The basic principle of thermocouple temperature measurement in the thermocouple circuit connected to the third metal materials, 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. The working principle of two kinds of different ingredients of conductor ( Called the thermocouple wire or hot electrode) Two ends synthesis loop, when the junction temperature is not at the same time, in the circuit will generate electromotive force, this kind of phenomenon called the thermoelectric effect, and the electromotive force called thermoelectric potential. Thermocouple is to use the principle of temperature measurement, which directly used for measuring the medium temperature is called at the end of the working side ( Also known as the measuring end) , on the other side called cold end ( Also known as compensation) ； The cold end connected to the display instrument or meter, display instrument will point out the thermocouple thermoelectric potential. Thermocouple is actually a kind of energy converter, it converts heat into electricity, using thermoelectric potential generated by the measuring temperature, the thermocouple thermoelectric potential. Characteristics of the assembly is simple, easy to change pressure reed temperature sensing element, seismic performance is good, large measuring range, high mechanical strength, good compression performance, high temperature resistant up to 2400 degrees. Thermocouple - Type and structure formation ( 1) Thermocouple types commonly used thermocouple could be divided into two types: standard thermocouple and non-standard thermocouple. Call standard thermocouple is refers to the standard stipulated by the 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 in order to keep the thermocouple is reliable, steady work, the structure of its requirement is as follows: (1) of the two thermocouple hot electrode welding must be strong; (2) 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 thermal electrodes should be able to make sufficient isolation and harmful medium.