Policy For Calibrating Thermocouples and How To Calibrate a Thermocouple

qu1nn

Involved In Discussions
#31
I have seen some very informative posts here in the elsmar forums regarding thermocouples and their verification.
I understand this is an old thread, but I figured this was the best place for my questions

We use ovens for some of our processes. The range in question is 100 - 250 deg F with a desired tolerance of +/- 10 deg F.
8 ovens temperatures are verified quarterly.
We use a Fluke 51 that is NIST calibrated yearly with k type thermocouples.


Ideally I would just purchase new thermocouples yearly and have them NIST calibrated, however I am part of a small company who is trying to keep costs down.
Again re-iterating acceptable range for the process is +/-10 deg F.
Would it be acceptable to do the following:


Purchase an Omega 5 pack of K type thermocouples (5SC-TT-K-30-36). Obtain a 3 point NIST cal on one of the 5 as they ??? typically calibrate one out of the pack of 5, which counts as a lot calibration.?

Question 1) Due to our infrequent use would it acceptable to keep a single thermocouple from the lot as a ?gold standard? which would be set off to the side and then only used when verifying yearly against one of the other thermocouples?

Question 2) Is it acceptable to use a concept of ?lot calibration? for thermocouples?

Question 3) if you have a ?gold standard? thermocouple that is sitting off to the side in ambient conditions, how long would you consider it be acceptable before having a NIST calibration performed again? (1 year, 2 year? )

Question 4)..... or would it be more preferable to not use the gold standard as is question 1, but instead to verify against crushed ice / boiling water as a verification of proper functioning?


Thank you for your time
Qu1nn
 
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Hershal

Metrologist-Auditor
Staff member
Super Moderator
#32
I have seen some very informative posts here in the elsmar forums regarding thermocouples and their verification.
I understand this is an old thread, but I figured this was the best place for my questions

We use ovens for some of our processes. The range in question is 100 - 250 deg F with a desired tolerance of +/- 10 deg F.
8 ovens temperatures are verified quarterly.
We use a Fluke 51 that is NIST calibrated yearly with k type thermocouples.


Ideally I would just purchase new thermocouples yearly and have them NIST calibrated, however I am part of a small company who is trying to keep costs down.
Again re-iterating acceptable range for the process is +/-10 deg F.
Would it be acceptable to do the following:


Purchase an Omega 5 pack of K type thermocouples (5SC-TT-K-30-36). Obtain a 3 point NIST cal on one of the 5 as they ??? typically calibrate one out of the pack of 5, which counts as a lot calibration.?

Question 1) Due to our infrequent use would it acceptable to keep a single thermocouple from the lot as a ?gold standard? which would be set off to the side and then only used when verifying yearly against one of the other thermocouples?

Question 2) Is it acceptable to use a concept of ?lot calibration? for thermocouples?

Question 3) if you have a ?gold standard? thermocouple that is sitting off to the side in ambient conditions, how long would you consider it be acceptable before having a NIST calibration performed again? (1 year, 2 year? )

Question 4)..... or would it be more preferable to not use the gold standard as is question 1, but instead to verify against crushed ice / boiling water as a verification of proper functioning?


Thank you for your time
Qu1nn
For question 1, the approach may be acceptable, but the calibration needs to be from an accredited calibration lab including getting the readings and uncertatinty, and the verification needs to be done at least monthly. Reason, thermocouples drift, and depending on several variables can drift a lot. See also question 2 below.

For question 2, the approach is valid ONLY if evidence is provided that the bunch is from the same roll. Absent that evidence, this approach is not valid.

For question 3, I would use 6 months, but not more than a year.

For question 4, it is a good sanity check. That can be done on the entire lot quarterly, but it is not a substitute for calibration.
 

BradM

Staff member
Admin
#33
Hello there!

We use ovens for some of our processes. The range in question is 100 - 250 deg F with a desired tolerance of +/- 10 deg F.
Is the ?10?F for each thermocouple, or is that the uniformity tolerance of the oven?

8 ovens temperatures are verified quarterly.
We use a Fluke 51 that is NIST calibrated yearly with k type thermocouples.
Off of the top of my head, those thermometers have around a ?2?F tolerance; so that needs to be added to your thermocouples.

Ideally I would just purchase new thermocouples yearly and have them NIST calibrated, however I am part of a small company who is trying to keep costs down.
Again re-iterating acceptable range for the process is +/-10 deg F.
Would it be acceptable to do the following:


Purchase an Omega 5 pack of K type thermocouples (5SC-TT-K-30-36). Obtain a 3 point NIST cal on one of the 5 as they ??? typically calibrate one out of the pack of 5, which counts as a lot calibration.?

Question 1) Due to our infrequent use would it acceptable to keep a single thermocouple from the lot as a ?gold standard? which would be set off to the side and then only used when verifying yearly against one of the other thermocouples?

Question 2) Is it acceptable to use a concept of ?lot calibration? for thermocouples?

Question 3) if you have a ?gold standard? thermocouple that is sitting off to the side in ambient conditions, how long would you consider it be acceptable before having a NIST calibration performed again? (1 year, 2 year? )

Question 4)..... or would it be more preferable to not use the gold standard as is question 1, but instead to verify against crushed ice / boiling water as a verification of proper functioning?
Honestly, what I would recommend, is to purchase a 500 foot roll of Type K special limits of error wire, and have both ends calibrated. Now... I say that with reference to AMS2750 (an aerospace specification).

What is your industry? Do you have additional regulatory and/or customer requirements that should be considered?

As far as your question 4, verifying a questionable reading using an ice/ boiling point method is OK... just for a quick check. Besides that, those two methods are pretty laden with error, and should not be used as a calibration method.

