Thermocouple can bend, high pressure resistance, heat fast response time and strong and durable many advantages, such as it and industrial prefabricated thermocouple, as measure temperature sensor, and usually display meter, recording meter and electronic regulator, at the same time, also can be used as a prefabricated thermocouple temperature sensing element. It can directly measure all kinds of production process from 0 ℃ ~ 800 ℃ in the scope of liquid, steam and gas medium, as well as the surface temperature of the solid. Compared with prefabricated thermocouple, thermocouple is flexible, high pressure resistance, thermal response time is short and strong and durable, etc. The working principle of thermocouple are two different components of conductor on both ends of the welding and form a loop, directly measuring WenDuan call work side, the terminal end is called the cold end, also known as the reference. When work end and reference end there is temperature difference, will produce thermal current in the circuit, connected to display instrument, the instrument will indicate the thermocouple thermoelectric emf generated by the corresponding temperature value. The thermocouple thermoelectric emfs growth over the course of measuring the temperature, the size of the thermoelectric emfs and thermocouple temperature difference on both ends of the conductor material and, only has nothing to do with hot electrode length and diameter. The characteristics of the thermocouple is thermocouple temperature measurement is widely used in the temperature of the device, it is the main characteristic of wide temperature range, stable performance, at the same time, simple structure, good dynamic response, more can far eastone - 4 20 ma electrical signals to facilitate automatic control and centralized control. Thermocouple temperature measurement principle is based on thermoelectric effect. Connect the two different conductor or semiconductor into closed loops, when the two contacts in temperature is not at the same time, the circuit will produce thermoelectric potential, this kind of phenomenon called the thermoelectric effect, also known as the seebeck effect. There are two kinds of closed loop of thermoelectric potential of electric potential; The thermoelectric potential and contact potential. Thermoelectric potential means the same at both ends of the conductor potential because of the different temperature, different conductors with different electron density, so they produced by electric potential is not the same, the contact potential as its name implies is refers to the two different conductor in contact, because of their different electron density so produce certain electronic diffusion, when they reach a certain potential, which is formed by the balance after the contact potential depends on the size of the two different conductor material properties and their temperature contact point. International application of thermocouple with a standard specification, the provisions on international thermocouple is divided into eight different degree, respectively b, r, s, k, n, e, j and t, its measuring temperature low measurable minus 270 degrees Celsius, up to 1800 degrees c, b, r, s belong to platinum series hot thermocouple galvanic couple, because platinum are precious metals, so they are also known as precious metal thermocouple and the remaining few called cheap metal thermocouple. There are two kinds of structure of the thermocouple, regular and type. Normal sexual thermocouples generally by the hot electrode, insulation tube, to protect casing and junction box parts, such as the type of thermocouple is the thermocouple wire, insulation material and metal protective casing combination after assembly, after stretching processing from a combination of a solid. But it takes a special electrical signal of thermocouple wires to pass, this kind of wire we called compensation conductor. Different thermocouples require different compensation conductor, its main function is connected to the thermocouple, away from the power of the reference junction of the thermocouple, so that the reference temperature stability. Compensation wire is divided into compensation type and extended type two kinds, extend the chemical composition of the wire and the compensation of thermocouple is the same, but in practice, to extend the type of wire is not used is the same as the thermocouple material of metal, usually use and thermocouple wires to replace with the same electron density. Compensation and thermocouple wires of the wire are generally not very clear, the positive connection of the thermocouple compensating lead red line, while the cathode connecting the rest of the color. Most of the material compensating lead wires are commonly used copper nickel alloy.