Thermocouple temperature sensor is one of the most commonly used temperature detecting element in industry, belongs to the contact temperature measurement range. Thermocouple temperature measurement principle is based on thermoelectric effect. Thermoelectric effect refers to the two different conductor or semiconductor connection form closed loops, when the two contacts in temperature is not at the same time, the loop will produce thermoelectric potential phenomenon. The thermoelectric potential also has two forms, is the temperature difference between potential and contact potential respectively. 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 have potential is different, the contact potential is when the two different conductor in contact, because of their different electron density and produce certain electronic diffusion, when reaching 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.
Thermocouple temperature sensor is so widely used lies in its wide range of temperature (270 ℃ to + 2800 ℃), high precision (direct contact with the object being measured, is not affected by the medium in the middle of the), performance is stable, at the same time, simple structure, good dynamic response, more able to 4-20 ma electrical signal remote transmission, convenient automatic control and centralized control. The structure of thermocouple there are two main types: regular and armor type. Ordinary thermocouples generally by the hot electrode, insulation tube, to protect casing and junction box parts, such as type and armoured thermocouple is the thermocouple wire, insulation material and metal protective casing combination after assembly, after stretching processing from a combination of a solid. In order to guarantee the thermocouple temperature sensor is reliable, steady work, the structure of its requirement is as follows: two of thermocouple thermal electrode welding must be strong; Two hot electrode between each other should have good insulation, prevent short circuit phenomenon; Compensation conductor should be connected to the free cod of a thermocouple to be convenient and reliable; Protective casing should ensure hot electrode and harmful medium sufficient isolation and so on.
The thermocouple of international application has a standard specification, it can be divided into eight different degree, respectively B, R, S, K, N, E, J and T, B, R, S belong to platinum series thermocouple, because platinum are precious metals, the temperature measurement points when using relatively far and the distance to the instrument, so the thermocouple signals need to use a special kind of wires to transfer, this kind of wire we called compensation conductor, the compensation conductor should be extended to the temperature of the cold junction of the thermocouple relatively stable control room, connected to the meter terminals. Different compensation wire, thermocouple temperature sensor need different compensation conductor should be mainly include 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, and commonly used 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. It must be pointed out that the role of the thermocouple compensation wire only extend thermal electrode, make moves to the control room of the cold junction of the thermocouple meter terminal, it itself does not eliminate the cold end temperature change on the influence of temperature, no compensation. So, still need other correction method is used to compensate the cold end temperature t0 indicates influence on measuring temperature 0 ℃.
In addition to the application of thermocouple as the temperature sensor of measuring temperature material, there are other thermistor, and so on, compared with thermocouple, heat resistance temperature measurement range is narrow, but it has a good linear relationship. When use, according to different conditions, select the appropriate measuring temperature sensor products.
