Strange jobs on the night shift
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During the night shift, a malfunction occurred with the temperature measurement of a certain device. When the cover of the wiring box was opened, it was found that the terminal board had been pushed up; ice was present inside the thermocouple wire, but not on its surface. It was determined that the protective sleeve was leaking, allowing water to enter the sleeve and freeze, which in turn pushed up the thermocouple wire and caused poor contact between the terminals. After reconnecting everything, normal operation was restored, but a new thermocouple needs to be installed. For now, the temperature is being monitored temporarily. Dear technicians, when the ambient temperature in winter is below 10 degrees Celsius or even 20 degrees Celsius, how should the error in measuring the millivolt value of thermocouples be calculated? Negative millivolt values are generated in such situations; under normal circumstances, the ambient temperature value should be added to this figure. For example, if the temperature at the hot end is 200 degrees Celsius and the ambient temperature is 20 degrees Celsius, the measured millivolt value should be increased by 20 degrees. But how should this be calculated when the temperature is below zero in winter? I’m not sure if I made myself clear; please bear with me, all of you masters!
When measuring the output of a thermocouple in a low-temperature environment, it is indeed necessary to provide appropriate compensation for the ambient temperature to ensure the accuracy of the measurement results. The output of a thermocouple is based on the temperature difference between two junctions (the hot end and the cold end). If the temperature of the cold end, which is the reference end, is known and stable, it is possible to calculate the actual temperature of the hot end. Generally, the reference terminal (i.e., the cold end or connection terminal of the thermocouple) should be at a known and stable temperature, which is usually 0°C (ice-water mixture) or room temperature (typically assumed to be 20°C). In practical applications, the temperature at the reference terminal may change due to variations in ambient temperature, so compensation is required. In cold environments (such as -10°C or lower), you need to measure the actual temperature at the reference terminal, and use an appropriate table or formula relating thermoelectric potential to temperature depending on the type of thermocouple, in order to perform the compensation calculation. These tables or formulas can be found in the technical manuals or standard books for thermocouples. Taking a standard reference terminal temperature of 0°C as an example for calculation, it is as follows: 1. Measure the actual reference terminal temperature (denoted as Tc, in °C). 2. Find the temperature-millivolt table corresponding to the model of the thermocouple, and determine the millivolt value (Mv_c) corresponding to the actual reference terminal temperature Tc. 3. Measure the output millivolt value (Mv_m) of the thermocouple. 4. Calculate the compensated millivolt value: Mv = Mv_m - Mv_c (if the temperature at the reference terminal is below 0°C, then Mv_c will be a positive value, and this value needs to be subtracted from the measured value). 5. Use the compensated millivolt value Mv to look up in the table or calculate the corresponding hot terminal temperature Th. Here, you mentioned that the normal procedure is to add the temperature of the reference terminal to the measured millivolt value; however, in reality, it is necessary to combine the millivolt value resulting from changes in the reference terminal’s temperature with the measured millivolt value in order to perform the appropriate compensation calculation. Please keep in mind that each type of thermocouple (K-type, J-type, T-type, etc.) has its own specific temperature-millivolt relationship; therefore, you need to ensure that the correct data and formulas are used for calculations. If you are unsure about how to handle these calculations, you can consult an experienced engineer or use a thermocouple reading device with automatic compensation features (such as a thermocouple temperature sensor or a multi-functional process calibrator). .
The mV value measured at sub-zero temperatures minus the mV value corresponding to the sub-zero ambient temperature