Reasons for leakage of armored thermocouples and prevention methods for leakage

Armored thermocouples have the advantages of bending, high voltage resistance, fast thermal response time, and durability. Like prefabricated thermocouples, armored thermocouples are commonly used as temperature sensors in conjunction with display instruments, recorders, and electronic regulators. Additionally, armored thermocouples can also serve as temperature sensing elements for prefabricated thermocouples. Armored thermocouples can directly measure the temperature of liquid, vapor, gas media, and solid surfaces in the range of 0 ℃ to 1200 ℃ during various production processes.
Armored thermocouples have a wide range of applications in production and have become commonly used temperature detection components in industry. They have the characteristics of high measurement accuracy, wide measurement range, simple structure, and easy use. We use various channels to understand and analyze products, presenting a wide range of industry knowledge to netizens.
Reasons for leakage of armored thermocouples:
(1) The insulation of refractory bricks on electric furnaces is poor at high temperatures. Inserting thermocouples with metal tubes into the furnace wall with furnace wires can easily make the wires come into contact with the thermocouples. Refractory bricks are good insulation materials at room temperature, but their insulation performance will significantly decrease with increasing temperature. Table 1 lists the electrical resistivity of three different refractory bricks at different temperatures. The lining of a resistance electric furnace is mostly made of alumina refractory bricks, and as shown in Table 1, its resistance value also significantly decreases with increasing temperature.
(2) The contact between the furnace wire and the thermocouple, the platinum rhodium thermocouple protection tube is an alumina ceramic tube, and the thermocouple wire is separated and insulated by threading the ceramic tube. The insulation performance also decreases at high temperatures. Table 2 lists the resistivity of AL203 at different temperatures. From Table 2, it can be seen that the insulation performance of refractory bricks decreases at high temperatures, causing leakage, which then leaks to the thermocouple, resulting in an AC leakage signal on the thermocouple.
Preventive methods for leakage of armored thermocouples
1. Keep the thermocouple out of contact with the refractory bricks.
2. Ground the negative terminal of the thermocouple to eliminate the leakage signal on the thermocouple.
3. Install a filtering device at the output end of the thermocouple.
4. Lead the grounding wire from the hot junction of the thermocouple.
5. Add a heat-resistant steel pipe or silicon carbide pipe to the thermocouple tube and ground it.