Thermocouple is a kind of temperature sensing element and a kind of instrument. It directly measures the temperature, and converts the temperature signal into a thermoelectromotive force signal, which is converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The basic principle of thermocouple temperature measurement is that two conductors of different components form a closed loop. When there is a temperature gradient at both ends, a current will flow in the loop. At this time, there is an electromotive force-thermoelectromotive force between the two ends. This is the so-called Seebeck effect. Two homogeneous conductors with different components are thermoelectrodes. The end with a higher temperature is the working end, and the end with a lower temperature is the free end. The free end is usually at a certain constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple index table is made; the index table is obtained when the free end temperature is at 0℃, and different thermocouples have different index tables.
When the third metal material is connected in the thermocouple circuit, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal in the circuit. Therefore, when the thermocouple measures the temperature, the measuring instrument can be connected, and the temperature of the measured medium can be known after the thermoelectromotive force is measured.
