3 WAYS TO DETERMINE THE TOLERANCE OF INSTRUMENTS IN A CALIBRATION CERTIFICATE – If a Tolerance For Pass Or Fail Is Not Given

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I was messaged by one of my reader about a question related to my previous post about “How to Interpret a Calibration Certificate”. See the post here.  It is a good question because I did not cover it on the previous post and it is a good thing to discuss and share it with you.

I will answer and discuss each question in this blog post. I believe that someone out there also needs this answer and I hope that it could help.

Below are the statement in a calibration certificate and the actual questions with a photo of the calibration certificate.

Actual Statement:

“Below is a calibration certificate for my digital caliper & there is a remark stated as

 ‘The user shall decide the usability of this instrument’.

Actual Questions:

  1. How do we know either this caliper is fit to use or not based on this statement & the calibration result?
  2. What shall we do with the instrument when we received the certificate with this mentioned statement?”
Calibration Results
Calibration Results with remarks.

First Question:

How do we know either this caliper is fit to use or not based on this statement & the calibration result?

Based on the calibration result, how do we determine if the Instrument has passed or failed?

Every calibration lab is required to put the statement ‘The user shall decide the usability of this instrument’ once calibration is already performed and results have been provided. The Calibration Lab will only reflect the measured value but will not instruct you on how to use it.

During calibration, the calibration lab follows a specific procedure in performing the calibration inside their lab. A certain condition is strictly followed inside the lab in order to achieve the best results.

Once the UUC (Unit Under Calibration) is released to the customer, a different condition in which the owner of the UUC has now the control.

The above statement means that the suitability of using the instruments is now into the user that He/She should make a proper decision based on the result that is provided by the lab.

And this is because only the user is the one who knows where to apply the given calibration results based on their day to day process, that is why “the user shall decide the usability of the instrument” but in consideration to the results provided by the lab.

But how do we decide on its usability or in other words suitability? Continue reading to learn more.

 Question: How do we know either this caliper is fit to use or not based on the above statement & the calibration result?

Now let us discuss the given questions related to the caliper based on the provided calibration certificate above.

Most labs will reflect tolerances of the Caliper on the calibration certificate based on its specifications from its manufacturer so it is not a problem to determine if the UUC has passed or failed, or acceptable or not.

Some customer will instruct the laboratory about their process tolerance and have it reflect it in their calibration certificate to easily determine a pass or failed result.

But since this is not the main requirement of a calibration certificate, it is not mandatory to include that is why there is that remarks that the lab has made because it is up to the user to determine this based on their process tolerance or how are you using it to provide a pass or fail decision.

This kind of instrument also is non-adjustable that is why you will rely on the result and use a correction factor just in case.

For more details on how to use a correction factor in a calibration certificate, visit this link.

Since it is up to you to determine if it is fit or not, we will use the calibration results in the certificate. We will check the given error and compare to a given tolerance or accuracy.

But how to check the accuracy or determine the tolerance?

 

3 Ways To Determine The Tolerance Of Instruments In a Calibration Certificate

Below are 3 ways to check the accuracy or determine the tolerance that you can implement. Just choose one where you are comfortable and very much applicable to your process.

FIRST, since you are the user, it is up to you how you provide the tolerances that are based on your process, the lab only reflects how accurate the caliper is. If the result of the calibration is within your process tolerances, then there is no problem.

For example, the error display in the calibration certificate on the  20 mm range is +0.8mm and your tolerance based on your process is +/-0.5mm, then this means that it is already out of tolerance.

So what can you do? Use the error to compensate the reading. Make it a correction factor by subtracting  -0.8mm every time you use the 20 mm range.

 But if the errors are very big and found in every range, then maybe it’s time to replace and have a new one.

SECOND, You can check the accuracy of the caliper in its specs on its user manual and see if the result of the measurement displayed in the certificate is within the provided specs of the manufacturer.

