Page 23 - Instrumentation and Measurement

The current flowing is the result of the difference in electromotive force developed at the two
junctions due to their temperature difference. In practice, the voltage difference between the two
junctions is measured; the difference in the voltage is proportional to the temperature difference
between the two junctions. Note that the thermocouple can only be used to measure temperature
differences. However, if one junction is held at a reference temperature the voltage between the
thermocouples gives a measurement of the temperature of the second junction.
2.3.3 Radiation Pyrometers
When temperatures are above the practical range of thermocouples, or the target is not accessible, or
an average temperature of a large surface is required then radiation pyrometers are used.
Methods used for the measurement of temperature which involve the radiation emitted by the body
include:
1. Optical pyrometer

This is based on comparing the brightness of the light emitted by the hot body with that from a
known standard.
2. Total radiation pyrometer
This involves the measurement of the total amount of radiation emitted by the hot body by a
resistance element or a thermopile.
The optical pyrometer, known generally as the disappearing filament pyrometer, involves just the
visible part of the radiation emitted by a hot object. The radiation is focused onto a filament so that
the radiation and the filament can both be viewed in focus through an eyepiece (Figure 2.16). The
filament is heated by an electrical current until the filament and the hot object seem to be the same
colour, the filament image then disappearing into the background of the hot object. The filament
current is then a measure of the temperature. A red filter between the eyepiece and the filament is
generally used to make the matching of the colours of the filament and the hot object easier. Another
red filter may be introduced between the hot object and the filament with the effect of making the
object seem less hot and so extending the range of the instrument.

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