What is a thermocouple

5.6 What is a thermocouple

If the junction of two dissimilar metals is heated, the heat energy forces the free electrons of one metal into the other, thus generating an e.m.f., see fig 5.6.1.  This e.m.f. produced depends on two factors:
a) The type of metal used in forming the junction.
b) The temperature difference between the hot and the cold junctions.
A single pair of junctions (cold and hot) produces very little electrical energy.  The pair is what forms the thermocouple.

To produce measurable electricity from thermocouples, individual thermocouples are sequentially arranged to form a thermopile.  During usage all the hot junctions are collectively placed close the area or substance whose temperature is being determined whereas the cold junctions are collectively placed within the fixed cold temperature.  This means that the cold junction is a reference or control temperature.  A simple hot and cold junctions’ circuit is shown in Fig. 5.6.2 for clarity.

Typical applications of thermocouples are: flame detectors, furnace controls and heat detectors.