Application principle of thermocouple

① High measurement accuracy. Since the thermocouple is in direct contact with the object being measured, it is not affected by the surrounding 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, chromium) and as high as +2800℃ (such as tungsten-rhenium).
③ Simple structure and convenient use. Thermocouples are usually composed of two different metal wires, and are not limited by size and termination. 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 Figure 2-1-1. 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, so a large current is formed in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by using this effect.
2. Types and structural composition of thermocouples
(1) Types of thermocouples
Common thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouples used refer to thermocouples whose national standards define the relationship between thermoelectric potential and temperature, allowable deviation, and have a unified standard scale. They have matching display instruments available for selection. Non-standard thermocouples are not as good as standardized thermocouples in terms of application range or quantity, and some have different scales. They are mainly used for measurements in certain special places.
Standardized thermocouples Since January 1, 1988, my country has produced all thermocouples and thermal resistors according to IEC standards, and the seven standardized thermocouples S, B, E, K, R, J, and T are my country's unified design thermocouples.
(2) Thermocouple structure In order to ensure that the thermocouple works reliably and stably, the following requirements are imposed on its structure:
① The welding of the two thermocouple electrodes must be firm;
② The two thermocouple electrodes should be well insulated from each other to prevent short circuit;
③ The connection of the free end of the thermocouple with the supplementary wire should be convenient and reliable;
④ The protective sleeve should ensure that the thermocouple is adequately isolated from the harmful medium.
3. Temperature compensation of the cold end of the thermocouple
Since the materials of thermocouples are generally relatively precious (especially when precious metals are used), and the distance between the temperature measuring point and the instrument is very far, in order to save thermocouple materials and reduce costs, supplementary wires are usually used to extend the cold end (free end) of the thermocouple to a control room with a relatively stable temperature 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 hot electrode, 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 it does not play a compensatory role. Therefore, other modification measures are required to compensate for the influence of the cold end temperature t0≠0℃ on the temperature measurement.
When using the thermocouple compensation wire, it is necessary to pay attention to the matching of the model, the polarity cannot be connected incorrectly, and the temperature of the connection between the compensation wire and the thermocouple cannot exceed 100℃