How to "Cleverly Distinguish" Thermal Resistors and Thermocouples

First, the nature of the signal. The thermal resistor itself is a resistor. The change in temperature causes the resistor to produce a positive or negative resistance change; while the thermocouple produces a change in induced voltage, which changes with the change in temperature... Although they are all contact temperature measuring instruments, their temperature measurement ranges are different. Thermocouples are used in high temperature environments, such as platinum rhodium 30---platinum rhodium 6 (type B) with a measurement range of 300 degrees to 1600 degrees, and can measure 1800 degrees in a short period of time. S type measures 20 to 1300 (short-term 1600), K type measures 50 to 1000, short-term 1200). XK type - 50 to 600 (800), E type - 40 to 800 (900). There are also J type, T type, etc.

　　This type of instrument is generally used for higher temperatures above 500 degrees. In the low temperature zone, the output thermoelectric potential is very small. When the potential is small, the anti-interference measures and secondary meters are very demanding, otherwise the measurement will be inaccurate. In addition, in the lower temperature area, the relative error caused by the change of the cold end temperature and the change of the ambient temperature is very prominent and difficult to be fully compensated. At this time, at medium and low temperatures, the temperature measurement range of the thermal resistor is generally -200~~500, and even lower temperatures can be measured (such as carbon resistors can measure low temperatures of about 1K). Now the platinum thermal resistor Pt100 is normally used (there are also Pt50, 100 and 50 to represent the resistance of the thermal resistor at 0 degrees. In the old graduation number, BA1 and BA2 are used to represent it. The resistance of BA1 at 0 degrees is 46 ohms. Copper resistors are also used in industry, with graduation numbers CU50 and CU100, but the temperature measurement range is smaller, between -50~~150. In some special occasions, there are also indium resistors, manganese resistors, etc.).

　　Second, the temperature ranges detected by the two sensors are different. Thermal resistors generally detect a temperature range of 0-150 degrees (of course, they can detect negative temperatures), while thermocouples can detect a temperature range of 0-1000 degrees (or even higher). Therefore, the former is low temperature detection, and the latter is high temperature detection.

　　Third, from the material point of view, thermal resistance is a metal material with temperature-sensitive changes, and thermocouple is a bimetallic material, that is, two different metals. Due to temperature changes, an electric potential difference is generated at both ends of the two different metal wires.

　　Fourth, on-site judgment at work

　　Thermocouples have positive and negative poles, and compensation wires also have positive and negative poles. First, ensure that the connection and configuration are correct. During operation. Common problems include short circuit, open circuit, poor contact (can be judged with a multimeter) and deterioration (identified by 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 the room temperature, then short-circuit the thermocouple terminal, the meter indicates the ambient temperature of the thermocouple (if not, the compensation wire is faulty), and then use the multimeter MV file to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).

　　The short circuit and open circuit of the thermal resistor can be judged by a multimeter. During operation, if a short circuit is suspected, just remove one wire end from the resistance end and look at the display instrument. If it reaches the maximum, the thermal resistor returns to zero when it is short-circuited, and the wire is short-circuited to ensure normal connection and configuration. If the meter value displays low or unstable, the protective tube may have been flooded and display the maximum value, and the thermal resistor is open and display the minimum value.