3 WAYS TO VERIFY THE MEASUREMENT RESULTS IN A CALIBRATION CERTIFICATE – If a Tolerance For Pass Or Fail Is Not Given

with No Comments

 

I was messaged by one of my subscribers 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 actual questions with a photo of the calibration certificate.

 “Below is a calibration certificate for my digital caliper & there is a remark stated as ‘The user shall decide the usability of this instrument’.

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

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

Calibration Results
Calibration Results with remarks.

‘The user shall decide the usability of this instrument’

This statement means that the suitability of using the instruments is still into the user that He/She should make a proper decision based on your process on how to use the result that is provided by the lab.

The Calibration Lab will only reflect the measurement value but will not instruct you on how to use it. Most remarks that you will read on a calibration certificate are mostly to support the results that they have gathered based on their calibration procedure.

And this is because only the user is the one who knows where to apply the given measurement value that is why “the user shall decide the usability of the instrument” but in consideration to the results provided by the lab.

Continue reading to learn how to decide.

 

3 WAYS TO DETERMINE THE TOLERANCE OF INSTRUMENTS IN A CALIBRATION CERTIFICATE

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

1st question:

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

Other 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 UUT has passed or failed, or acceptable or not.

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 result of the measurement 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?

Below are some check items 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.

 

WHY DO WE NEED TO VERIFY THE RESULTS IN A CALIBRATION CERTIFICATE UPON RECIEVE?

Check the answer in the second question below.

2nd question:

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

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 verification 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.

 

 

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 the 3 ways on how to determine the tolerance of a calibration result just in case it is not given by the calibration lab, and use this result to determine a PASS or FAIL decision.

I also discussed that this is an SOP in order to perform a quality control during the receiving process.

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

 

 

Leave a Reply