Thermocouple is one of the most commonly used temperature measuring devices. It directly measures the temperature and converts the temperature signal into thermoelectric EMF signal. The thermocouple is converted into the temperature of the measured medium through electrical instruments. Because of the advantages of wide measuring range, high precision and fast response time, thermocouple temperature sensor is widely used.
It is very important to choose the temperature measuring point of thermocouple. For the production process, the location of temperature measurement point must be typical and representative, otherwise it will lose the significance of measurement and control. When the thermocouple is inserted into the measured site, heat flow will be generated along the length direction of the sensor. When the ambient temperature is low, there is heat loss. As a result, the temperature of thermocouple temperature sensor is not consistent with the measured object, resulting in temperature measurement error.
In short, the error caused by heat conduction is related to the depth of insertion. The insertion depth is related to the material of the protective tube. Because of its good thermal conductivity, the insertion depth of metal protective tube should be deeper, and the ceramic material has good thermal insulation performance and can be inserted shallowly.
For engineering temperature measurement, the insertion depth is also related to the static or flowing state of the measured object. For example, the measurement of the temperature of flowing liquid or high-speed air flow will not be subject to the above restrictions. The insertion depth can be shallower, and the specific value should be determined by experiments. Therefore, it is necessary to keep a certain time in the temperature measurement to make the two reach the thermal balance. The holding time is related to the thermal response time of the temperature measuring element. However, the thermal response time mainly depends on the structure and measurement conditions of the sensor, and the difference is great. For the gas medium, especially for the static gas, it should be kept at least 30 min to reach the equilibrium; for the liquid, it should be more than 5 min soon.
The response time of the sensor is required to be in the order of millisecond when the temperature changes continuously, especially in the transient process. Therefore, the ordinary temperature sensor not only can not keep up with the temperature change speed of the measured object, but also can't reach the thermal balance and produce measurement error. It's a good choice for fast response sensors. For the thermocouple, in addition to the influence of the protection tube, the diameter of the measuring end of the thermocouple is also the main factor, that is, the thinner the wire is, the smaller the diameter of the measuring end is, the shorter the thermal response time is.
On 2018-08-08 11:45:46
