Discussing the temperature measurement principle of thermocouple

Thermocouples are commonly used temperature measuring elements in temperature measuring instruments. When the two ends of two conductors of different components are connected to form a loop, when the temperatures of the two junctions are different, a thermal current will be generated in the loop. If there is a temperature difference between the working end and the reference end of the thermocouple, the display instrument will indicate the temperature value corresponding to the thermoelectric potential generated by the thermocouple. The thermoelectric heat of the thermocouple will increase as the temperature of the measuring end increases. Its size is only related to the thermocouple material and the temperature of the two ends, and has nothing to do with the length and diameter of the thermoelectrode. The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same. They are usually composed of main parts such as thermoelectrode, insulating sleeve protection tube and junction box, and are usually used in conjunction with display instruments, recording instruments and electronic regulators.

Its advantages are:

①High measurement accuracy. Since the thermocouple is in direct contact with the object being measured, it is not affected by the intermediate medium.
②Wide measurement range. Commonly used thermocouples can measure continuously from -50" to +1600℃. 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).
③Simple structure and easy to use.

Thermocouples are usually made of two different metal wires and are not restricted by size or opening. They are covered with a protective sleeve and are very convenient to use.

1. Basic principles of thermocouple temperature measurement

Weld two conductors or semiconductors A and B made of different materials together 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, thus forming a current of a certain magnitude in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by utilizing this effect.

2. Types and structures of thermocouples

(1) Types of thermocouples

Commonly used 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-standardized 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 structural requirements for it are as follows:

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 be able to ensure that the thermocouple is fully isolated from harmful media.

3. Temperature compensation of the thermocouple cold end

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 into 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 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 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, it is necessary to pay attention to the matching of the models, the polarity cannot be connected incorrectly, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100℃.

4. Summary

This article discusses the basic application principles of thermocouples, an electronic component, its structure formation, and its classification and application. Through this article, you can learn the basic application knowledge of thermocouples. This article analyzes thermocouples very thoroughly, and those who accumulate knowledge carefully will definitely gain something from it.