Thermocouples basic circuit

Fig. 5. Basic thermocouple circuit. A and B are wires of different materials.

An elementary thermocouple circuit is shown in Fig. 16.16. The EMF generated in this circuit is a function of the materials used and the temperatures of the junctions. It is useful to describe briefly the basic thermoelectric phenomena or effects that are related to the Seebeck effect and are present in thermocouple measurements. They include two well-known irreversible phenomena—Joule heating and thermal conduction—and two reversible phenomena—the Peltier effect and the Thompson effect. 

Consider a basic thermocouple circuit with one measuring junction, as shown in Fig. 16.17. The two thermoelements A and B are joined at point c to form the measuring junction at temperature T. The thermoelements are connected to wires C at points b and d, both immersed in an ice bath (liquid water and ice in equilibrium) at T0. The two wires C are connected to the input of an EMF measuring device. The input ports, a and e, are maintained at temperature 7j. Applying Eq. 16.18 over the various legs of the circuit gives... 

Ideal case. The thermoelectric junction constitutes the basic system in the domain of thermoelectricity. It is not utilized alone as in the case study treated here but with another junction for making up a thermocouple, each junction being raised to a different temperature. The reason is quite logical In order to electrically connect the two materials of a junction with the exterior and make an electric circuit, it is compulsory to use somewhere at least another junction, if only for closing the circuit with the same materials used in the junction. 

Thermocouple (TC) temperature sensors are also known as thermoelectric transducers a basic TC circuit is shown in Fig. 4.10. 

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