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technical article
The role and working principle of thermocouples
2011-12-24 11:07:06

的应用 Application of multi-point thermocouple

Applicable to the production site where the temperature gradient is not significant, you must measure multiple locations or multiple measurements at the same time. Widely used in large fertilizer synthesis towers, storage tanks and other devices

选型须知: Multi-point thermocouple selection instructions:

1.Selection
2. Index number
3.Accuracy level
4.Thermocouple points
5. Installation and fixing form
6.Material of protection tube
7.Length or insertion depth

Thermocouples are one of the most commonly used temperature sensing elements in the industry. Its advantages are:

High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.

Wide measurement range. Commonly used thermocouples can be continuously measured from -50 to + 1600 ° C. Some special thermocouples can measure as low as -269 ° C (such as gold-iron-nickel-chromium) and as high as + 2800 ° C (such as tungsten-rhenium).

Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and they are not limited by size and opening. They have protective sleeves and are very convenient to use.

First, the basic principle of thermocouple temperature measurement

Solder two conductors or semiconductors A and B of different materials to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, and a large current is formed in the circuit.

This phenomenon is called the pyroelectric effect. Thermocouples use this effect to work.

的种类及结构形成 Second, the type and structure of the thermocouple

1.The type of thermocouple

Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose national standard specifies the relationship between thermoelectric potential and temperature, the allowable error, and has a unified standard index table.

It has optional display instruments to choose from. Non-standardized thermocouples are inferior to standardized thermocouples in the scope or magnitude of use. Generally, there is no unified indexing table, which is mainly used for measurement in some special occasions. Standardized thermocouple

Since January 1, 1988, all thermocouples and thermal resistors have been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J, T (that is, graduation numbers) have been designated as China. Unified design thermocouple.



2. Structural form of thermocouple In order to ensure the reliable and stable operation of thermocouple, its structural requirements are as follows:

The welding of the two hot electrodes that make up the thermocouple must be firm;

The two hot electrodes should be well insulated from each other to prevent short circuit;

The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;

The protective sleeve should be able to ensure that the hot electrode is sufficiently isolated from the harmful medium.


3. Temperature compensation for the cold junction of a thermocouple

Because the materials of thermocouples are generally relatively expensive (especially when noble metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save the thermocouple materials and reduce costs, compensation wires are usually used to connect the cold end of the thermocouple (free end)

It extends into the control room where the temperature is relatively stable and is connected to the instrument terminals. It must be pointed out that the function of the thermocouple compensation wire is only to extend the hot electrode, so that the cold end of the thermocouple is moved to the instrument terminal in the control room. It cannot eliminate the cold end temperature by itself.

The effect of changes on temperature measurement cannot compensate. Therefore, other correction methods are needed to compensate for the effect on the temperature measurement when the cold junction temperature t0 ≠ 0 ° C.

When using the thermocouple compensation wire, you must pay attention to the matching of the model, the polarity must not be wrong, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100 ° C.



Platinum-rhodium 10-platinum thermocouple (S-type thermocouple) is a precious metal thermocouple. The diameter of the wire is specified as 0.5mm, and the allowable deviation is -0.02mm. The nominal chemical composition of the positive electrode (SP) is platinum-rhodium alloy, which contains 10% rhodium and 90% platinum.

The negative electrode (SN) is pure platinum, so it is commonly called single platinum rhodium thermocouple. The long-term maximum use temperature of this thermocouple is 1300 ° C, and the short-term maximum use temperature is 1600 ° C.

The S-type thermocouple has the highest accuracy, the best stability, the wide temperature range and long service life among the thermocouple series. It has good physical and chemical properties, good thermoelectric 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, the S-type thermocouple has long been used as an interpolated instrument of the international temperature scale. Although "ITS-90" stipulates that it will no longer be used as an internal inspection instrument of the international temperature scale, But the International Temperature Advisory Committee (CCT)

It is believed that S-type thermocouples can still be used to approximate the international temperature scale.

The shortcomings of S-type thermocouples are thermoelectric potential, small thermoelectric potential, low sensitivity reading, low mechanical strength at high temperature, very sensitive to pollution, expensive precious metal materials, and therefore a large one-time investment.