Thermocouple types and structures

Thermocouples are 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 object being measured, it is not affected by the intermediate medium.
② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600℃, and some special thermocouples can measure as low as -269℃ (such as gold, iron, nickel, and chromium) and as high as +2800℃ (such as tungsten-rhenium).
③ Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and are not limited by size and opening. There is a protective sleeve outside, which is very convenient to use.
1. Basic principle of thermocouple temperature measurement
Weld two conductors or semiconductors A and B of different materials to form a closed loop, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between the two, and thus a current of a certain size is formed in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by using this effect.

2. Types and structures of thermocouples
(1) Types of thermocouples
Common thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple refers to the thermocouple whose thermoelectric potential and temperature relationship, allowable error, and unified standard scale are specified by the national standard. It has a matching display instrument for selection. Non-standard thermocouples are not as good as standardized thermocouples in terms of use range or order of magnitude, and generally do not have a unified scale. They are mainly used for measurements in certain special occasions. Standardized thermocouples Since January 1, 1988, all thermocouples and thermal resistors in my country have been produced in accordance with IEC international standards, and seven standardized thermocouples, S, B, E, K, R, J, and T, have been designated as my country's unified design thermocouples.
(2) Thermocouple structure In order to ensure that the thermocouple works reliably and stably, the following structural requirements are imposed on it:
① The welding of the two thermocouples must be firm;
② The two thermocouples 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 ensure that the thermocouple is fully isolated from harmful media.
3. Temperature compensation of the cold end of the thermocouple
Since the materials of thermocouples are generally expensive (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to the control room where the temperature is relatively stable and connect it to the instrument terminal. It must be pointed out that the role of the thermocouple compensation wire is only to extend the thermocouple so that the cold end of the thermocouple moves to the instrument terminal in the control room. It itself cannot eliminate the influence of the cold end temperature change on the temperature measurement and does not play a compensation role. Therefore, other correction methods are required to compensate for the influence of the cold end temperature t0≠0℃ on the temperature measurement. When using thermocouple compensation wires, you must pay attention to matching the model, the polarity cannot be connected incorrectly, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100℃.