Accuracy Problem on the Calibrator - Two Labs with different Results

C

Calsun

#1
Hi all,
I?m new guy for working with calibration laboratory. Now I have a question to enquiry expert to solve my problem.
The question is that, we have a calibrator for external calibration in each of year. It?s calibrated by Manufacturer (Lab A). Now, due to the cost problem, we have selected another Lab B for doing calibration on this year. After calibration from Lab B, they points out some of test point were out of manufacturer?s stated 1-year relative accuracy limits. However, Lab A was no problem in the past.
Does anyone know what the problem here is? Both lab A and B are accreditation laboratory. Why their result has such as big difference? How can we judge which lab has problem?
Thanks.
 
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Jerry Eldred

Forum Moderator
Super Moderator
#3
There is not a definitive easy answer. It could have been out of tolerance the second year. If it is a certain high quality brand name calibrator - okay, I'll go ahead and say it - if it is a Fluke calibrator and you sent it to their standards lab, I have great trust in them. If it is a high quality OEM (Original Equipment Manufacturer) lab in the first attempt at calibrating your calibrator, I would call into question the second lab's assessment. If Lab A was a lesser quality OEM calibration standards lab, that may call them into question.

There is always also the possibility that your calibrator went out of tolerance.

If you don't feel comfortable putting actual details on the forum, that is understandable. If you wish, you may send me a PM with actual manufacturer and lab names, along with what was out of tolerance, and I can give a more detailed opinion.
 

BradM

Staff member
Admin
#4
Just a few thoughts in addition to the ones from my esteemed colleague from Texas. :bigwave:

Jerry is right; the instrument might have just been out of tolerance. But your situation is one I have seen before. Consider...

1. Two labs may have different uncertainty calculations. The Lab B may have a larger uncertainty than the mfg. and thus, may have more error contributing to the measurement.
2. Unless two labs use the identical procedure for calibration, it may be that the error was detected using a different method/ test points than was used at the mfg.

I have seen some 3rd party calibrations that I had more confidence in than the mfg. However, some mfg. have a pretty decent metrology program, with many of them being accredited under ISO 17025.

All that said, it would be difficult (I think) to say the Lab B calibration is suspect just because they found the instrument out of tolerance. I would be more interested in determining if Lab B is competent.
 
M

Murphys Law

#5
How much is it out of calibration by? If marginally over, it maybe within measurement uncertainty.
 

Jerry Eldred

Forum Moderator
Super Moderator
#6
Good point Murphy's Law. Depending on how far out of tolerance the calibrator is, this may not even be an issue (although the normal requirements for how you deal with an out of tolerance measurement standard is its own issue - potential implications to instruments calibrated by it - another topic).
 
C

Calsun

#7
Thanks for all.
I have uploaded the error comparison table for both Lab. Please see attachment.
I have missing one point to mention that the Lab A was calibrated and do with adjustment for each time and Lab B haven’t done for this issue due to have not software for doing that. And then, checked the report, the uncertainty of Lab B is smaller than Lab A.
Question:
1) Does anyone know how to determine the problem? What kind of method can I to do?
2) Could the uncertainty larger than error?
3) How can i ensure the history of stability is not broken when the calibrator was adjusted and calibrated at each time?
And the last, since I’m new guy for touching the DMM calibration, does any expert have some reference book to recommend for me?
Thank you very much for all.

P.S. The attachment file of the Uncertainty for Lab A have some thing wrong. the correct value is 2.6ppm + 1.4uV for 100mV to 200mV range and 200V to 1000V is 3.3ppm.
 

Attachments

Last edited by a moderator:

Jerry Eldred

Forum Moderator
Super Moderator
#8
The amount of errors does not strongly point to either lab being bad. One good way to establish which one is bad - if either - would be to re-send the calibrator back to lab A and have them re test the measurands in question. Or find a third lab to send to and have it re-tested. You need to have confidence that were ever you are sending your calibrator, you are getting legitimate readings. If you are strongly concerned about a quality issue with one or both of the labs, you could communicate your concern with either or both of them.

There is a definite possibility that neither lab is bad. Something could have happened during the year that shifted the output of the calibrator.

Also, if it is one of the Fluke series multifunction meter calibrators, there is a zero adjust done after warm up that could possibly cause a shift. I don't know if that would indeed be the issue, but one of the possibilities.

I guess I would like to know the names and locations of the two labs, and models of the calibrators. It is understandable if you don't want to publicly post those names - and I wouldn't post the names either. But I could take a look at their accreditation information and maybe have some more ideas. I would not share any information publicly.
 
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