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technical article
How to classify thermocouples
2011-7-21 8:36:57

The basic principle of temperature measurement is the thermoelectric effect. Metal conductors of two different materials are connected end to end to form a closed loop. If the temperature of the contact changes, a thermal current will be generated in the loop. This is the thermoelectric effect.
A thermocouple is made by welding one end of two different metal materials. The welded end is the measurement end and the unwelded end is the output end. The magnitude of its thermoelectric potential is related to the characteristics of the two metal materials and the temperature of the measuring end, and has nothing to do with the thickness and length of the thermocouple.
First, the type of thermocouple
Commonly used thermocouples are mainly the following categories.
1. Platinum-rhodium 10-platinum thermocouple. The division number is S, the long-term maximum use temperature is 1300 ° C, and the short-term maximum use temperature is 1600 ° C. It is suitable for temperature measurement in an oxidizing atmosphere and not suitable for use in a reducing atmosphere, but it can be used for temperature measurement in a vacuum in the short term.
2. Platinum rhodium 30-platinum rhodium 6 thermocouple. The index number is B. The long-term maximum use temperature is 1600 ° C, and the short-term maximum use temperature is 1800 ° C. It is suitable for temperature measurement in an oxidizing atmosphere and not suitable for use in a reducing atmosphere, but it can be used for temperature measurement in a vacuum in the short term.
3. Nichrome-NiSi thermocouple. The division number is K. According to the diameter of the thermocouple, the temperature range is 0 to 1300 ° C. It is suitable for measuring temperature in a neutral atmosphere and not suitable for measuring temperature in a reducing atmosphere.
4. Nichrome-constantan thermocouple. The division number is E, and the temperature range is -200 ~ + 900 ℃, which is suitable for temperature measurement in an oxidizing and weak reducing atmosphere.
Second, use precautions
安装和使用不当,不但会增大测量误差,还可能降低热电偶的使用寿命。 If the thermocouple is not installed and used properly, it will not only increase the measurement error, but also reduce the service life of the thermocouple. Therefore, the thermocouple should be correctly installed and used reasonably according to the measured temperature range and working environment.
1. The appropriate installation site should be selected. Due to the uneven temperature distribution in the heating furnace and the temperature measured by the thermocouple in a local area, a suitable measurement point should be selected to place the thermocouple. Thermocouples can usually be installed in locations where the temperature is more uniform and can represent the temperature of the workpiece. They cannot be installed near the furnace door or too close to the heating source.
2. The location of the thermocouple should be as far away from the strong electromagnetic field as possible to prevent the temperature measuring instrument from introducing additional interference signals.
3. The depth of the thermocouple inserted into the furnace should not be less than 8 to 10 times the outer diameter of the thermocouple protection tube. The hot end should be as close to the workpiece as possible, but it must be ensured that the thermocouple is not damaged when the workpiece is loaded and unloaded.
4. The junction box of the thermocouple should not be close to the wall of the furnace, so as to avoid the high temperature of its cold end. Generally should be about 200mm away from the furnace wall.
5. The thermocouple should be used as vertically as possible to prevent deformation of the protective tube at high temperature. For horizontal installation, the insertion depth should not be greater than 500mm, and the exposed part should be secured by a shelf. After a period of use, it should be rotated 180 °; to prevent the temperature of the thermocouple junction box from becoming too high, a rectangular thermocouple can also be used.
6. The gap between the thermocouple protection tube and the furnace wall should be tightly blocked with refractory material to prevent air convection from affecting the accuracy of temperature measurement. The gap between the compensation wire and the connection hole of the junction box should also be tightly plugged with asbestos wire so that it faces down to prevent dirt from falling into it.
7. When measuring the furnace temperature with a thermocouple, direct flame spraying should be avoided, because the temperature of the flame spraying point is higher than the actual temperature in the furnace and unstable.
8. When measuring at low temperature, in order to reduce the thermal inertia of the thermocouple, a thermocouple with a protective tube opening or without a protective tube can be used.
9. Thermocouples in the service period should always check whether the heating electrode and the protection tube are good. If you find pits, stains, thinning of the local diameter or serious corrosion on the surface of the protection tube, stop using and repair Or replace with a new thermocouple.