp.
When
the intensity of illumination from the standard source is equal to the intensity of
illumination from the hot body (source measured), you can assume that both bodies
are at proportionate temperatures.
The proportionate temperature of the hot body
is determined by the graduations on the pyrometer scale.
(2) The type of pyrometer you use will vary with the need.
Optical
pyrometers may have serial numbers such as 8621, 8622, 8623, and 8626.
These
pyrometers may have a suffix C, indicating that the instrument is designed for
centigrade temperature measurements. In all other respects the capabilities of the
instrument are the same as those using the Fahrenheit scale.
Temperature ranges
for three of these instruments are as shown in Table 7.
(3) The two major parts of this instrument are the telescope and a control
box.
The telescope is designed to fit the hand.
The telescope has an eyeshield
which is used when sighting a body. A flexible cable connects the control box to
the telescope.
The control box contains a galvanometer, a standard cell, and a
breather. The telescope consists of a lamp, a switch, and a means of focusing the
lamp filament and the image of the hot body. The pyrometer measurement principle
is included in the steps of the measurement procedure which follow.
Examine the
steps to see if you can identify the measurement principle.
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