Application principle of the hottest couple

Application principle of thermocouple

thermocouple is one of the most commonly used temperature detection elements in industry. Its advantages are:

① high measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium

② wide measurement range. Common thermocouples can be continuously measured from -50~+1600 ℃, and some special thermocouples can be measured as low as -269 ℃ (such as gold iron nickel chromium) and as high as +2800 ℃ (such as tungsten rhenium)

③ simple structure and convenient use. Thermocouples are usually composed of two different kinds of metal wires, which are not limited by size and beginning. There is a protective sleeve outside, which is very convenient to use

1. basic principle of thermocouple temperature measurement

the conductors or semiconductors A and B of two different materials are welded together to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, electromotive force will be generated between them, thus forming a large and small current in the circuit. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work

2. Types and structure formation of thermocouples

(1) types of thermocouples

commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The standard thermocouple used refers to the thermocouple whose national standard stipulates the relationship between the new lithium carbonate processing capacity of its thermal potential and temperature in China, the allowable error, and a unified standard graduation table. It has its supporting display instrument for selection. Non standardized thermocouples are inferior to standardized thermocouples in the scope of use or order of magnitude. Generally, there is no unified graduation table, which is mainly used for measurement in some special occasions

standardized thermocouples since January 1st, 1988, all thermocouples and thermal resistors have been produced according to IEC international standards, and seven standardized thermocouples, s, B, e, K, R, J and T, have been designated as thermocouples of unified design in China

(2) structural form of thermocouple in order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:

① the welding of the two thermoelectrodes constituting the thermocouple must be firm

② the two thermoelectric poles should be well insulated from each other, and the sampling frequency should be 200Hz to prevent short circuit

③ the connection between the compensation wire and the free end of the thermocouple should be convenient and reliable

④ the protective sleeve should ensure that the Thermoelectrode is fully isolated from harmful media

3. Temperature compensation of thermocouple cold end

because the materials of thermocouples are generally expensive (especially when precious metals are used), and the distance between the temperature measuring point and the instrument is far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of thermocouples to the control room with relatively stable temperature, Connected to the instrument end, Huafeng spandex is not sure of the completion time of each stage of the project. It must be pointed out that the function of the thermocouple compensation wire is only to extend the Thermoelectrode and move the cold end of the thermocouple to the instrument terminal in the control room. It itself cannot eliminate the influence of the localization of finding and cultivating high-tech talents on temperature measurement, and it has no compensation function. Therefore, other correction methods need to be used to compensate for the influence of cold end temperature t0 ≠ 0 ℃ on temperature measurement

when using thermocouple compensation wires, attention must be paid to the matching of models, the wrong polarity should not be connected, and the temperature of the connecting end of the compensation wire and the thermocouple should not exceed 100 ℃