Industrial Furnace Thermocouple Working Principle And Installation Precautions

Thermocouple: is the most widely used contact temperature sensor, which consists of two uniform metal wires of different compositions. One end of them is welded together, which is called the measuring end (hot end); the other end is connected to the electric furnace of the measuring instrument, which is called the reference end (cold end). The measuring end produces different thermoelectric potentials with temperature changes.

In addition to considering the temperature range, the selection of thermocouples should pay attention to its adaptability to the heat treatment atmosphere. For example, in a reducing atmosphere, the J type is superior to the K type, and in an oxidizing atmosphere, the K type is superior to the J type, and the K type is superior to the sulfur type. Contamination is very sensitive, and in an atmosphere with low oxygen content, the chromium-containing thermocouple wire will oxidize preferentially, producing green corrosion and reducing the output signal.

Installation Precautions:
According to the temperature measurement range and working environment, it is very important to install and use the thermocouple properly.

The thermocouple should choose a suitable installation location. Typically, thermocouples are installed where the temperature is more uniform and representative of the co-build temperature, rather than beside the furnace door or too close to the heating source.

The installation position of the thermocouple should avoid strong magnetic field and electric field as much as possible, such as should not be close to the electrode of the salt bath furnace, so as to avoid introducing additional interference signal to the temperature measuring instrument.
The depth of the thermocouple inserted into the furnace is generally not less than 8 to 10 times the outer diameter of the thermocouple protection tube. The hot end should be as close to the heated workpiece as possible, but it must be ensured that the thermocouple will not be damaged when loading and unloading the workpiece.

The junction box of the thermocouple should not lean against the furnace wall to avoid the temperature of the cold end being too high. Generally, the junction box should be about 200mm away from the furnace wall.

Thermocouples are used as vertical as possible to prevent deformation of the protective tube at high temperatures. If it needs to be installed horizontally, the insertion depth should not be greater than 500mm, the exposed part should be supported by a shelf, and after a period of use, it should be rotated 180℃. When measuring the temperature of the salt bath furnace, in order to prevent the temperature of the thermocouple junction box from being over High, right-angle thermocouples are often used.

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