是工业上最常用的温度检测元件之一。 Thermocouples are one of the most commonly used temperature sensing elements in the industry. Its advantages are:
1. High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
2. Wide measurement range. Common thermocouples can be continuously measured from -50 to + 1600 ° C, and some special thermocouples can be measured down to -269 ° C (such as gold iron nickel
Chromium), up to + 2800 ° C (such as tungsten-rhenium).
3. Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and are not restricted by size and opening, and are protected outside
Sleeve is 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 two attachment points 1 and 2 of conductors A and B
When there is a temperature difference, an electromotive force is generated between the two, so a large current is formed in the circuit. This phenomenon is called the thermoelectric effect. Thermocouples use this effect to work.
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 indexing table. It has a display instrument supporting it.
Available. 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 S, B, E, K, R, J, T seven standardized thermocouples have been designated as China's unified design thermocouples.
的结构形式为了保证热电偶可靠、稳定地工作，对它的结构要求如下： 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 ensure that the hot electrode is fully isolated from the harmful medium
Third, the temperature compensation of the cold end of the thermocouple
Because the materials of thermocouples are generally relatively expensive (especially when noble metals are used), and the distance from the temperature measurement point to 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) extended to temperature comparison
Connected to the instrument terminal in a stable control room. 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 effect of the cold junction temperature change on the temperature measurement.
Compensation effect. Therefore, other correction methods are needed to compensate for the effect on the temperature measurement when the cold junction temperature t0 ≠ 0 ° C.
补偿导线时必须注意型号相配，极性不能接错，补偿导线与热电偶连接端的温度不能超过100℃。 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.