A thermocouple temperature sensor is actually an energy converter that converts thermal energy into electrical energy and measures the temperature using the resulting thermoelectric potential. A thermocouple is a meter, which directly measures the temperature and converts the temperature signal into a thermo-electromotive force signal, which is converted into the temperature of the measured medium by an electrical meter (secondary instrument). What are the principles and factors that influence the operation of the thermocouple temperature sensor in the measurement?
The basic principle of thermocouple temperature measurement is that two different compositions of material conductors form a closed loop. When there is a temperature gradient at both ends, current will flow through the circuit. At this time, there is an electromotive force-thermal EMF between the two ends. This is The so-called Seebeck effect.
Two different components of the conductor (called a thermocouple wire or a hot electrode) are joined together at both ends. When the junction temperature is different, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect. The electromotive force is called thermoelectric power.
Thermocouples use this principle for temperature measurement, in which the end that is used directly to measure the temperature of a medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end); the cold end and the display The meter or associated meter is connected and the meter indicates the thermoelectric potential generated by the thermocouple.
There are many factors affecting the operation of the thermocouple temperature sensor. Among them, the insertion depth, response time, thermal resistance increase, and heat radiation are the four most important factors. The following is a detailed description.
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 materials, thermal insulation performance is good, can be inserted more shallow. For engineering temperature measurement, the insertion depth is also related to the state of the measurement object being stationary or flowing. For example, the measurement of the flowing liquid or high-speed airflow temperature will not be limited to the above, and the insertion depth may be shallower. The specific value should be determined experimentally.
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 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.
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