Thermocouple calibration device is a commonly used temperature measuring element in temperature measuring instruments. When two different conductors are connected to the synthetic circuit, when the temperature of the two junction points is different, the thermal current will be generated in the circuit. If there is a temperature difference between the working end and the reference end of the thermocouple, the display instrument will indicate the temperature value corresponding to the thermoelectric potential generated by the thermocouple. The thermoelectric heat of the thermocouple calibration device will increase with the increase of the temperature at the measuring end. Its size is only related to the thermocouple material and the temperature at both ends, and has nothing to do with the length and diameter of the thermocouple. The shapes of various thermocouples are often very different according to their needs, but their basic structures are roughly the same. They are usually composed of thermoelectric poles, insulating sleeve protective tubes and junction boxes. They are usually used in conjunction with display instruments, recording instruments and electronic regulators.
Although the structure of thermocouple calibration device is simple, there are still many problems in its application. For example, improper installation or use of methods will lead to greater measurement errors, and even qualified thermocouples will be unqualified due to improper operation, when used, in reducing atmosphere such as carburizing, if not noticed, K-type thermocouples will also be over-poor due to selective oxidation. In order to improve measurement accuracy, reduce measurement errors and prolong the life of thermocouples, users should not only have operational skills in instrumentation, but also have knowledge of physics, chemistry and materials.
Shunt error of thermocouple verification device
The so-called shunt error is that when the temperature of furnace is measured by armored thermocouple, when the temperature distribution in the middle part of the thermocouple exceeds 800 C, the insulation resistance of the thermocouple decreases and the indication value of the thermocouple appears abnormal, which is called shunt error. According to the law of homogeneous circuit, the temperature measurement with thermocouple is only related to the temperature at both ends of the measuring end and the reference end, but not to the temperature distribution in the middle. However, as the insulator of armored thermocouple is powdered magnesium oxide, the insulation resistance decreases by an order of magnitude for every 100 C rise in temperature. When the temperature of the middle part is higher, leakage current must occur, resulting in shunting error in the output potential of thermocouple.
Application of Thermocouple Verification Device and Treatment of Leakage
1. When using thermocouple compensating conductor, attention must be paid to the type matching, the polarity can not be misconnected, and the temperature of the compensating conductor and the thermocouple connecting end can not exceed 100 C.
2. When the temperature is measured or controlled by a thermocouple with a large time constant, the temperature shown by the instrument fluctuates very little, but the actual temperature of the furnace may fluctuate greatly. In order to measure temperature accurately, thermocouples with small time constants should be selected.
3. After power-on according to the instrument wiring diagram, the instrument first displays the thermocouple indexing number of the instrument, then displays the range of the instrument, and then measures the setting temperature of the digital tube display in the lower row of the instrument, and the digital tube display in the upper row of the instrument.
4. The thermocouple can not be installed in the area where the measured medium seldom flows. When measuring the gas temperature in the tube with the thermocouple, the thermocouple must be installed in the opposite direction of the flow velocity and be in full contact with the gas.
5. In order to make full heat exchange between the measuring end of thermocouple and thermal resistance and the medium under test, the location of the measuring point should be reasonably selected.
6. According to the law of intermediate conductor, in the practical temperature measurement application of thermocouple verification device, hot-end welding and cold-end open circuit are often adopted, and the cold-end is connected with the display instrument through connecting wires to form a temperature measurement system.
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