1. Types of thermocouples
Commonly used thermocouples can be divided into two categories: Standard thermocouples and non-standard thermocouples. Standard thermocouple refers to a thermocouple whose national standard specifies the relationship between its thermoelectric potential and temperature, the allowable error, and has a unified standard graduation meter. It has a matching display instrument for selection. Since January 1, 1988 in my country, thermocouples and thermal resistances have all been produced in accordance with IEC international standards, and seven standardized thermocouples, S, B, E, K, R, J, and T, are designated as my country’s unified design thermocouples. Non-standardized thermocouples are inferior to standardized thermocouples in the scope of use or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions.
2. The structure of the thermocouple In order to ensure the reliable and stable operation of the thermocouple, its structure requirements are as follows:
(1) The welding of the two thermoelectrodes forming the thermocouple must be firm;
(2) The two hot electrodes should be well insulated from each other 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 be able to ensure that the hot electrode is fully isolated from the harmful medium.
3. Temperature compensation of the cold junction of the thermocouple
Since the materials of thermocouples are generally more expensive (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to connect the cold end of the thermocouple (free End) extend into the control room where the temperature is relatively stable, and connect to the terminal of the instrument. It must be pointed out that the function of the thermocouple compensation wire is only to extend the hot electrode and move the cold end of the thermocouple to the instrument terminal of the control room. By itself, it cannot eliminate the influence of the temperature change of the cold end on the temperature measurement and cannot compensate. Therefore, other correction methods are needed to compensate for the influence of the cold junction temperature t0≠0°C on the temperature measurement. When using the thermocouple compensation wire, you must pay attention to the matching of the model, the polarity can not be wrong, and the temperature of the compensation wire and the thermocouple connection end can not exceed 100 ℃.
