Thermocouples: When two dissimilar metals are joined together to form a junction, then an electromotive force exists between the junctions and the connecting wires that forms the circuit. It leads to a current flow in the circuit due to the potential difference of voltage that comes from two different sources: from contact of two dissimilar metals at the junction temperature and from the temperature gradients along the conductors in the circuit. The first one is known as Peltier effect which is due to the temperature difference between the junctions. The voltage difference due to temperature gradients along the conductors in the circuit is known as Thomson effect . In most of the cases, the Thomson emf is quite small relative to Peltier emf and with proper selection of conductor materials, the Thomson emf can be neglected. With this principle, a thermocouple is prepared between two conductors of different materials resulting two junctions p and q as shown in Fig. 7.1.6. If the junction temperatures are equal, then the emfs will balance and no current will flow. When there is temperature difference between the junctions, then there will be net emf and a current will flow in the circuit. When the thermocouple is used to measure a unknown temperature, then temperature of one of the junction is known and termed as reference junction and by measuring the net emf in the circuit, the unknown temperature can be measured. The thermocouple circuits also follow certain laws in addition to Seebeck effect;

Fig. 7.1.6: Schematic representation of a thermocouple junction.

 

Fig, 7.1.7: Illustration of thermocouple law of intermediate metal.

 

Law of intermediate metals : Insertion of an intermediate metal into a thermocouple circuit does not affect the net emf, provided the two junctions introduced by a third metal are at identical temperatures. (Fig. 7.1.7)

Law of intermediate temperatures : If a simple thermocouple circuit develops an emf when its junctions are at temperatures and emf of when the junctions are at temperatures , then the same thermocouple will develop an emf when the junctions are at temperatures , respectively.

Depending on the combination of materials used for the conductors, thermocouples are classified in different types. The selection of the material is based on cost, availability, convenience, melting point, chemical properties, stability, and output. They are usually selected based on the temperature range and sensitivity needed. Some of the classifications are given in Table 7.1.1.

Table 7.1.1: Classifications of thermocouples