How to choose thermal resistance or thermocouple for the same temperature measurement location?
In daily work, we often encounter the use of temperature sensors. Although thermocouples and thermal resistances are used as temperature sensing elements, their principles and functions are different. Should we choose thermal resistance or thermocouple for the same temperature measurement location? Today we have a comprehensive analysis.
1. Difference in working principle
A thermocouple is formed by welding or twisting two different conductors or semiconductor materials. It is divided into a hot end and a free end. The hot end is inserted into the device that needs temperature measurement, and the cold end is placed outside the device. If the temperature is different, the thermoelectric power will be generated in the thermocouple circuit. Since the thermoelectric power is a function of the measured temperature, the measured value of the electromotive force can be converted into a temperature value.
Thermal resistance is based on the property that the resistance value of a conductor changes with temperature, and the change in resistance is converted into an electrical signal to perform temperature measurement.
2. Structure difference
The structure of thermocouple
There are 3 types of thermocouple tip joint shapes, as shown in the figure below. It can be joined by gas welding, butt welding, resistance welding, arc welding, silver welding and other methods according to the type of thermocouple, wire diameter, and operating temperature.
In industrial applications, in order to facilitate installation and prolong the service life of thermocouples, an external sleeve is usually used. Casing is generally divided into protective tube type and armored type.
The structure of thermal resistance
There are three types of thermal resistance element shapes, and the ceramic package type is currently dominant. Ceramic package type is used for thermal resistance with protective tube and armored thermal resistance. The diameter of the bare platinum wire of the ceramic and glass package type is about tens of microns, and the diameter of the mica plate type is about 0.05mm. The lead wire uses platinum alloy wire that is much thicker than the component wire.
In industrial use, the appearance of thermocouple and thermal resistance protection sleeve is almost the same. How to identify it when there is no nameplate and the signal is unknown?
The first is to look at the lead wires of the temperature measuring element. Usually a thermocouple has only two lead wires. If there are three lead wires, it is a thermal resistance. But for the four lead wires, the resistance value needs to be measured to determine whether it is a dual thermocouple or a four-wire thermal resistance. If it is infinite, it is a double thermocouple, and a pair of lead wires with almost zero resistance is a thermocouple. If the resistance of the two pairs of lead wires is between 10 and 110, it is a single four-wire thermal resistance, and its resistance value is closest to the thermal resistance of which graduation number, then it is the thermal resistance of that graduation number .
If there are only two lead wires, you can use a digital multimeter to measure the resistance value to judge. Because the resistance value of the thermocouple is very small, the thermal resistance is almost zero; if the resistance value is very small during the measurement, it may be a thermocouple.
At room temperature, the minimum resistance value of the thermal resistance will also be greater than 10. Commonly used thermal resistances are Pt10, Pt100 platinum thermal resistance, Cu50, Cu100 copper thermal resistance with four graduation numbers, the resistance value of Pt10 is 10 at room temperature of 20℃. 779, Pt100 is 107. 794, Cu50 is 54. 285, Cu100 is 108. 571. The resistance value is greater when the room temperature is greater than 20°C, and most of the resistance values can be judged by comparing the two resistance values. If it is a thermal resistance, you can also know what the graduation number of the thermal resistance is.
How to judge in the work site?
Thermocouple: The thermocouple has positive and negative poles, and the compensation wire also has positive and negative points. First of all ensure the connection, the configuration is correct. In 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). During inspection, the thermocouple should be separated from the secondary meter. The method I have judged in practice is for your reference: short-circuit the compensation line on the secondary meter with a tool, the meter indicates room temperature (if not, the meter is broken), and then short-circuit the thermocouple terminal, indicating the environment where the thermocouple is located Temperature (no, the compensation line is faulty), and then use the multimeter mv file to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).
Thermal resistance: It is nothing more than short circuit and open circuit, which can be judged with a multimeter. During operation, if you suspect a short circuit, just remove a wire from the resistance end and look at the display meter. If it reaches the maximum, the thermal resistance is short-circuited, zeroed, and the wire is short-circuited. When the normal connection and configuration are ensured, the meter value shows low or unstable. The pipe may be flooded. Display maximum, thermal resistance open circuit, display minimum, short circuit. Generally speaking, thermal resistance is used for temperature below 300 degrees, and thermocouple is used for temperature above 300 degrees. As the temperature changes, the resistance of the thermal resistor will change, and the thermoelectric potential of the thermocouple will change.
In conclusion, how to choose thermocouple and thermal resistance?
Choose according to the temperature measurement range: generally choose thermocouple above 500℃, generally choose thermal resistance below 500℃;
Choose according to measurement accuracy: select thermal resistance for higher accuracy requirements, and select thermocouples for low accuracy requirements;
Choose according to the measurement range: the temperature measured by the thermocouple generally refers to the "point" temperature, and the temperature measured by the thermal resistance generally refers to the average temperature in the space.