Problems that are easily overlooked when testing thermocouples

Thermocouples need to be inspected regularly during use so that problems can be discovered and solved in time. However, there are some problems that are often overlooked during inspection. Here are some of them:

1. Thermocouple length

Thermocouples are required to be no less than 750mm in length. The reason for the length is that the thermocouple must have a sufficiently wide temperature gradient zone after leaving the temperature measurement zone. Thermoelectric potential is also generated in this area. To effectively prevent the heat from the hot end from being transferred to the cold end, the basic method is that the cold end of the thermocouple must be far enough away from the hot end.

2. Thermocouple wire bending

Thermocouple wires are thin and soft and are very easy to deform. When the thermocouple wires are folded, twisted, or other plastic deformations occur, causing stress in the thermocouple wires, the thermoelectric properties are changed, thereby affecting the accuracy of the deformation measurement results. Therefore, the thermocouple wires must be straightened before calibration.

3. Thermocouple wire is contaminated

When the thermocouple wire is contaminated or even oxidized, the surface of the thermocouple wire will be dull, dark and black. At this time, the thermoelectric characteristics of the thermocouple are extremely unstable, and the accuracy of the measured data is poor. Therefore, the contaminated electrode should be cleaned to eliminate the contamination layer.

4. The influence of response time

The basic principle of contact temperature measurement is that the temperature measuring element must reach thermal equilibrium with the object being measured. Therefore, it is necessary to maintain a certain time during temperature measurement to achieve thermal equilibrium between the two. The length of the holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure of the sensor and the measurement conditions, which varies greatly. Therefore, in the daily calibration process, it is necessary to select the appropriate heating rate and thermal equilibrium time according to different types of thermocouples.

5. The influence of insulation resistance

When the thermocouple is at high temperature, its insulation resistance decreases sharply with the increase of temperature, so a leakage current will be generated. This current flows into the instrument through the insulation whose insulation resistance has dropped, making the instrument indication unstable or causing measurement errors. Therefore, do not neglect to test the insulation resistance of the thermocouple before installing it in the furnace. Only when the requirements of the verification regulations are met can the temperature tolerance verification be carried out.