Problem of high readings on thermocouples
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Is it possible for a thermocouple to give a reading that is higher than the actual value? I don’t think it’s possible; those in charge of production say it’s 60 to 70 degrees. I think a thermocouple is either short-circuited or open-circuited. It can cause a low MV value. How could it be that high?
Thermocouples may also indicate higher values; for example, if the compensation wires are connected in reverse, if the control unit sets a maximum value in case of a broken wire, or if the thermoelectric properties of the thermocouple change due to corrosion. You can measure the mV value of the thermocouple and use a table to determine the corresponding temperature; be sure to add the mV value corresponding to the temperature of the controller as well. If the display on the DCS matches this value, then it can be concluded that the temperature indication is accurate. If the process engineer is still skeptical, and after checking that the circuit is fine, the only option is to use a new thermocouple for comparison.
It is possible that, if the meter shows excessive errors, the performance of the thermocouple deteriorates, or incorrect or reversed use of the compensation wires can all cause the readings to be too high...
First, post the installation wiring diagram to take a look
Once, the issue was caused by slight oxidation of the terminals of the safety barrier; after replacing those terminals, everything worked properly. The temperature at the site was 900 degrees, and the reading was 50 degrees higher than normal. The oxidation of the terminals wasn’t severe – they were just a little darker than the others, but it wouldn’t be noticeable without close inspection.
There are still many possibilities; it is recommended to use a signal generator to rule them out one by one.
It is possible to first measure at the component end on-site, check the temperature value using the temperature potential reference table, and see how much the difference is from the display on the control panel; however, temperature compensation must be applied when making measurements on-site.
In that case, it needs to be taken apart to take a look. Don’t you have a SIS system installed there? Use the bypass to bypass these process conditions, and then remove it to see if it works properly.
Thermocouples have many problems. Short-circuit the safety barrier and check whether the display on the control panel shows the room temperature. When you don’t know where to start, checking the issues step by step from beginning to end is the simplest yet most effective method.
I’m also looking at it; I’m also concerned about this issue