Basic principles of thermocouple temperature measurement

A closed loop composed of a homogeneous conductor does not generate thermoelectric potential regardless of the cross-sectional area, length and temperature distribution of the conductor. If the two thermocouples are composed of two homogeneous conductors, the thermoelectric potential of the thermocouple is only related to the temperature of the two junctions and has nothing to do with the temperature distribution of the thermocouple; if the thermocouples are non-homogeneous electrodes and are in a temperature field with a temperature gradient, additional potential will be generated. If the temperature is judged only from the thermoelectric potential of the thermocouple, it will cause errors.

1. Homogeneous conductor rule: In the actual temperature measurement circuit, there must be connecting wires and display instruments. If the connecting wires and display instruments are regarded as the third conductor, as long as the temperature of their two ends is the same, it will not affect the total thermoelectric potential.

2. Intermediate conductor rule: If a conductor of a third material is connected to the thermocouple circuit, as long as the temperature of the two ends is equal, the connection of the conductor will not affect the total thermoelectric potential of the thermocouple circuit. According to this rule, one contact of the thermocouple can be disconnected and connected to a third conductor, or one conductor of the thermocouple can be disconnected and connected to a third conductor. As long as the temperatures of both ends of each conductor are the same, the total thermoelectric potential of the circuit will not be affected.

3. Reference electrode rule: Two conductors A and B form a thermocouple with reference electrode C (or standard electrode). If the thermoelectric potential generated by them is known, the thermoelectric potential of A and B after pairing can be obtained by the following formula. It can be seen that as long as the thermoelectric potential of the two conductors when they form a thermocouple with the reference electrode is known, the thermoelectric potential of the two conductors when they form a thermocouple can be calculated according to the reference electrode rule. This simplifies the selection of thermocouples. Since platinum has stable physical and chemical properties, high melting point, and is easy to purify, people often use high-purity platinum as a reference electrode.