Do Thermocouples need to be Calibrated?

I

Inunez

#11
i myself have been tasked with calibrating an immersion probe with a thermocouple on the end in the past. here's something...if you cant adjust anything in a thermocouple, how can it be calibrated? this is where the words calibration and verification should be used carefully. just my 2 cents
 
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Jim Wynne

Staff member
Admin
#12
i myself have been tasked with calibrating an immersion probe with a thermocouple on the end in the past. here's something...if you cant adjust anything in a thermocouple, how can it be calibrated? this is where the words calibration and verification should be used carefully. just my 2 cents
Calibration is comparison to a standard, which can be done whether or not the thing can be adjusted. "Verification" in this sense is a type of calibration, not something different from calibration.
 
I

Inunez

#13
Calibration is comparison to a standard, which can be done whether or not the thing can be adjusted. "Verification" in this sense is a type of calibration, not something different from calibration.
thank you Jim :applause:
but i'll be honest, i'm one of those people who likes to call things what they are, so as an entry level tech, i'm still getting used to the lingo of the business. i wont start this discussion here, but in the clear definitions of the words verifying is not calibrating.
its just hard to start calling a chicken a duck when you know otherwise :frust: lol
either way, thank you for your help. :agree:
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#14
i myself have been tasked with calibrating an immersion probe with a thermocouple on the end in the past. here's something...if you cant adjust anything in a thermocouple, how can it be calibrated? this is where the words calibration and verification should be used carefully. just my 2 cents
The reason is, if you have a perfect calibration, there is no adjustment, but the traceability chain and the uncertainty will be clearly defined and documented.

Thermocouple calibration does not require nor truly allow adjustment. However, it does require the uncertainties to be calculated and the traceability chain to be documented.

For thermocouple calibration, just like for microwave thermistor calibration, you should chart the readings at each point you need it to measure. You should also trend the uncertainties.

The idea is to catch and quantify the drift.

This can be done through Excel, an expensive Monte Carlo program is not needed for such a simple application.

Hope this helps.
 

Jim Wynne

Staff member
Admin
#15
thank you Jim :applause:
but i'll be honest, i'm one of those people who likes to call things what they are, so as an entry level tech, i'm still getting used to the lingo of the business. i wont start this discussion here, but in the clear definitions of the words verifying is not calibrating.
its just hard to start calling a chicken a duck when you know otherwise :frust: lol
either way, thank you for your help. :agree:
It's a good thing to use terms that everyone understands, and if that involves a corruption (or expansion) of definitions, that's OK. Nonetheless, as Hershal has pointed out, "calibration" can and should be used to describe the "full" process for devices that can't be adjusted. If you want to differentiate between calibration and verification, whether something can be adjusted or not shouldn't enter into it.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#16
The reason is, if you have a perfect calibration, there is no adjustment, but the traceability chain and the uncertainty will be clearly defined and documented.
Exactly - calibration is the comparison to a standard to determine the bias - or deviation from the standard. If the deviation is in the acceptable range,it is calibrated.

Adjustment is not calibration, although some gaging allows you the option to adjust to bring a gage back into calibration, but adjustment itself is not a requirement of calibration.
 

Big Jim

Super Moderator
#17
Calibration is comparison to a standard, which can be done whether or not the thing can be adjusted. "Verification" in this sense is a type of calibration, not something different from calibration.
May I add that the wording of the standard helps with this understanding.

"Where necessary to ensure valid results . . . measuring equipment shall be calibrated or verified, or both . . . "

In my mind, that pretty much tells you that whatever you call it, it has to be done. I think it also pretty much tells you that verification is a calibration activity.
 

Jerry Eldred

Forum Moderator
Super Moderator
#18
I'm not sure how mouch of a settled issue this is. I am way too busy lately, and don't have much time to post. So let me appease my guilt a l ittle.

Thermocouples need to be periodically calibrated (verified, as you can't adjust them) where they are used to measure with any needed accuracy. Where they are installed in systems such as oven chambers, you may verify them by comparison while they are still connected to the oven. If they are removable, you can verify them in a lab.

Using the initial certificate for the roll of TC wire may be okay to fulfill documentation requirements on new TC's, but the final thermocouples need to be verified periodically (again, if you care about the accuracy). I have calibrated thermocouples for many years (particularly those used in high temperature environments, and they absolutely do drift over time. A base metal TC used in relatively low temperatures will not drift very much, but higher temp applications change metal composition through a baking process. Also, buildup of corrosion in the connector can cause additional error, and so must be accounted for as well.

No, you can't adjust them, but you can periodically certify that their output emf is still withing specified limits. And in some circumstances (which I occasionally have to do), you can supply the user with a correction table as appropriate for their use.

Again, this applies linearly to higher temp usage. A type K TC used to periodically check 100 Deg C (for example) may see very little drift over many years. A type R used at 1200 Deg C almost continuously will likely drift 1 or 2 percent within a year or two (depending on particulars of use).
 

BradM

Staff member
Admin
#19
Jerry, I for one am extremely grateful each time you post. You have a profound amount of knowledge, and I learn something each time I read your posts. :agree1:

Thanks for taking the time to contribute as much as you do. :agree1:
 
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