Factors affecting measurement of thermal resistance temperature sensors
Insertion depth
The selection of temperature measurement points for thermal resistance 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 the thermocouple is inserted 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 temperature sensor 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. Metal protective tubes should be inserted deeper due to their good thermal conductivity, while ceramic materials have good insulation properties and can be inserted shallower. For engineering temperature measurement, the insertion depth is also related to whether the measurement object is in a static or flowing state. For example, the measurement of the temperature of a flowing liquid or high-speed airflow will not be subject to the above limitations. The insertion depth can be shallower, and the specific value should be determined by experiments.
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 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.
Increased thermal impedance
The thermal resistance temperature sensor used at high temperatures, if the measured medium is in a gaseous state, the dust and other particles deposited on the surface of the protective tube will melt on the surface, increasing 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. Therefore, in addition to regular calibration, regular spot checks are also necessary to reduce errors.
thermal radiation
The thermal resistance temperature sensor inserted into the furnace for temperature measurement will be 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. In general, in order to reduce thermal radiation errors, it is necessary to increase heat conduction and make the furnace wall temperature as close as possible to the temperature of the thermocouple. In addition, the installation position of thermocouples should avoid thermal radiation emitted from solids as much as possible, so that it cannot radiate to the surface of thermocouples; Thermocouples are best equipped with thermal radiation shielding sleeves.
The above are the four factors that affect the measurement of thermocouple temperature sensors. When using them, we should pay attention to ensuring the best measurement effect according to the actual situation.
