The factors that affect the measurement of the thermocouple temperature sensor mainly include response time, thermal impedance increase and thermal radiation. Let's take a look at it in detail below.
Response time
The basic principle of contact temperature measurement is that the temperature measuring element must reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain period of 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 vary greatly. For gaseous media, especially static gas, it should be kept for at least 30 minutes to reach equilibrium; for liquids, the fastest should be at least 5 minutes. For the place under test with constantly changing temperature, especially the instantaneous change process, the whole process is only 1 second, and the response time of the sensor is required to be in the millisecond level. Therefore, the ordinary temperature sensor not only can't keep up with the temperature change speed of the measured object, but also has a measurement error due to the inability to reach the thermal balance. It is best to choose a sensor that responds quickly. 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 coupling wire, the smaller the diameter of the measuring end, and the shorter the thermal response time.
Increased thermal resistance
For thermocouple temperature sensors used at high temperatures, if the measured medium is gaseous, the dust deposited on the surface of the protective tube will melt on the surface, increasing the thermal resistance of the protective tube; if the measured medium is a melt, Slag deposits during use, which not only increases the response time of the thermocouple, but also lowers the indicated temperature. Therefore, in addition to regular inspections, in order to reduce errors, frequent random inspections are also necessary. For example, the imported copper smelting furnace is not only equipped with a continuous temperature measuring thermocouple temperature sensor, but also equipped with a consumable thermocouple temperature measuring device, which is used to calibrate the accuracy of the continuous temperature measuring thermocouple in time.
Heat radiation
The thermocouple temperature sensor inserted into the furnace for temperature measurement will be heated by the thermal radiation emitted by the high-temperature object. It is assumed that the gas in 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 the thermal radiation error, heat conduction should be increased, and the furnace wall temperature should be as close as possible to the temperature of the thermocouple. In addition, the installation position of the thermocouple should be as far away as possible from the heat radiation emitted from the solid, so that it cannot radiate to the surface of the thermocouple; the thermocouple should preferably be equipped with a heat radiation shielding sleeve.
