How to check the quality of thermal resistor

　　Temperature detection is a circuit solution often used in electronic engineering, and the most common temperature detection circuit is the thermistor solution. So how do we check the quality of the thermistor before using it?

　　What is thermal resistance?

　　Thermal resistors are the most commonly used temperature detectors in the medium and low temperature range. Thermal resistor temperature measurement is based on the property that the resistance value of a metal conductor increases with the increase of temperature to measure temperature. Its main features are high measurement accuracy and stable performance. Thermal resistors are mostly made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, materials such as nickel, manganese and rhodium are now being used to manufacture thermal resistors. There are many types of temperature sensing materials commonly used in metal thermal resistors, and the most commonly used is platinum wire. In addition to platinum wire, metal thermal resistor materials used for industrial measurement include copper, nickel, iron, iron-nickel, etc. Among them, platinum thermal resistors have the highest measurement accuracy. They are not only widely used in industrial temperature measurement, but also made into standard reference instruments.

　　How to check the quality of thermal resistors

　　Of course, we also have a benchmark requirement for temperature measurement accuracy here, that is to say, before checking the quality of the thermistor, we need to judge based on demand how much accuracy my temperature measurement circuit requires. Here we need to judge the quality of the thermistor.

　　To check the quality of the thermistor, we must first check the resistance and temperature correspondence table provided by the manufacturer. According to the comparison table, we measure the resistance of the thermistor to see if it is within the normal error range. If we use a multimeter to detect its resistance value, and it corresponds to the resistance value at the temperature of the environment in which the thermistor is located during measurement, it can be judged that the thermistor is good.

　　If according to the corresponding table, there is a big difference between the resistance value and the temperature it should reflect, it means that the quality is not good enough, or if the thermal resistor is unstable, it means that the quality is poor.

　　If it is found that the thermistor cannot be used, it means that the problem with the components will cause the temperature measurement circuit to be invalid. At this time, it is necessary to check the quality of the thermistor.

　　How to check the quality of thermal resistor

　　The general simple verification method is to place the thermistor in hot water, or heat it with a lighter, but if the resistance value does not change and the temperature does not change, then it is definitely broken.

　　Common thermal resistors include copper resistors, platinum resistors, and thermistors. For copper resistors and platinum resistors, if the resistance value is measured, infinite resistance means that the wire is broken or the resistor is broken, and zero resistance between the three wires means a short circuit.

　　If you measure the two ends of the resistor and find an open circuit, it is bad! If there is no open circuit, but the resistance value is very large, it is also bad!

　　1. Use the resistance range of the multimeter to measure the resistance signal.

　　2. Assuming PT100: 100 ohms at 0 degrees Celsius, 107.7935 ohms at 20 degrees Celsius. The differential resistance is about 0.39 ohms/degree Celsius. Calculate the resistance value based on your on-site ambient temperature, and then measure it with the resistance range of a multimeter. Then use a lighter to heat the thermal resistor to see if its resistance value rises.

　　3. The accuracy and other parameters of thermal resistors need to be tested by special instruments, and a multimeter can only detect whether it is good or bad. In other words, as a user, as long as it is determined that the thermal resistor is not short-circuited or open-circuited, and the resistance value changes with temperature, the thermal resistor can be considered to be good.

　　How to check the quality of pt100 thermal resistor

　　The main signs of a PT100 thermal resistor are broken wire, short circuit, low insulation and degraded performance. Broken wire and short circuit are relatively easy to check and can be solved by testing with a multimeter. The insulation quality can be checked remotely through a megohmmeter.

　　The so-called good or bad is mainly judged by short circuit and open circuit, that is, whether a thermistor lead wire is short-circuited or broken due to mechanical vibration, temperature baking, or wiring reasons during use. This can be detected with an ordinary multimeter.

　　Furthermore, it is necessary to check whether the correspondence between the resistance value and the temperature value of the thermal resistor is linear and accurate. This can be divided into rough judgment and accurate calibration. Rough judgment is mainly used for on-site fault judgment and troubleshooting. The resistance value is measured by a multimeter and then compared with the estimated value of the measured temperature to see if there is a large gap. After the thermal resistor is pulled out and cooled naturally, the resistance value is measured to see if it corresponds to the ambient temperature. If accurate judgment is required, it is necessary to take it to the calibration room for 0 degrees and 100 degrees for calibration to see if the error meets the accuracy requirements.

　　How to check the quality of PT100 thermal resistor

　　The characteristic of PT100 thermal resistance sensor is that the resistance is 100Ω at 0℃, the resistance value decreases at low temperature and increases at high temperature. The good or bad can be judged by multimeter.

　　You can first disconnect the PT100 wiring in the loop, and then use the resistance range (200Ω) of the multimeter to measure its resistance value. However, the measurement result can only judge whether the PT100 is good or bad, and cannot be used as a basis for verifying its accuracy.

　　The resistance of PT100 is 100Ω at 0℃. As the temperature rises, the resistance gradually increases. The specific corresponding relationship can be found in its graduation table.

　　1. The temperature range of PT100 thermal resistor is T = 0 ~ 100 degrees, its output is I = 4 ~ 20ma, and the relationship between I and T is: I = 4 + (4/25) * T

　　For example, when T=0 degrees, I=4ma, when I=100 degrees, I=20ma. When T=25 degrees, I=8ma

　　2. Remove the PT100 thermal resistor and place it in a water container. Connect a 25 (or 50) mA range ammeter in series to its external output point. Use a thermometer to measure the water temperature. Heat the water to change its temperature. If the changes in the indicated value of the thermometer and the indicated value of the current scale conform to the relationship of formula (1), the thermal resistor is good. If the indicated value of the current meter remains unchanged or the change in the indicated value of the current scale and the indicated value of the thermometer do not conform to the relationship of formula (1), the thermal resistor is bad.

　　Professional operators' suggestions on measuring the quality of thermocouples and thermal resistors

　　Use a multimeter to measure the two output terminals (sometimes multiple terminals) to see if they are connected (although there is a certain resistance value). If the circuit is open, it must be broken. This is the first step in actually judging whether it is good or bad. The resistance value of thermal resistors is certain, such as PT100 at room temperature is about 110 ohms, and CU50 at room temperature is around 55 ohms. Thermocouples output voltage values, which are generally several to tens of millivolts at a certain temperature. The voltage range of the multimeter can be used to measure. The output voltage of thermocouples is only a few mV, depending on the accuracy of the multimeter. A digital multimeter can be used for rough measurement to determine whether it is good or bad. The output of thermocouples is at the millivolt level. It is unlikely to detect its output with a multimeter, but its continuity can be measured. In most cases, as long as the thermocouple part (at the intersection of the two wires) is connected, not oxidized, and not broken, it is generally fine. So it can also be pulled out of the sheath for appearance inspection. To really check, you need to use a standard thermocouple to compare and measure the millivolt value it outputs. This requires measurement personnel to do it.

　　Some multimeters have a special function for measuring thermocouples. If not, observe the surface of the thermocouple and use the ohm function to check if it is broken. If both are good, you can put the thermocouple in two places with a large temperature difference and use the MV function of the multimeter to test it. When it is placed in two different places, the mV is obviously different (generally difficult to measure accurately), so it can be considered that the thermocouple is fine. As long as the thermal resistor is placed in two places with a significant temperature difference, measure its resistance value, and compare it with the scale to know whether it is good or bad. If you want high-precision confirmation, you need special equipment or send it to the Quality Supervision Bureau and other departments for testing.