Thermocouple RTD single point check
Thermocouple RTD single point check
(OP)
Does anyone use a "single point check" to verify the output of their thermocouples and RTDs (basically checking to make sure the element is reading something), then use a TC/RTD simulator to check the rest of the loop? Everything I've read indicates that RTD's drift, and thermocouples drift even moreso, which leads me to believe that this method is practically useless. Can anyone suggest any published standards for TC/RTD loop calibration?
Thanks
Thanks





RE: Thermocouple RTD single point check
the isa is a good source for such specialized issues as are rtd-t/c manufacturers
RE: Thermocouple RTD single point check
Using an oil bath may not be so easy on a thermocouple, which may be measuring many hundreds or even thousands of degrees.
RE: Thermocouple RTD single point check
RE: Thermocouple RTD single point check
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(2B)+(2B)' ?
RE: Thermocouple RTD single point check
Drift of thermocouples was/is not an issue. The MV output is a function of two dissimilar metals at the given temperature. The same with RTDs: the ohmic value is a function of temperature, period.
If anything has gone wrong with the TC or RTD (burn-out, ground fault, etc) there will be a sudden large change in the reading and the measuring instrument will show this. In fact with TCs there was always a burn-out circuit in th instrument that drove the reading up to the end of the scale (safely shutting off the heat source) in case the TC went open circuit.
But with the actual measuring instruments themselves, drift can occur and therefore such instruments were calibration checked at regular intervals, anywhere from once every month up to once per year, depending on the criticality of the application.
Unfortunately I think youngblood has gotten the wrong information from somewhere.
rasevskii
RE: Thermocouple RTD single point check
I've attached my references that initially supported my suspicion that thermocouples and RTD's drift. Can anybody attach other references/data to the contrary, because I would seriously love to see them?
Thanks,
RE: Thermocouple RTD single point check
RE: Thermocouple RTD single point check
Thanks
RE: Thermocouple RTD single point check
Good on ya,
Goober Dave
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RE: Thermocouple RTD single point check
It looks like RTD's have very good temperature stability. If we look at Dave's number "common" RTD's 0.05C per 5 years... would take 100 years to drift 1C. It is really irrelevant for the applications I'm concerned with (primarily motor bearing and winding temperature sensors).
I see much higher rates mentioned for thermocouples, primarily related to oxidation/corrosion which are temperature dependent and naturally accelerate at higher temperatures. Mentioned temperatures of 800F are nowhere near what I'm interested in (again, motors). It's not clear to me exactly how big the drift is in the range up to let's say 250F which is about the highest winding temperature I ever see.
So as expected it probably depends on your application (max temprature) and requirements (is 1C change significant?).
I can see the logic coming from the perspective I had before that thread. That is: each RTD or thermocouple comes with a calibration table that tells us resistance vs temperature (rtd) or voltage vs temperature (thermocouple). If the stated accuracy for the table is well within our requirements, then the main thing we want to double check is that we have the right sensor installed... which single point check will do. Sure, you get better check with two points, but still not perfect if you have a non-linear table lookup function for thermocouple. More points is always better, you can still question the in between points to some extent not matter how many points you check. How valid are the questions depends on how exact you want to be or need to be.
I can also see where coming from your viewpoint (that the drift is significant for your application), this approach would not be valid.
Thanks again. There is always something to learn here.
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(2B)+(2B)' ?
RE: Thermocouple RTD single point check
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(2B)+(2B)' ?
RE: Thermocouple RTD single point check
Thanks