Thermocouple principle and structure

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 object to be 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.

When using thermocouple compensation wires, you must pay attention to the matching of models, the polarity cannot be connected incorrectly, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100℃.

For example: S-type thermocouple) platinum rhodium 10-platinum thermocouple

Platinum rhodium 10-platinum thermocouple (S-type thermocouple) is a precious metal thermocouple. The wire diameter is specified as 0.5mm, with an allowable deviation of -0.02mm. The nominal chemical composition of the positive electrode (SP) is a platinum-rhodium alloy, which contains 10% rhodium and 90% platinum. The negative electrode (SN) is pure platinum, so it is commonly known as a single platinum-rhodium thermocouple. The long-term maximum operating temperature of this thermocouple is 1300℃, and the short-term maximum operating temperature is 1600℃.

The S-type thermocouple has the advantages of the highest accuracy, the best stability, a wide temperature measurement range, and a long service life in the thermocouple series. It has good physical and chemical properties, good thermoelectric potential stability and good oxidation resistance at high temperatures, and is suitable for oxidizing and inert atmospheres. Because the S-type thermocouple has excellent comprehensive performance and conforms to the international temperature scale, it has long been used as an interpolation instrument for the international temperature scale. Although "ITS-90" stipulates that it will no longer be used as an internal inspection instrument for the international temperature scale in the future, the International Temperature Consultative Committee (CCT) believes that the S-type thermocouple can still be used to approximately achieve the international temperature scale.

1. Basic principle of thermocouple temperature measurement

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

2. Types and structural formation 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 a thermocouple whose national standard stipulates the relationship between its thermoelectric potential and temperature, the allowable error, and has a unified standard scale. It has a matching display instrument for selection. Non-standard thermocouples are inferior to 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 (i.e., graduation numbers), 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 that make up the thermocouple must be firm;

② The two thermocouples should be well insulated from each other to prevent short circuits;

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, a compensation wire is 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 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.