Factors affecting thermocouple measurement

Thermocouples are one of the most commonly used temperature measuring devices, which directly measure temperature and convert temperature signals into thermoelectric signals. In some cases, thermocouples produced by thermocouple manufacturers are converted into the temperature of the measured medium through process electrical instruments. Due to the advantages of wide measurement range, high accuracy, and fast response time of thermocouple temperature sensors, they have lost their widespread application.

The choice of thermocouple temperature measurement point is the most important. The position of temperature measurement points must have representativeness and representativeness for the production process, otherwise the meaning of measurement and control will be lost. When the thermocouple is pulled out of the measured field, heat flow will occur along the length of the sensor. When the temperature is low, there will be heat loss. As a result, temperature deviation occurs between the thermocouple temperature sensor and the measured tool due to temperature deviation.

In short, the deviation caused by thermal conduction is related to the depth of extraction. The depth of extraction is also related to the material of the maintenance tube. Metal maintenance pipes should be pulled out deeper due to their good thermal conductivity, while ceramic materials have good insulation properties and can be pulled out shallower.

For engineering temperature measurement, the extraction depth is also related to the movement or activity of the measuring tool. For example, the measurement of the temperature of moving liquids or high-speed airflow will not be subject to the above limits. The extraction depth can be shallower, and the detailed value should be determined by experiments.

Therefore, it is necessary to persist for a certain amount of time when measuring temperature in order to achieve thermal equilibrium between the two. The right and wrong that persist in time are related to the thermal response of temperature measuring elements to time. The thermal response time depends heavily on the structure and measurement conditions of the sensor, with significant differences. For gas media, especially moving gases, it is necessary to maintain equilibrium for at least 30 minutes; For liquids, the fastest time should be at least 5 minutes.

For situations where the temperature is constantly changing, especially in the case of sudden changes, if the entire process takes only 1 second, the response time of the sensor request should be in milliseconds. Therefore, typical temperature sensors not only lag behind the temperature change rate of the measured tool, but also experience measurement deviations due to the inability to achieve thermal equilibrium. Choose sensors that respond quickly. For thermocouples, in addition to the influence of maintenance tubes, the measuring end diameter of the thermocouple is also an important factor, that is, the thinner the thermocouple wire, the smaller the measuring end diameter, and the shorter the thermal response time.