Thermocouple temperature sensors have the advantages of wide measurement range, high accuracy, and fast response time, so they are widely used. This article mainly discusses the factors that affect the measurement of thermocouple temperature sensors. In summary, there are the following points:
The choice of thermocouple temperature measurement point is the most important. The location of the temperature measurement point must be typical and representative for the production process, otherwise the meaning of measurement and control will be lost. When a thermocouple is inserted into the measurement site, a heat flow is generated along the length of the sensor. There is heat loss when the ambient temperature is low. As a result, the temperature of the thermocouple temperature sensor is inconsistent with the temperature of the measured object, resulting in a temperature measurement error. In short, the error caused by heat conduction is related to the insertion depth. The insertion depth is related to the material of the protective tube. Due to its good thermal conductivity, the metal protective tube should have a deeper insertion depth, and the ceramic material has good thermal insulation properties 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 a flowing liquid or high-speed airflow will not be limited by the above. The insertion depth may be shallower. The specific value should be determined experimentally.
The basic principle of temperature measurement by contact method is that the temperature measuring element must achieve thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain 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 depends on the structure of the sensor and the measurement conditions. For gaseous media, especially stationary gas, it should be maintained for at least 30 minutes to reach equilibrium; for liquids, the fastest should be more than 5 minutes. For the measured place where the temperature changes continuously, especially the transient change process, the whole process is only 1 second, and the response time of the sensor is required 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 cause measurement errors due to failure to achieve thermal equilibrium. It is best to choose a sensor that responds quickly. For thermocouples, in addition to the protection 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.
Increase in thermal resistance
For thermocouple temperature sensors used at high temperatures, if the measured medium is in a gaseous state, dust and the like deposited on the surface of the protection tube will melt on the surface, increasing the thermal resistance of the protection tube. If the measured medium is a melt, the 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 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 measurement thermocouple temperature sensors, but also equipped with consumable thermocouple temperature measurement devices for timely calibration of the accuracy of continuous temperature measurement thermocouples.
Thermocouple temperature sensor inserted into the furnace for temperature measurement will be 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 location of the thermocouple should be avoided as much as possible from the heat radiation from the solid, so that it can not radiate to the surface of the thermocouple; the thermocouple is best equipped with a heat radiation shield.
The above are the four factors that affect the measurement of the thermocouple temperature sensor. When using it, we should pay attention to ensure the best measurement effect according to the actual situation.