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Thermocouple precautions

A. The thermocouple should be placed as close as possible to the temperature control point to be measured. To prevent heat from passing along the thermocouple or to prevent the protective tube from affecting the temperature being measured, the thermocouple should be immersed in the fluid being measured to a depth of at least 10 times the diameter. When measuring the solid temperature, the thermocouple should be placed against or in intimate contact with the material. In order to minimize thermal conduction errors, the temperature gradient near the junction should be reduced.

B. When selecting the temperature measurement point, it should be representative. For example, when measuring the fluid temperature in the pipeline, the measuring end of the thermocouple should be at the maximum flow velocity in the pipeline. In general, the end of the protective sleeve of the thermocouple should cross the centerline of the flow rate.

 

C. When measuring the temperature of a gas in a pipe with a thermocouple, if the wall temperature is significantly higher or lower, the thermocouple will radiate or absorb heat, thereby significantly changing the measured temperature. At this time, a radiation shield can be used to bring the temperature close to the gas temperature, using a so-called screen type thermocouple.

D. Pay special attention to the use of compensation wires when actually using it. The compensation wire usually connected between the meter and the junction box has the same thermoelectric properties as the thermocouple used, and does not generate a large additional thermoelectric potential when connected to the thermocouple, and does not affect the total thermoelectric potential of the thermocouple circuit. If the ordinary wire is used instead of the compensation wire, the compensation effect will not be obtained, thereby reducing the accuracy of the temperature measurement. Therefore, when using the unit to install the instrument, it should be noted that when the compensation wire is connected with the thermocouple, the polarity should not be reversed, otherwise the temperature measurement error will increase.

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