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How to choose between thermocouple temperature sensor and RTD temperature sensor?

by:Rika Sensors     2021-11-08
How to choose between thermocouple temperature sensor and RTD temperature sensor?
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. We choose thermal resistance or thermocouple for the same temperature measurement location Woolen cloth? Today we have a comprehensive analysis.

1. The difference in working principle

The 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 equipment that needs temperature measurement. The cold junction is placed outside the device. If the two ends are at different temperatures, a thermoelectric potential will be generated in the thermocouple circuit. Since the thermoelectric potential is a function of the measured temperature, after the measured value of the electromotive force, it can be converted into temperature value.

Thermal resistance is based on the property that the resistance value of the conductor will change with the change of temperature, and the change of resistance is converted into an electrical signal for temperature measurement.

2. Structure difference

The structure of the thermocouple

There are 3 types of thermocouple front 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 the thermal resistance

As shown in the figure below, there are three types of component shapes of the thermal resistance, 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 a 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 not known?

First of all, look at the lead wires of the temperature measuring element. Usually there are only two lead wires for a thermocouple. 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 the resistance value of which is closest to the thermal resistance of the graduation number is the thermal resistance of the 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. At a room temperature of 20℃, the resistance value of Pt10 is 10.779, Pt100 is 107.794, and 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 u200bu200bcan 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 and the configuration is correct. During operation, the common ones are short circuit, open circuit, poor contact (to be judged by a multimeter) and deterioration (identified by the surface color). When checking, 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 to indicate the environment where the thermocouple is located. Temperature (no, the compensation line is faulty), and then use the mv file of the multimeter 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 the end of the resistance 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, and protects The pipe may be flooded. Display maximum, thermal resistance open circuit, display minimum, short circuit. Generally speaking, a thermal resistance is used for temperatures below 300 degrees, and a thermocouple is used for temperatures above 300 degrees. As the temperature changes, the resistance of the thermal resistance will change, and the thermoelectric potential of the thermocouple will change.

Finally, how to choose thermocouple and thermal resistance?

Choose according to the temperature measurement range: generally choose thermocouple above 500℃, and generally choose thermal resistance below 500℃;

choose according to measurement accuracy: choose thermal resistance for higher accuracy requirements, and choose for low accuracy requirements Thermocouple;

Select 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.

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