The choice of thermal resistance temperature measurement point 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 meaning of measurement and control. When a thermocouple is inserted into the test site, heat flow will occur along the length of the sensor. There will be heat loss when the ambient temperature is low. As a result, the temperature of the thermocouple temperature sensor and the measured object are inconsistent and a temperature measurement error occurs. In short, the error caused by heat conduction is related to the insertion depth. The insertion depth is related to the material of the protection tube. Due to its good thermal conductivity, the metal protection tube should have a deeper insertion depth, and the ceramic material has good thermal insulation performance and can be inserted shallower. For engineering temperature measurement, the insertion depth is also related to whether the measurement object is stationary or flowing. For example, the measurement of the temperature of flowing liquid or high-speed airflow will not be restricted by the above. The insertion depth may be shallower. The specific value should be determined by experiment.   

Response time   

The basic principle of temperature measurement by contact method is that the temperature measuring element should reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain time when measuring temperature, so that the two can reach thermal equilibrium. 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, and varies greatly. For gaseous media, especially still gas, it should be maintained for at least 30 minutes to reach equilibrium; for liquids, it should be at least 5 minutes. For the measured place where the temperature is constantly changing, especially the instantaneous change process, the whole process is only 1 second, the response time of the sensor is required to be in the order of milliseconds. Therefore, the common temperature sensor not only can't keep up with the temperature change rate of the measured object, but also can cause measurement error due to the failure 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 its main factor. That is, the thinner the wire, the smaller the diameter of the measuring end, and the shorter the thermal response time.   

Increased thermal impedance   

Thermal resistance temperature sensor used at high temperature, if the measured medium is gaseous, the dust deposited on the surface of the protective tube will burn on the surface, increasing the thermal resistance of the protective tube; if the measured medium is molten The slag will be deposited during use, which not only increases the response time of the thermocouple, but also lowers the indicated temperature. Therefore, in addition to regular verification, in order to reduce errors, frequent spot checks are also necessary. For example, imported copper smelting furnaces are not only equipped with continuous temperature measuring thermocouple temperature sensors, but also equipped with consumable thermocouple temperature measuring devices, which are used to calibrate the accuracy of continuous temperature measuring thermocouples in time.   

Heat radiation   

Thermistor temperature sensor inserted into the furnace for temperature measurement is heated by the heat radiation from high temperature objects. 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, a temperature measurement error will occur due to energy exchange. In general, in order to reduce the heat radiation error, the 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 avoid the heat radiation emitted from the solid as much as possible, so that it cannot radiate to the surface of the thermocouple; the thermocouple is best equipped with a heat radiation shielding sleeve.