 

You can use this as your tolerance also.  Just compare the error on the specifications. Check the accuracy on the specs of the caliper and see if the 0.01 error is within the accuracy range of the caliper.

THIRD and final, check the uncertainty results. The uncertainty result is the combination of all or most of the possible sources of errors that the caliper can receive or encounter. So you can make the uncertainty result as the tolerance. If the readings of the caliper is within this uncertainty value, then it is a good measurement and the caliper is still fit for use.

 

You can do any one of the above check items as part of your quality control. For me, the easiest is to use the second option because that is already the capability of the instrument based on manufacturer’s recommendation, but as I said, it is still up to you as long as it is aligned in your process.

 

2nd Question:

What shall we do with the instrument when we received the certificate with this mentioned statement?

What can we do after we received the calibrated instrument?

In addition to the determination of pass and fail results in the calibration certificate, there is a mandatory procedure that we need to do during the receiving process.

Every measurement result provided by the calibration lab is taken inside their laboratory which means that it has different environmental conditions, etc., and once the instrument is brought back into your facility, it is now exposed to different environment and handling. Moreover, there is a tendency that a mishandling or some outside factors like vibrations or accidental drop may occur.

Once the Instrument is released or out of the calibration lab, it is now up to the user to determine the validity and the usability of the instruments as stated in the calibration certificate remarks.

So during the receiving process, or upon receipt of the instruments, make sure that you handle this properly and use the appropriate procedure for quality control to verify that it is still in good condition.

Usually, the laboratory will report or display in the certificate if the instrument is already out-of-tolerance or defective. Else, just use the check item that I have presented above then the instrument is good to be used if it has passed your review or assessment.

Also as an SOP for quality control check regarding the instruments that are shipped back to your company after calibration, perform what we call a functional check.

Even it is newly calibrated, the handling as  I stated above, during the transport can have an effect on its calibration, therefore it is mandatory to perform a functionality check once you have received the caliper.

Verification of a caliper using a gauge block
Verification of a caliper using a gauge block. Note, you can use other samples with a known value in order to verify the caliper’s performance.

 

 

Use a known unit with a known data where the caliper is being used before it was shipped out for calibration and measure it again. If the result is within your expectation then it is ok.

Do not forget to document this. You may also include in your documentation on how you perform a review or assessment regarding the calibration certificate.

( learn more about the functional check or intermediate check in this link Intermediate Check)

 

I have included below note in this post for additional knowledge.

The NOTE: “Above 300mm range not in accreditation scope “

The statement where the 300 mm range not in their accredited scope means that they do not have the capability to provide you an accredited result. If you do not use this range in your caliper then it is not a problem. But if you are using it, then it should be included in the calibration report.

Before you give them the calibration work or before calibration takes place for your caliper, you should be informed in any way. This is an SOP for an accredited  Calibration laboratory but sometimes you need to ask or inform them.

I Hope that I answered your questions, please do not hesitate to answer me back if you encounter any more concerns about my blog post.

Conclusion

An ISO 17025 accredited calibration lab follows the format of an accredited calibration certificate. It is a must that a calibration lab will provide remarks regarding the observe parameters during the calibration process and reflect it on the certificate if those remarks support a certain measurement result.

I have discussed in this post about the remarked The user shall decide the usability of this instrument’, provided the 3 ways on how to determine the tolerance of a calibration result to determine a PASS or FAIL decision and answered the question ‘Why do we need to verify the results in a calibration certificate upon receive?’

If you have determined your acceptable instrument tolerance and the correction factor to be used, then I can say that you have already determined the usability or suitability of your instrument for your intended use.

For more details on what to review on a calibration certificate, visit this post: How to Properly Use and Interpret an ISO 17025 Calibration Certificate

Do you have other ways on how to determine tolerances or how to determine a Fit or No Fit status of an instrument based on its calibration certificate?

Please do not hesitate to comment.

And do not forget to subscribe and share.

 

Best Regards,
Edwin

 

 

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