When measuring temperature with a thermocouple, there is a complex heat exchange process. Due to multiple temperature transfers, the temperature at the measuring end is not completely consistent with the temperature of the measured medium. Therefore, in temperature measurement, thermocouples cause errors in the surrounding environment's heat dissipation (radiation heat dissipation and thermal conduction heat dissipation). In this article, we will analyze and introduce the correct reduction of measurement errors when using thermocouples for temperature measurement.

When using temperature measurement, the temperature indicated by the measuring instrument is essentially the temperature at the measuring end of the thermocouple. It is generally believed that this is the temperature of the measured medium. However, in actual side temperature, there is a complex heat exchange process. For example, when the temperature of the measured medium is higher than the ambient temperature, the medium transfers heat to the thermocouple, which in turn transfers heat to the surrounding environment (including the inner wall of the medium container and the environment outside the container) due to thermal radiation and conduction, making the temperature at the measuring end of the thermocouple unable to reach the temperature of the measured medium in any case. Therefore, the temperature at the measuring end is not completely consistent with the temperature of the measured medium, resulting in measurement errors. The greater the temperature difference between the measured medium and the surrounding environment, the greater the error. It is mainly caused by the heat dissipation of the surrounding environment by thermocouples.

The measurement errors caused by heat exchange include:

1. Temperature measurement error caused by radiation heat dissipation;

2. Temperature measurement error caused by thermal conduction and heat dissipation.

There are basically two methods to overcome errors caused by heat exchange: one is to calculate and determine the size of heat transfer errors, and then make corrections; The second is to take measures to reduce the heat transfer error to the allowable range.

To reduce errors caused by radiation heat dissipation, the following measures can be taken:

1. Install an insulation layer on the outer surface of the pipe wall to reduce the temperature difference between the pipe wall and the measured medium.

2. Minimize the outer diameter of the protective tube and the blackness (radiation) coefficient of the protective tube and thermoelectric electrode as much as possible.

3. Install a radiation shield between the thermocouple and the pipe wall to reduce direct radiation between the thermocouple and the pipe wall.

4. Increase the flow rate of the measured medium passing through the measuring end of the thermocouple to increase convective heat transfer between the measured medium and the thermocouple.

To reduce thermal conductivity errors, the following measures can be taken:

1. Increase the insertion depth of the thermocouple and reduce the length exposed outside the pipe wall.

2. Reduce the diameter and wall thickness of the protective tube.

3. Use protective tubes with low thermal conductivity. This can reduce thermal conductivity error, but it will increase thermal inertia and increase dynamic error, so it needs to be handled appropriately.

4. Wrap insulation materials (such as asbestos, fiberglass, etc.) outside the pipeline and thermocouple support to reduce the temperature difference at both ends of the protective tube.