One, the application principle of thermocouple thermocouple is one of the industry commonly used temperature detecting element. Its advantages are: (1) high accuracy of measurement. Because of thermocouple direct contact with the object being measured, not affected by intermediate medium. (2) the measurement range. The commonly used thermocouple from - 50 ~ 1600 ℃ can renew edge measurement, some special thermocouple low measurable to - 269℃（ Such as gold iron nickel chrome) Up to 2800 ℃ ( Such as tungsten-rhenium thermocouples) 。 3. Simple structure, easy to use. Thermocouple is usually composed of two different kinds of metal wire, but is not limited by the size and the beginning of, outside has protective casing, which are very convenient. （ 1) Thermocouple types commonly used thermocouple could be divided into two types: standard thermocouple and non-standard thermocouple. Invokes standard thermocouple is refers to the national standard specifies its thermoelectric potential and the relationship between temperature, permissible error, and a unified standard score table of thermocouple, it has with matching display instrument to choose from. Non-standard thermocouple or on the order of magnitude less than the range using standardized thermocouple, generally no unified indexing table, mainly used for the measurement of some special occasions. Standardized thermocouple since January 1, 1988 in our country, the thermocouple and thermocouple were manufactured according to IEC international standard, and specify the S, B, E, K, R, J, T seven standardization thermocouple type thermocouple for our country unified design. ( 2) The structure of thermocouple thermocouple is reliable, steady work, to ensure that the requirements for its structure is as follows: (1) of the two thermocouple electrode welding must be strong (2) the two hot electrode should be well insulated between each other, in case of short circuit (3) compensation conductor should be connected to the free cod of a thermocouple to be convenient and reliable (4) protect casing should ensure hot electrode and harmful medium fully isolate the application principle of thermocouple, thermocouple is commonly used in the low temperature zone of a temperature detector. Its main characteristic is high accuracy, stable performance. Among them is platinum thermal resistance measurement accuracy is high, it is not only widely used in industrial temperature measurement, and is made into standard benchmark instrument. Thermocouple temperature measuring thermocouple temperature measurement principle and the material is based on the metal conductor resistance value increases with the increase of the temperature of the properties for temperature measurement. Thermocouples are mostly made of pure metal materials, the application is a lot of platinum and copper, in addition, now has started using survey, nickel, manganese and rhodium thermocouple materials such as manufacturing. Thermocouple temperature measurement system of thermocouple temperature measurement system in general by thermocouple, connecting wires and display instrument, etc. Must pay attention to the following two points: (1) the thermocouple and the index number of the display instrument must agree (2) in order to eliminate the influence of the connection wire resistance change, must use three wire connection. Specific content see this chapter 3.