Advantages of thermocouples
1. High measurement accuracy. Because the thermocouple is in direct contact with the object being measured, it is not affected by the intermediate medium.
2. Wide measurement range. Commonly used thermocouples can continuously measure from -50 to +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 easy to use. Thermocouples are usually composed of two different metal wires and are not limited by size or shape. They are also protected by a sheath, making them very convenient to use.
Basic principle of thermocouple temperature measurement
When two conductors or semiconductors, A and B, made of different materials are welded together to form a closed circuit, an electromotive force is generated between them when a temperature difference exists between the two junctions, 1 and 2, thus creating a current in the circuit. This phenomenon is called the thermoelectric effect. Thermocouples utilize this effect to function.
Types and structural formation of thermocouples
1. Types of thermocouples
Commonly used thermocouples can be divided into two main categories: standard thermocouples and non-standard thermocouples. Standard thermocouples are those whose thermoelectric potential-temperature relationship, permissible error, and standardized calibration tables are specified by standards, and they have matching display instruments available for selection. Non-standard thermocouples are less widely used or numerous than standardized thermocouples, and generally do not have standardized calibration tables; they are mainly used for measurements in certain special applications.
Standardized thermocouples: Since January 1, 1988, all thermocouples and resistance thermometers in my country have been manufactured in accordance with IEC international standards, using seven standardized thermocouple types: S, B, E, K, R, J, and T, which are the unified design thermocouples for my country.
2. Thermocouple structure
To ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:
① The welding of the two thermocouple electrodes must be firm.
② The two thermoelectrodes should be well insulated from each other to prevent short circuits;
③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable.
④ The protective sleeve should be able to ensure that the thermoelectric electrode is fully isolated from the harmful medium.
Thermocouple cold junction temperature compensation
Because thermocouples are generally made of expensive materials (especially precious metal thermocouples like S-type thermocouples), and the distance between the temperature measuring point and the instrument is often considerable, compensating wires are typically used to extend the cold junction (free end) of the thermocouple to the more temperature-stable control room and connect it to the instrument terminals to save on thermocouple materials and reduce costs. It must be pointed out that the function of the thermocouple compensating wire is only to extend the thermocouple electrodes, moving the cold junction to the instrument terminals in the control room; it does not eliminate the influence of cold junction temperature changes on temperature measurement and therefore does not provide compensation. Therefore, a thermocouple cold junction temperature compensation correction method is needed to compensate for the influence of cold junction temperature t0 ≠ 0℃ on temperature measurement.
When using thermocouple compensating wires, it is essential to ensure that the models are compatible, the polarity is correct, and the temperature at the connection point between the compensating wire and the thermocouple does not exceed 100℃.
