Using the thermal effect of different metals, the potential difference is generated. The temperature range is very wide, and it is generally used to measure the temperature of several hundred degrees.

Thermocouples are one of the most commonly used temperature detection devices in the industry. Its advantages are:

1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.

2 wide measurement range. Common thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).

3 simple structure, easy to use. Thermocouples are usually composed of two different types of wire, and are not limited by the size and the beginning. They have a protective sleeve and are very convenient to use.

(1) Basic principle of thermocouple temperature measurement
Conductors of two different materials, such as being connected to each other at a certain point, heat the connection point, and a potential difference occurs at the point where they are not heated. The value of this potential difference is related to the temperature of the measurement point of the non-heating part and is related to the material of these two conductors. This phenomenon can occur over a wide temperature range. If the potential difference is accurately measured and the ambient temperature of the unheated part is measured, the temperature of the heating point can be accurately known. Because it must have two different material conductors, it is called a "thermocouple." Thermocouples made of different materials are used in different temperature ranges and their sensitivity varies. The sensitivity of a thermocouple is the amount of change in the output potential difference when the temperature of the heating point changes by 1°C. For most metal-supported thermocouples, this value is approximately between 5 and 40 mV/°C.

Thermocouple sensors have their own advantages and disadvantages. They have low sensitivity, are easily affected by environmental interference signals, and are susceptible to preamplifier temperature drift. Therefore, they are not suitable for measuring small temperature changes. Since the sensitivity of the thermocouple is independent of the thickness of the material, a very thin material can be used to make the temperature sensor. Also due to the excellent ductility of the metal materials used to make the thermocouples, this subtle temperature measurement element has a very high response speed and can measure rapidly changing processes.
Thermocouple
(2) Types of thermocouples
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to thermocouples whose national standards stipulate the relationship between thermoelectric potential and temperature, allowable errors, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there are generally no uniform indexing tables, which are mainly used for measurement in certain special occasions. Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples. Measurement range and advantages and disadvantages of various thermocouples:

S-type thermocouple: platinum-rhenium 10-platinum thermocouple, temperature range 0-1600°C, old index number LB-3. Advantages: heat resistance, stability, good reproducibility and superior accuracy; good oxidation resistance, rot resistance; can be used as a standard. Disadvantages: small value of the thermo-electromotive force; relatively fragile gas environment (especially hydrogen, metal vapor); compensating wire error; expensive.

R-type thermocouples: platinum-rhodium 13-platinum thermocouples, temperature range 0-1600°C. Advantages: heat resistance, stability, good reproducibility and superior accuracy; good oxidation resistance, rot resistance; can be used as a standard. Disadvantages: small value of the thermo-electromotive force; relatively fragile gas environment (especially hydrogen, metal vapor); compensating wire error; expensive.

Type B thermocouples: platinum rhodium 30-platinum rhodium 6 thermocouples, temperature range 600 to 1800°C, old index number LL-2, free ends in 0 to 50°C without compensation wires. Advantages: Suitable for temperatures from 1000°C to 1800°C; Very low thermo-electromotive force at room temperature, no compensation of wires; Excellent resistance to oxidation, rot and turbidity; Excellent heat resistance and mechanical strength compared to R-type. Disadvantages: In the low-temperature region, the thermal EMF is extremely small, and the measured temperature below 600°C is inaccurate; the value of the thermoelectric EMF is small; the linearity of the EMF is not good; the price is high.

K-type thermocouples: Nichrome-nickel-silicon thermocouples, nickel-chromium-nickel-aluminum thermocouples, temperature range -200 to 1300°C. Advantages: Good linearity of the thermo-electromotive force; good resistance to oxidation below 1000°C; good stability in metallic thermocouples. Disadvantages: It is not applicable to the restitution gas environment, especially carbon monoxide, sulfur dioxide, hydrogen sulfide and other gases; the thermo-electromotive force and precious metal thermocouples change greatly compared with the time; affected by the short range order will produce errors.

N-type thermocouple: nickel-chromium silicon - nickel-silicon thermocouple, the temperature range of -270 ~ 1300 °C. Advantages: The linearity of the thermo-electromotive force is good; the oxidation resistance below 1200°C is good; the improved type of the K-type is less affected by the GreenRot and the heat-resistant temperature is higher than that of the K-type. Disadvantages: It is not applicable to the restitution gas environment; the thermo-electromotive force changes significantly with the precious metal thermocouple over time.

E-type thermocouple: nickel-chromium-silicon-constantan thermocouple, temperature range -270 ~ 1000 °C. Advantages: The best sensitivity among existing thermocouples; Good heat resistance compared to J thermocouples; Non-magnetic on both feet; Suitable for oxidizing gas environment; Disadvantages: not applicable to restitution gas environment; with a little history.

Type J thermocouple: iron-constantan thermocouple, temperature range -210 ~ 1200 °C. Advantages: Can be used for recharging gas environment; thermo-electromotive force is 20% larger than K thermocouple; the price is cheaper and it is suitable for medium-temperature area. Disadvantages: The (+) foot is rusted; poor reproducibility.

T-type thermocouple: copper-constantan thermocouple, temperature range -270 ~ 400 °C. Advantages: Good linearity of the thermo-electromotive force; good characteristics at low temperatures; good reproducibility, high accuracy; can be used in restitution gas environments. Disadvantages: The use of low temperature limit; (+) copper oxidation of the foot; heat conduction error.

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