thermocouple leakage of main reason and prevention method

With the rapid development of chemical industry, petroleum, metallurgy, machinery, scientific research, military industry and other departments, high-precision and high-sensitivity instruments are widely used. The anti-interference problem of instruments has become an indispensable research topic, especially the interference caused by industrial electric furnace, silicon carbide rod furnace, salt bath furnace, etc., which results in abnormal operation of the instrument, increase in error, instability, only slow down the speed, etc 。 It must be solved, otherwise the product quality can not be guaranteed and the loss will be caused.

Causes of thermocouple leakage

(1) The insulation of refractory brick on electric furnace is poor at high temperature. Inserting thermocouple with metal tube on furnace wall with furnace wire can make the wire contact with thermocouple easily. Firebrick is a good insulating material at room temperature, but the insulation performance will decrease significantly with the increase of temperature. The resistivity of three kinds of refractory brick at different temperature is listed in Table 1.

(2) For the contact between the furnace wire and the thermocouple, the protective tube of platinum rhodium thermocouple is alumina ceramic tube, and the couple wire is put on the porcelain tube for separate insulation, so the insulation performance will be reduced at high temperature. Table 2 shows the resistivity of Al203 at different temperatures. It can be seen from table 2 that under high temperature, the insulation performance of refractory brick is reduced, resulting in leakage, which will leak to the thermocouple, so that there is AC leakage signal on the thermocouple.

Prevention of electric leakage on thermocouple

(1) Keep thermocouple away from firebrick.

(2) The negative pole of the thermocouple is grounded to eliminate the leakage signal on the thermocouple.

(3) A filter is installed at the output of thermocouple.

(4) Ground wire shall be led out from the hot contact of thermocouple.

(5) Heat resistant steel pipe or silicon carbide tube is added on the thermocouple tube and grounded.