As far as your "golden" standard, you really need to purchase a standard significantly more accurate than the instruments you are verifying. :)
 

qu1nn

Involved In Discussions
#34
Hello,

First off, thanks for the quick responses.
The desired tolerance that I stated (+/-10 deg F) is the tolerance of the oven temperature.


I work for a company that manufactures load cells / electronics for use in weigh devices. Specifically the ovens are used in a process where the strain gage is bonded to a load cell. There are inspection stages (visual and test) throughout manufacturing that will determine conformity of the product to the customer / engineering requirements.

Regarding the 500 foot roll of Type K special limits of wire?
(1) Knowing that I am really only using the thermocouple to test quarterly, and we only use 3 ft thermocouples, why 500 ft?

(2)Would you agree with Hershal that the calibration of this wire would only then be good for one year?

Regarding the ice/boiling point method?
(1)What would the typical +/- deg F error be for this method?


It wouldn?t be used as a calibration method, just a verification that the entire setup (meter, probe, method) is within OUR expected range. So if the Fluke 51 II has a +/-2 deg F error and lets say that a roll of special limits wire has an error of +/- 2 deg F (http://www.omega.com/pptst/SLE_Wire.html), thus there would be a total of +/- 4 deg F. Hopefully the ice/boiling method is less than +/-5 deg F so that the entire set of combined errors is less than +/- 10 deg F.


again thank you for your time
qu1nn
 

BradM

Staff member
Admin
#35
Hello,

First off, thanks for the quick responses.
Hey... thanks for the well worded replies. It really helps in answering the questions.

The desired tolerance that I stated (+/-10 deg F) is the tolerance of the oven temperature.
OK. I just didn't know if you performed uniformity surveys on the ovens.

I work for a company that manufactures load cells / electronics for use in weigh devices. Specifically the ovens are used in a process where the strain gage is bonded to a load cell. There are inspection stages (visual and test) throughout manufacturing that will determine conformity of the product to the customer / engineering requirements.
Ah. Ok. That helps. :agree1:

Regarding the 500 foot roll of Type K special limits of wire?
(1) Knowing that I am really only using the thermocouple to test quarterly, and we only use 3 ft thermocouples, why 500 ft?

(2)Would you agree with Hershal that the calibration of this wire would only then be good for one year?
1) Ah... yes. That would be overkill. I just didn't understand your application.

2) Yes. I agree with Hershal on most everything; he is a sharp man. :yes:I would suggest, though, that the wire on the roll has unlimited life until it is cut from the roll. Then it has a shelf life. However, given how little you are going to use, I wouldn't buy that much on a roll anyway.

Regarding the ice/boiling point method?
(1)What would the typical +/- deg F error be for this method?


It wouldn?t be used as a calibration method, just a verification that the entire setup (meter, probe, method) is within OUR expected range. So if the Fluke 51 II has a +/-2 deg F error and lets say that a roll of special limits wire has an error of +/- 2 deg F (http://www.omega.com/pptst/SLE_Wire.html), thus there would be a total of +/- 4 deg F. Hopefully the ice/boiling method is less than +/-5 deg F so that the entire set of combined errors is less than +/- 10 deg F.
Good question, but I couldn't even venture a guess. There are so many variables involved. There is a document here on building an ice mantle, but even then...

With boiling water, you would need to determine depth, consistency, etc.

What I might do is contact Pyromation. Get them to spec you out a thermocouple for your use. Also, if you don't have ports in your oven, I would consider adding them. That way, you're not slamming doors on the wires all the time. But.. sometimes you don't have a choice.

http://www.pyromation.com/Catalog/ac14.pdf

I would get a MGO stainless steel probe (1/8 or 1/4 thick for your application) with plenty of lead wire. Make sure everything is rated for your temperatures. Have it calibrated by Pyromation for the temperatures you use. I would set a one year interval for it; then order a new one.

I would start a spreadsheet, and record the offset for the probes at the temperatures. Then when a new probe comes in, put it in the oven side by side with the "old" probe and verify the readings. You can at least tell if the probes are consistent and haven't varied greatly.

The third option: find a local competent calibration source that calibrates temperature. Carry them the Fluke meter with the thermocouple attached and have them calibrate it as a loop. I would have a spare meter/ thermocouple to use while that one is out for calibration. I think this would give you the best measurement solution at the lowest cost. :)
 

qu1nn

Involved In Discussions
#36
2) Yes. I agree with Hershal on most everything; he is a sharp man. :yes:I would suggest, though, that the wire on the roll has unlimited life until it is cut from the roll. Then it has a shelf life. However, given how little you are going to use, I wouldn't buy that much on a roll anyway.

I am kind of confused on the above statement: this is how I read it:
If I have a roll of thermocouple wire, the wire has been NIST calibrated, it will have an unlimited shelf life.
when I cut off a piece and make a thermocouple the shelf life is 1 year max (prior post from Hershal)

Did I get that right?


Regards
Qu1nn
 
Last edited:

BradM

Staff member
Admin
#37
I am kind of confused on the above statement: this is how I read it:
If I have a roll of thermocouple wire, the wire has been NIST calibrated, it will have an unlimited shelf life.
when I cut off a piece and make a thermocouple the shelf life is 1 year max (prior post from Hershal)

Did I get that right?


Regards
Qu1nn
That is one way to approach it. However, I wouldn't push it with a 15 year old roll of wire! :) the principle is that you buy a roll and cut off to make a T/C. That T/C will have a shelf life of six months or less, if they are destroyed during the survey process. If you don't think you would use up a roll of wire in a couple of years, it may not be worth it.

In general, a "Calibration" performed on something does not last forever; it needs to be re-certified.
 
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