A thermocouple is one of the most commonly used temperature measuring devices. It directly measures the temperature and converts the temperature signal into a thermo-electromotive force signal, which is converted to the temperature of the measured medium by an electrical instrument. Because thermocouple temperature sensors have the advantages of wide measurement range, high accuracy, and fast response time, they are widely used. This article mainly discusses the factors that affect the measurement of thermocouple temperature sensors. To sum up, the main points are as follows:

Depth of insertion

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 it will lose the significance of measurement and control. When the thermocouple is inserted into the location to be measured, heat flow will occur along the length of the sensor. When the ambient temperature is low, there will be heat loss. Resulting thermocouple temperature sensor does not match the temperature of the measured object to generate temperature error. In short, the error caused by heat conduction depends on the insertion depth. The insertion depth is related to the protection tube material. Metal protection tube because of its good thermal conductivity, its insertion depth should be deeper, ceramic material insulation performance is good, you can insert a little lighter. For engineering temperature measurement, the insertion depth is also related to the state of the measurement object such as static or flowing. If the measurement of the flowing liquid or high-speed airflow temperature is not limited to the above, the insertion depth may be shallow, and the specific value should be determined by experiments.

Response time

The basic principle of temperature measurement by contact method is that the temperature measuring element must reach thermal equilibrium with the measured object. Therefore, it takes a certain period of time for the temperature measurement to reach a 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 mainly depends on the sensor's structure and measurement conditions. For gas media, especially stationary gas, at least 30 minutes or more should be maintained to achieve equilibrium; for liquids, it should be more than 5 minutes at the fastest. For a test site where the temperature is constantly changing, especially during the transient process, the entire process is only 1 second, and the response time of the sensor is required to be in the order of milliseconds. Therefore, an ordinary temperature sensor not only fails to keep up with the lag of the temperature change of the measured object, but also causes measurement error because the heat balance is not reached. It is best to choose a sensor that responds quickly. For the thermocouple, except for the effect of the protective tube, the diameter of the measuring end of the thermocouple is also the main factor. That is, the finer the twin wire, the smaller the diameter of the measuring end and the shorter the thermal response time.

Increased thermal resistance

If a thermocouple temperature sensor is used at a high temperature, if the measured medium is in a gaseous state, dust deposited on the surface of the protection tube will be fused on the surface to increase the thermal resistance of the protection tube; if the measured medium is a melt, There will be slag deposition during use, which not only increases the response time of the thermocouple, but also makes the indicated temperature low. Therefore, in addition to regular inspections, frequent inspections are also necessary in order to reduce errors. For example, imported copper smelting furnaces are not only equipped with a continuous temperature thermocouple temperature sensor, but also equipped with a consumable thermocouple temperature measuring device for the timely calibration of the accuracy of the thermocouples for continuous temperature measurement.

Heat radiation

The thermocouple temperature sensor inserted into the furnace for temperature measurement heats the heat radiation emitted by the hot object. It is assumed that the gas in the furnace is transparent, and if the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange. In general, in order to reduce the thermal radiation error, heat conduction should be increased and the 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 far as possible from the heat radiation emitted from the solid, so that it can not be radiated to the surface of the thermocouple; the thermocouple is best equipped with a heat radiation shield.

This article describes the four factors affecting the thermocouple temperature sensor measurement, we should pay attention when using, according to the actual situation, to ensure the best measurement results.