Thermocouples

When two dissimilar metal wires are joined at the ends and are kept at different temperature TR and TS, a continuous thermoelectric current flows owing to the Seebeck effect as shown in Fig. 5.31a. If the wires are cut, an open-circuit Seebeck voltage is measured which is proportional to the temperature difference.

If TR is constant, the Seeback voltage is proportional to Ts. The associated Peltier effect is the increase or decrease of the junction temperature when an external current flows in the thermocouple (TC) wires. Reference 77 gives a description of these effects and their interrelationships.

Figure 5.31 Thermocouples. (a) Seebeck effect. (b) Measurement setup for remote sensing.

Figure 5.31b shows an arrangement when the measuring equipment is remote from the sensing TC. Matched extension TC wires are used and the reference junctions are formed at the measuring equipment. The reference junctions must be kept at a constant temperature, mostly at ambient room temperature TA.

This requirement can also be satisfied by electronically providing a reference voltage compensation.

Thermocouples used in industry are made from several combinations of metals.

Table  summarizes the properties of TCs that are commonly used, and a complete set of thermocouple reference tables is given in Ref. 76. The basic measuring instrument used with these thermocouples is the potentiometer, but direct-reading, analog, and digital meters of many kinds are also available from manufacturers.