Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures the temperature and converts the temperature signal into a thermoelectromotive force signal, which is converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The basic principle of thermocouple temperature measurement is that two conductors of different components form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there will be an electromotive force-thermoelectromotive force between the two ends. This is the so-called Seebeck effect. Two homogeneous conductors with different compositions are thermoelectrodes, the end with a higher temperature is the working end, the end with a lower temperature is the free end, and the free end is usually at a certain constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple index table is made; the index table is obtained when the free end temperature is at 0℃, and different thermocouples have different index tables. The differences between thermocouple and thermal resistance are: 1. The nature of the signal. The thermal resistance itself is a resistance. The change in temperature causes the resistance to produce a positive or negative resistance change; while the thermocouple produces a change in the induced voltage. Change with temperature. .Although they are all contact temperature measuring instruments, their temperature measurement ranges are different. Thermocouples are used in higher temperature environments, such as platinum rhodium 30---platinum rhodium 6 (B type), and the measurement range is 300 degrees ~ ~ 1600 Degree, short-term measurable 1800 degrees. S type measures a 20~~1300 (short term 1600), K type measures a 50~~1000, short term 1200), XK type a 50~~600 (800), E type a 40~~800 (900). There are also J-type, T-type and so on. This type of instrument is generally used for higher temperatures above 500 degrees, and the output thermoelectric potential is very high in the low temperature area. When the potential is small, the anti-interference measures and the secondary meter and the requirements are very high, otherwise the measurement is inaccurate. In the temperature region, the relative error caused by the change of the cold junction temperature and the change of the ambient temperature is very prominent, and it is not easy to be fully compensated. At this time, in the middle and low temperature, the temperature measurement range of the thermal resistance is generally from 200 to 500, and even lower temperatures can be measured (for example, a low temperature of about 1K can be measured with a carbon resistance). The platinum thermal resistance Pt100 is normally used now. (There are also Pt50, 100 and 50 representing the resistance of the thermal resistance at 0 degrees. In the old graduation number, it is represented by BA1, BA2. The resistance of BA1 at 0 degrees is 46 ohms. Copper resistors are also used in industry. The graduation numbers are CU50 and CU100, but the temperature measurement range is small, ranging from one to 50 to 150. In some special occasions, there are indium resistors, manganese resistors, etc.). 2. The temperature ranges detected by the two sensors are different. The thermal resistance generally detects the temperature range of 0-150 degrees (of course it can detect negative temperatures), and the thermocouple can detect the temperature range of 0-1000 degrees (or even higher). Therefore, the former is Low temperature detection, the latter is high temperature detection. 3. From the point of view of material, thermal resistance is a metal material with temperature-sensitive changes, and thermocouples are bimetallic materials, which are two different metals. Due to temperature changes, the two different metal wires A potential difference is generated at the end. 4. On-site judgment during work, thermocouples have positive and negative poles, and compensation wires also have positive and negative points. First of all, ensure that the connection is made and the configuration is correct. During operation. The common ones are short circuit, open circuit, poor contact (can be judged by a multimeter) and deterioration (identified according to the surface color). When checking, separate the thermocouple from the secondary meter, use a tool to short-circuit the compensation wire on the secondary meter, the meter indicates room temperature, and then short-circuit the thermocouple terminal, the meter indicates the ambient temperature where the thermocouple is located (not that the compensation wire is faulty) ), and then use the mv file of a multimeter to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).
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