的应用原理 Application principle of thermal resistance
Thermal resistance is the most commonly used type of temperature detector in low and medium temperature areas. Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.
1. Thermal resistance temperature measurement principle and material
Thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with temperature. Most of the thermal resistances are made of pure metal materials, and platinum and copper are currently the most widely used. In addition, materials such as copper, nickel, manganese and rhodium have been used to make thermal resistances.
2. 的结构（1）精通型热电阻 Structure of thermal resistance (1) Proficient thermal resistance
From the temperature measurement principle of the thermal resistance, it can be known that the change of the measured temperature is directly measured by the change of the resistance value of the thermal resistance. Therefore, changes in the resistance of various wires such as the lead wire of the thermal resistance body will affect the temperature measurement. In order to eliminate the influence of the lead resistance, a three-wire system or a four-wire system is generally used.
1. 测温基本原理 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 them, and a large current is formed in the circuit. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Type and structure of thermocouple
(1) 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 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 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.
(2) The structure of the thermocouple In order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:
① The welding of the two hot electrodes that make up the thermocouple must be firm;
② The two hot electrodes should be well insulated from each other to prevent short circuit;
③ The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should be able to ensure that the hot electrode is fully 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 precious metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to cool the thermocouple's cold end End) extends into the control room where the temperature is relatively stable and is connected to the instrument terminals. 补偿导线的作用只起延伸热电极，使热电偶的冷端移动到控制室的仪表端子上，它本身并不能消除冷端温度变化对测温的影响，不起补偿作用。 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.