Problems with using thermocouples to measure temperature
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1. What impact does the thermocouple compensation wire have on the measurement results? 2. How to determine the accuracy of temperature measurement using a thermocouple? For example, if the DCS shows a furnace temperature of 800°C, and the process team doubts the accuracy of this temperature, how can the instrumentation technician determine whether it is correct?
1. You will understand it once you look at the principle of cold-junction compensation for thermocouples. 2. Check the MV value corresponding to 800 degrees; apply an MV signal with that value at the thermocouple at the site, and see if the display in the control room shows 800 degrees
If in doubt, measure the millivolt voltage and check accordingly
1. Why is compensation necessary? Because thermocouples measure not temperature but the temperature difference; therefore, it is necessary to measure the temperature of the cold end in order to perform compensation. 2. This issue is rather troublesome; measuring the millivolt value is one approach, but the best way is to verify it through redundant temperature sensors.
The MV value of temperature measurement using a thermocouple on-site is simply E(t,t1). And what is the value of t? It’s unknown t1 is the ambient temperature at the field junction box. Extended to the control room via compensation wires, it also involves the ambient temperature t2 at the cabinet. It is certainly possible to use a thermocouple calibration table, but what is the calculation method?
2 table lookups for calculation. As for calculating the temperature corresponding to the millivolt value, I think it would be better to find a location at the same point and place another thermocouple there to take measurements.
Are there any good ways to determine the accuracy of thermocouple temperature measurement?
Replace it and carry out calibration, that is, compare it against the regulations and standards. I can only say that the calibration furnace is very important.
Is there a way to determine accuracy in the absence of calibration conditions?
Well, it’s still the same method: find a reliable one to use for comparison. If the thermocouple is damaged, the millivolt values obtained will be inaccurate.