What issues should we pay attention to when understanding thermal resistors?

1. For the thermoelectric potential of thermocouples, the following issues should be noted:

1: The thermoelectric potential of a thermocouple is the difference between the temperature functions of the working end and the cold end of the thermocouple, not the function of the temperature difference between the cold end and the working end of the thermocouple;

2: The size of the thermoelectric potential generated by the thermocouple is independent of the length and diameter of the thermocouple when the material of the thermocouple is uniform.

3: When the material composition of the two thermocouple wires of the thermocouple is determined, the size of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple is kept constant, the thermoelectric potential of the thermocouple is only a single-valued function of the working end temperature. Weld two conductors or semiconductors A and B of different materials together to form a closed loop. The two conductors of different components (called thermocouple wires or hot electrodes) are connected at both ends to form a loop. When the temperature of the junction is different, an electromotive force will be generated in the loop. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential.
When measuring temperature, the thermocouple requires that the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through the lead is called the cold end) remain unchanged, so that its thermoelectric potential is proportional to the measured temperature. If the (ambient) temperature of the cold end changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold end to compensate for the impact caused by the change in cold end temperature is called cold end compensation of the thermocouple.

Appendix: Calculation method of thermocouple cold end compensation: From millivolts to temperature: measure the cold end temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert it to temperature. From temperature to millivolts: measure the actual temperature and the cold end temperature, convert them to millivolt values ​​respectively, subtract them to get the millivolt value, and then get the temperature.
Eg. If the indicating instrument has cold end compensation, the actual temperature is 520 degrees; if the indicating instrument does not have cold end compensation, first check the K-type thermocouple graduation table to get: 520 degrees corresponds to a voltage of 21.497mv, 25 degrees corresponds to a voltage of 1mv; 21.497mv+1mv=22.497mv; then check the K-type thermocouple graduation table to get that 22.497mv corresponds to a temperature value of 543.47 degrees, that is, the actual temperature is 543.47 degrees. PS: The graduation number is a standard series used to reflect the temperature change of the temperature sensor within the measurement temperature range corresponding to the sensor voltage or resistance change, that is, the temperature value corresponding to the thermal resistor, thermocouple, resistor, and potential.

Second, the difference between thermocouple and thermal resistor:

First, the nature of the signal. The thermal resistor itself is a resistor. The change of temperature causes the resistor to produce positive or negative resistance changes; while the thermocouple produces changes in induced voltage, which changes with the change of 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 term. S type measures 20~1300 (short term 1600), K type measures 50~1000, short term 1200). XK type 50~600 (800), E type 40~800 (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 fully compensate. 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, negative temperatures can be detected), while thermocouples can detect a temperature range of 0-1000 degrees (or even higher). Therefore, the former is a low-temperature detection, while the latter is a high-temperature detection.

Third, from the material point of view, thermal resistors are a metal material with temperature-sensitive changes, while thermocouples are bimetallic materials, that is, two different metals. Due to temperature changes, a potential difference is generated at both ends of 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 the connection and configuration are correct. In operation. Common ones are short circuit, open circuit, poor contact (can be judged by 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 line on the secondary meter, and the meter indicates the room temperature. Then short-circuit the thermocouple terminal, and the meter indicates the ambient temperature of the thermocouple (if not, the compensation line is faulty). Then use the multimeter's MV gear 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 with a multimeter. During operation, if a short circuit is suspected, just remove a wire end from the resistor end and look at the display instrument. If **** is reached, the thermal resistor short circuit returns to zero, the wire is short-circuited, and normal connection and configuration are guaranteed. If the meter value is low or unstable, the protective tube may have water in it.