Thermocouple is a temperature sensing element, is an instrument. It is directly measured temperature and the temperature signals into a thermoelectric emfs, after electric meter ( Secondary instrument) Convert the temperature of the measured medium. The basic principle of thermocouple temperature measurement are two different ingredients of the material of conductor closed loop, when when there is a temperature gradient at both ends, will have a current through the loop. Existed between the electromotive force on both ends - at this time - Thermoelectric emf, this is the so-called seebeck effect. Access the third metal materials in the thermocouple loop when only two contacts at the same temperature, this material produced by the thermocouple thermoelectric is set to the same, which is not affected by the third metal access in the loop. Therefore, when the thermocouple temperature measurement, can access measurement instrument, measured after the thermoelectric emfs, can know the temperature of the measured medium. Thermal resistance material used at present, there are mainly platinum, copper, nickel, etc. , using these materials mainly is that they are in common temperature period of temperature and the resistance is the ratio of the linear relationship, we mainly introduced here platinum resistance thermometer. Platinum is a precious metal, its physical and chemical performance is stable, especially the oxidation resistance ability is very strong, it is easy to be purified, good manufacturability. Can be made into very fine platinum, and copper, nickel and other metals, compared with high resistivity, high reproducibility, is a kind of ideal heat resistance materials, defect is temperature coefficient of resistance, the smaller the restore work easy to brittle medium, the price is more expensive. Second, thermocouple theory is a kind of energy converter, it converts heat into electricity, using the thermoelectric potential measurement temperature.