3 WAYS TO DETERMINE THE TOLERANCE OF INSTRUMENTS WITH A CALIBRATION CERTIFICATE – If the Tolerance 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 believe that someone out there also needs this answer and I hope that it could help.

And here is His 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?” Below is the remark with a photo.

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

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

Calibration Results
Calibration Results with remarks.

Now, let us discuss…

First Question:

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

Passed or failed remarks are usually termed in calibration as “In-Tolerance” or “Out-of-Tolerance”. This is not usually displayed in most calibration certificates if it is not relevant or appropriate.

Why? Because an out-of-tolerance condition is only determined and decided by the user of the instrument. His decision could be from a process requirement or an international standard requirement.

The lab will only reflect the result, and based on this result, it is up to the user to decide its suitability or acceptability if still fit to use.

 In any case, if an Out-of-Tolerance condition is observed, it will be reflected in the calibration report but the customers are first informed.

But If tolerance is not provided, how can you determine or decide if your instrument is still fit for use or not based on the calibration result? Read further…

Below are 3 ways to determine if your instrument is fit to use or not based on the calibration certificate. Just choose one where you are comfortable and very much applicable to your process.

  1. Determine your process tolerance;
  2. Check the specifications of the instrument in the user manual;
  3. Use the uncertainty results.

I will explain further in more details….

3 Ways To Determine If The Instruments is Within the  Tolerance based on a Calibration Certificate

First, refer to your process tolerance, it is determined during the design of your measurement process (it should be available). 

During calibration, the lab will determine how accurate the caliper is by basing it on the reference value. By having a reference value, an error (Measured value-True Value) will be computed and reflected in the calibration certificate.

For example, in a caliper, the error displayed in the calibration certificate during the calibration on the  20 mm range is +0.06 mm.

If the user has a tolerance based on their process which is +/-0.04mm, then, this means that it is already “out-of-tolerance”.

This kind of instrument 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.

 

accuracy specs of a caliper based on its user manual
accuracy specs of a caliper based on its user manual

Second, check the specifications of the caliper in the user manual, look for the accuracy specification and get the value (you need a little research).

Again, determine the error displayed in the calibration certificate. Compare results if the 0.01 error (based on the sample certificate above) is within the accuracy range of the caliper (which is 0.03 in the above photo).

As per the result of the comparison, 0.01 is within the limits of 0.03 so it is in “Tolerance” or fit for use.

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 basis for tolerance. If the readings of the caliper are within this uncertainty value, then it is a good measurement and the caliper is still fit for use.

Since uncertainty results are usually very small, that will result in a strict tolerance, the possibility of a failed result is higher. 

What you can do is to multiply the uncertainty results by a factor of 2 to expand it to a 95% confidence. Ensure to document this procedure to support an audit question just in case.

based on the above result, the tolerance would be:

 tolerance = 0.014×2 = 0.028 mm or 0.03mm

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 it is still up to you as long as it is properly documented and aligned in your process.

2nd Question:

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

‘The user shall decide the usability of this instrument’

Every calibration lab is required to put their remarks based on their opinions and interpretations once calibration is already performed and results have been provided.

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.

So how do we decide the usability?

Below are my suggestions:

1.      Review the results and see if it is within your applicable tolerance or specifications (refer to question number 1 answers).

2.      Use the indicated correction factor or compensate for the error using the correction factor every time you use the UUC for measurement.

3.      Perform a functionality check once received from the cal lab. Read more below..

What can we do after we received the calibrated instrument?

In addition to the determination of tolerance 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.,

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, 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.

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 sample, 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)

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

During accreditation, the accreditation body will check all your capability and reflect the result on the scope of the accreditation certificate.

In this case, 300 mm range is not included in that scope. This means that the lab is not accredited to perform the calibration.

Before you give the calibration work to a lab, or before calibration takes place for your caliper, you should be informed in any way and have an approval or agreement from them.

This is an SOP as an Accredited Calibration laboratory but sometimes you need to ask or inform them.

There is no problem here if you are not using that range, or it is not that critical that you can support during an audit.

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 the3 Ways To Determine If The Instruments is Within the  Tolerance based on a Calibration Certificate and answered the question ‘What shall we do with the instrument when we received the certificate with this mentioned statement?’

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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