The influence of the insertion depth of thermocouples on temperature measurement

In industrial temperature measurement, thermocouples are generally selected as instrument temperature measuring tools for temperatures above 0-400 ℃. The installation position of thermocouples, that is, the selection of temperature measurement points, is the most important. The position of the temperature measurement point must be typical and representative for the production process, otherwise it will lose the significance of measurement and control.
When inserting a depth thermocouple into the tested location, heat flow will be generated along the length direction of the sensor. When the ambient temperature is low, there will be heat loss. Causing temperature measurement errors due to temperature inconsistency between the thermocouple and the measured object. In short, the errors caused by heat conduction are related to the insertion depth. The insertion depth is related to the material of the protective tube. Due to its good thermal conductivity, the insertion depth of metal protective tubes should be deeper (about 15-20 times the diameter). Ceramic materials have good insulation performance and can be inserted shallower (about 10-15 times the diameter).
The basic principle of contact temperature measurement based on the influence of response time is that the temperature measuring element needs to reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain period of time during temperature measurement in order to achieve thermal equilibrium between the two. The length of holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure and measurement conditions of the sensor, with significant differences. For gas media, especially stationary gases, equilibrium should be maintained for at least 30 minutes or more; For liquids, the fastest time should be at least 5 minutes. For the temperature constantly changing test site, especially the instantaneous change process, if the entire process is only 1 second, the response time of the sensor is required to be in the millisecond level. Therefore, ordinary temperature sensors not only fail to keep up with the temperature change rate of the measured object, but also produce measurement errors due to the inability to achieve thermal equilibrium. It is best to choose a sensor with a fast response. For thermocouples, in addition to the influence of protective tubes, the diameter of the measuring end of the thermocouple is also the main factor, that is, the thinner the thermocouple wire, the smaller the diameter of the measuring end, and the shorter its thermal response time. In order to improve the efficiency of calibration, many enterprises use automatic calibration devices to calibrate incoming thermocouples. However, this device is not very complete.
The influence of thermal radiation is inserted into the thermocouple used for temperature measurement in the furnace, which is heated by the thermal radiation emitted by high-temperature objects. Assuming that the gas inside the furnace is transparent, and when the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange. Therefore, in order to reduce thermal radiation errors, heat conduction should be increased to be as close as possible to the temperature of the thermocouple. In addition, during installation, attention should also be paid to: ① The installation position of the thermocouple should avoid the thermal radiation emitted from the solid as much as possible, so that it cannot radiate to the surface of the thermocouple; ② Thermocouples are best equipped with thermal radiation shielding sleeves.
The effect of increased thermal impedance on thermocouples used at high temperatures. If the measured medium is in a gaseous state, the dust deposited on the surface of the protective tube will melt on the surface, causing an increase in the thermal impedance of the protective tube; If the measured medium is a melt, there will be slag deposition during use, which not only increases the response time of the thermocouple, but also makes the indicated temperature lower. In addition to regular calibration, regular spot checks are important to reduce errors