Introduction to Thermocouple Related Knowledge
Thermocouple is a commonly used temperature measuring element in the industry. It directly measures the temperature and converts the temperature signal into a thermo-electromotive force signal. It is converted into the temperature of the measured medium through an electrical instrument (secondary instrument).
1) Basic principle of thermocouple temperature measurement
When the temperature of the two junctions is different, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential. Thermocouples use this principle for temperature measurement. Among them, one end that is directly used as the temperature of the temperature medium is called the measurement end, and the other end is called the cold end (compensated end). The thermoelectric potential generated by the thermocouple will be indicated. It is actually an energy converter that converts thermal energy into electrical energy and measures the temperature with the generated thermoelectric potential.
2) The advantages of thermocouples
1. Wide measuring range, commonly used thermocouples can be measured continuously from -50 to + 1600 ° C. Some special thermocouples can measure as low as -269 ° C and as high as + 2800 ° C.
2. High measurement accuracy, because the thermocouple is in direct contact with the measured object, and is not affected by the intermediate medium.
3. 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.
3) Types of thermocouples
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, the allowable error, and has a unified standard index table. It has a matching display instrument for selection. 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 thermocouples in China 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, and T have been designated as China's unified design type Thermocouple.
4) Temperature compensation of the cold junction of the thermocouple
Because the materials of thermocouples are generally relatively expensive (especially when precious 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 cool the cold end of the thermocouple (free End) 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 only extends 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 influence of the cold junction temperature change on the temperature measurement, and it does not 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. ^ ^
5) Structural requirements of thermocouple:
1. The two hot electrodes should be well insulated from each other to prevent short circuit;
2. The welding of the two hot electrodes that make up the thermocouple must be firm;
3. The protective sleeve should be able to ensure that the hot electrode is fully isolated from the harmful medium.
4. The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;