Can Uncertainty be larger than the Specification on a Certification

adamt

Involved In Discussions
#1
I have a certification on an electrical device that states a larger measurement uncertainty than the specification of the UUT. I have not seen this before. Can someone give me guidance.

I have always been under the impression that if your uncertainty was larger than your specification that you could not be sure of your measurement results were within the stated tolerance allowed.
 
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Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#2
At the face of it my answer would be "no." But the subject is not so simple. An article titled "Uncertain about uncertainty?" is available to (I hope) help understand the subject. An excerpt from the article is as follows:
Uncertainty, on the other hand, is a range of values likely to be encountered during a measurement. A simple example is a format such as 10 mm ± 0.005 mm. This means that a measurement of 10 mm will have values ranging from 9.995 to 10.005 mm. And that range of results may be made up of both systematic and random errors.
The literature by itself is not all there is to the subject - I think you would be more interested in Test Uncertainty Ratio (TUR) as described in the paper TEST UNCERTAINTY & TEST UNCERTAINTY RATIO (TUR), and A2LA has put out a piece of guidance titled Guide for Estimation of Uncertainty of Dimensional Calibration and Testing Results that I hope will also be of some use.
 

adamt

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#3
Thank you for your reply. I have read the documents you posted. It is interesting that you bring up A2LA do to it is an A2LA accredited certification that is in question. It still does not answer the question out right.

I don't understand the uncertainty on this issue. :mg:

Maybe I should be asking for a opinion pole on this issue.
 

Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#4
I was concerned about whether or not my reply answered your question. Let me make sure I understand you: are you saying your instrument's stated uncertainty (as listed on calibration certificate) exceeds your dimension acceptable tolerance window?

If so, let us begin again with a paper I found on the uncertainty subject, titled IMPORTANT FEATURES OF ESTIMATION & INTERPRETATION OF MEASUREMENT UNCERTAINTY. Sections of particular interest to us are 3.1 and 3.2.

Uncertainty is a big subject that is beyond what most of us do; A2LA (I like A2LA for putting out these nice guidance documents) has published a guidance document titled A2LA Guide for Estimation of Measurement Uncertainty In Testing that goes into more depth than the first paper.

The uncertainty scenario you describe seems very unusual. So I have questions:

Does your instrument have a resolution at least 10 times greater than the increments of your specification tolerance?

What is the range of your instrument as compared to the specification window?
 

adamt

Involved In Discussions
#5
Thanks for your quick response.

The instrument is a temperature calibrator with a 30 to 400 deg C range. Resolution is .1 deg C accuracy is +/- 2.1 deg C.

The uncertainty on the cert is 2.66 deg C. There are four other data points listed and all have uncertainties greater than the specification of the UUT.
 

BradM

Staff member
Admin
#6
Thanks for your quick response.

The instrument is a temperature calibrator with a 30 to 400 deg C range. Resolution is .1 deg C accuracy is +/- 2.1 deg C.

The uncertainty on the cert is 2.66 deg C. There are four other data points listed and all have uncertainties greater than the specification of the UUT.
I would suggest the 2.66 deg C is the total uncertainty calculated by the vendor for performing the calibration on the temperature calibrator. It takes several variables into account when estimated. I would hope their uncertainty would be better than that.

It could be the uncertainty for your Unit Under Test though, and may include additional factors other than the calibrator itself. The best way to find out would be contact the vendor and see how they estimated that value.

Without seeing the certificate it's difficult to say.
 

adamt

Involved In Discussions
#7
Hi Brad,

I have talked to the lab in question and they said that an uncertainty for a measurement data point can be larger than the specification. I asked them how and I did not agree and now am asking for advice and trying to get documentation that tells me if I am right or not.

Believe me I have been wrong before. I would like a more definitive answer before I take them off our vendor list. If I am wrong them I will eat crow. I am a CCT and have done lots of uncertainty budgets for our CMCs and UUT. I believe the more information I have the better I can understand this point.
 

Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#8
It's plain enough that I'm in over my head with this one. I have PMed two fellow Moderators with 17025 specialty, to see if they can lend a hand with this question. :cfingers:
 

BradM

Staff member
Admin
#9
Ok, I'm still unsure what uncertainty the vendor is reporting.

Are they reporting their testing uncertainty or an uncertainty for the UUT?

Either way, that's not a very good uncertainty they report. :)

What standard are they using?
 

adamt

Involved In Discussions
#10
They say that it is the uncertainty of each calibration test point with standard and test included. They are using a Hart 1502A and 5618B. The accuracy for these is very good.

I believe they are using the right equipment per manufactures procedure. It is the uncertainty they list on the certification that I have a problem with.

The question still looms can you list an uncertainty larger than the specification and have confidence that the unit is still in tolerance.
 
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