Thermistor Temperature Sensor - Factors Affecting Measurement
Insertion depth
The selection of temperature measurement points for thermal resistance is the most important. The location 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 a thermocouple is inserted into the measured 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 between the thermocouple temperature sensor and the measured object due to temperature inconsistency. In short, the error caused by thermal conduction is related to the insertion depth. The insertion depth is also 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 the stationary or flowing state of the measurement object. For example, the measurement of the temperature of flowing liquids or high-speed airflows will not be limited by the above. The insertion depth can be shallower, and the specific value should be determined experimentally.
response time
The basic principle of contact temperature measurement is that the temperature measuring element needs to reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain amount of time during temperature measurement in order to achieve thermal equilibrium between the two. The duration 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 and measurement conditions of the sensor, with significant differences. For gas media, especially stationary gases, equilibrium should be maintained for at least 30 minutes; For liquids, the fastest time should be at least 5 minutes. For the measured location with constantly changing temperature, especially during instantaneous changes, if the entire process takes only 1 second, the response time of the sensor is required to be in the millisecond range. Therefore, ordinary temperature sensors not only lag behind 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 sensors with fast response. For thermocouples, in addition to the influence of the protective tube, 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 the thermal response time.
Increased thermal impedance
The thermal resistance temperature sensor used at high temperatures, if the measured medium is in a gaseous state, will burn and melt the dust and other particles deposited on the surface of the protective tube, 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 lowers the indicated temperature. Therefore, in addition to regular inspections, regular sampling is also necessary to reduce errors. For example, imported copper smelting furnaces are not only equipped with continuous temperature measurement thermocouple temperature sensors, but also with consumable thermocouple temperature measurement devices for timely calibration of the accuracy of continuous temperature measurement thermocouples.
thermal radiation
Insert a thermal resistance temperature sensor into the furnace for temperature measurement, which 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 location 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 and ensure the best measurement effect according to the actual situation.
