Effect of Thermocouple Insertion Depth on Measured Temperature

1. The influence of the insertion depth of the thermocouple 1. The selection of the temperature measurement point The installation position of the thermocouple, that is, the selection of the 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 significance of measurement and control will be lost. 2. Insertion Depth When a thermocouple is inserted into the site under test, a heat flow will be generated along the length of the sensor. Heat loss occurs when the ambient temperature is low. As a result, the temperature of the thermocouple 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. Because of its good thermal conductivity, the metal protection tube should be inserted deeper (about 15-20 times the diameter), and the ceramic material has good thermal insulation performance and can be inserted shallower (about 10-15 times the diameter). For engineering temperature measurement, the insertion depth is also related to whether the measurement object is still or flowing. For example, the measurement of the temperature of flowing liquid or high-speed airflow will not be limited by the above, and the insertion depth can be shallower, and the specific value should be determined by experiment. 2. Influence of response time The basic principle of temperature measurement by contact method is that the temperature measurement element should reach thermal equilibrium with the measured object. Therefore, it needs to be kept for a certain period of time during temperature measurement 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 and measurement conditions of the sensor, which varies greatly. For gaseous media, especially static gas, it should be maintained for at least 30min to reach equilibrium; for liquid, the fastest time should be more than 5min. 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 millisecond level. Therefore, ordinary temperature sensors not only cannot keep up with the temperature change speed of the measured object, but also cause measurement errors due to the failure to reach thermal equilibrium. 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 pair of wires, the smaller the diameter of the measuring end, and the shorter the thermal response time.

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