Working principle of the thermocouple are two different components of homogeneous conductor of a closed loop, when the temperature gradient is at both ends, the loop will have an electric current passes through, the existed Seebeck electromotive force between two ends -- - Thermoelectric emf, this is the so-called seebeck effect. Two different components of homogeneous conductor electrode as heat, the temperature is higher for work at the end of the end, one end of the low temperature as the free end, usually free end under a constant temperature. According to the thermoelectric emf as a function of temperature, the thermocouple indexing table; Indexing table is free end temperature at 0 ℃, under the condition of different thermocouples with different indexing table. Access in the thermocouple loop when the third metal material, the two contacts at the same temperature as long as the material, produced by the thermocouple thermoelectric is set to remain the same, which is not affected by the third metal access in the loop. Therefore, when the thermocouple temperature measurement, by compensating conductor access digital display instrument, measured after the thermoelectric emfs, can know the temperature of the measured medium.