Thermocouple belongs to contact temperature measuring instrument is one of the temperature measuring instrument is commonly used in industrial production. Thermocouple principle of thermocouple is a thermal device, which converts the signal temperature thermoelectric emf signal, through electric instrument converts the temperature of the measured medium. Thermocouple temperature measurement is the basic principle of two kinds of different ingredients of homogeneous conductor closed loop, when 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, can be connected to the measuring instrument, measured after the thermoelectric emfs, can know the temperature of the measured medium. Thermocouple is the temperature of the temperature measuring device is commonly used in industry, has the following characteristics: (1) high accuracy of measurement: thermocouple direct contact with the object being measured, is not affected by the medium in the middle of the. (2) thermal response time fast: thermocouple is responsive to temperature changes. (3) large measuring range: thermocouple from - 40 ~ + 1600 ℃ can be continuous temperature measurement (4) reliable performance, good mechanical strength. 5. Long service life, easy installation.