As a Calibration Technician, an Equipment Engineer, a QA or just a user of a calibration certificate, let me ask you below questions:
Q1: Do you know how to interpret a calibration certificate during measurements and/or preventive maintenance?
Q2: If you receive a newly delivered calibration certificate, what would you do?
Q3: What are the important parts of a calibration certificate we need to fully understand?
Q4: What are the requirements of a calibration certificate based on ISO 17025?
Q5:How can you determine and understand if the instruments you are using are performing as expected?
In this post, I will share with you the importance of a calibration certificate review, discuss its true purpose and how to use it properly. I will also explain and answer the questions I listed above.
Once you understand its features (I will call it features because of its application), you will appreciate the importance of a calibration certificate while performing measurements, preparing for an audit, or just simply evaluating the performance of your measuring instruments.
I will also share the requirements of a calibration certificate based on ISO 17025 as applied to an accredited calibration laboratory that every Calibration Technician must know.
What is an Accredited Calibration Certificate?
I believe most instrument users are aware of what a calibration certificate is. But I am not sure if all who uses a measuring instrument with a calibration certificate knows how to read calibration certificate content.
A calibration certificate is a record of the results of calibration performed on a specific Instrument or measuring equipment. This is also known as calibration report.
An accredited calibration certificate has the below information:
- Issued by an accredited calibration laboratory
- It has traceability information, these are:
- The details of the reference standard used
- The measurement results
- The estimated measurement uncertainty results
- The name of the Higher Lab or Organization where traceability originates (ex: NIST)
- It has all the elements that I listed below. It complies with the requirements of ISO 17025:2017 under clause 7.8, reporting the results.
- And of course, it has the logo of the accreditation body.
To ensure that the lab is accredited under ISO 17025, we should have a copy of their accreditation certificate and scope as evidence that they are accredited.
This is to differentiate it from other non-accredited calibration records where there is no measurement uncertainty result and with limited information.
These kinds of certificates are just a verification result/record.
Mostly, what I observe in most users (quality personnel, safety officer, or other technicians) is that they only use the calibration certificate as evidence that the measuring instruments that they are using are calibrated.
Only a show part for the auditors telling ..“Hey Mr. Auditor, our Instruments are calibrated, we have here the calibration certificate, we have complied with your requirements”.
And some technicians, only use the calibration certificate to verify the date and due dates if the instruments are still within the said calibration date and nothing follows.
Sometimes, calibration certificates are kept directly without reviewing them.
We need to change this habit and start using the calibration certificate for its intended purpose.
Below are the Calibration Certificates Specific Requirements Based on ISO 17025:
- A title (calibration certificate).
- The name and the Address of the Calibration Laboratory.
- Unique identification number of the calibration certificate and the pages until the last page of the calibration certificate ( an example is a serial number; page 2 of 2).
- The name and contact details of the customer.
- Identification of the calibration method.
Other evidences of traceability that can be seen in the calibration certificate are:
- The LOGO of the accreditation body
- Measurement Uncertainty results
- The details of the reference standard used
- The Measurement results
>> It means that the calibration certificate cannot be reproduced (to make another copy) in part only where some page is missing (in cases where there are multiple pages). Certificates are allowed to be reproduced considering that it was reproduced with a complete number of pages. This will ensure that the user will get the complete information, and therefore, proper interpretation of data results. In any case, you want to reproduce it with one part only (1 page only), ask first for the approval of the laboratory.
>> To understand easily, let us take the opposite statement, it can be: “The calibration certificate can only be reproduced in complete pages only.”
We should consider the above list when creating a format for a calibration certificate, following the requirements above will ensure that all relevant details for a calibration certificate will be covered.
Their location in the calibration certificate is not the same, some certificates are one-paged only while others are 2 pages or more. It depends on the measuring instruments being calibrated.
Furthermore, as per Clause 188.8.131.52, which states that “When agreed with the customer, the results may be reported in a simplified way”.
This means that some Information stated above can be omitted or not reflected in the calibration certificate as long as it is agreed with the customers or users. But make sure that the omitted information is available in the records or raw data.F
What To Do Once You Have Received a Calibration Certificate?
Definitely, we review them. But what needs to review, or in simple terms, do you know how to read a calibration certificate? We can use the above list as a checklist when reviewing a calibration certificate or verify the completeness of a calibration certificate.
If you received a calibration certificate from a calibration laboratory, it should contain the above list or ensure that it follows the above requirements. More specifically when the calibration laboratory that we have outsourced is not accredited to ISO 17025.
If you are under quality or handling calibration, one of your main concerns is to check all the above requirements of a calibration certificate to verify if complete or properly labeled and spelled.
One mistake for example, on a serial number, will invalidate the traceability of the instrument, traceability in a sense that the identification on the specific instrument is lost.
In addition to most details, I want to emphasize the Number 12 in the list above which is the “Statement of compliance with requirements and specification”.
This should be reviewed immediately to ensure that a newly calibrated instruments have passed the calibration and are safe or acceptable to use. Below is a sample statement.
There are some cases where you will encounter an “Out of Tolerance” results upon receipt from the external calibration lab.
By determining this in the first place will prompt you to apply the corrective actions. (please see below info on how to check the results or data under as found/as left)
If ever you encounter an OOT (Out-of Tolerance) results, one corrective action is to adjust or reduce the calibration interval. How to perform this? Check out my other post here>> calibration interval adjustment
What to Inspect in a Calibration Certificate During Measurement and/or Preventive Maintenance?
Many of the technicians engaged in measurements or preventive maintenance do not check or review the calibration certificate in terms of the result of calibrations.
They are already confident that the calibrated instruments they use are accurate. This is what I usually hear, ”My caliper is accurate because this is newly calibrated!”.
Yes, they are correct at some point, but not always…if this is how you think about calibration then you are missing something. Not all calibrated instruments are being adjusted during calibration.
Once a newly calibrated Instruments are received with a calibration certificate, of course, one must know how to review and interpret the content of the calibration certificate.
This is not just a job of quality personnel (calibration technician) but specifically, the one who directly uses the instrument, the technician, or the operator.
But if you are a technician or an engineer directly using the instruments, in addition to the above requirements, concentrate to review on below checkpoints:
- As found /as left data
- Traceability information
- Uncertainty results
Usually, the above contents can be seen on the second page of the calibration certificate, we commonly call it calibration results (for the traceability details mostly it is on the first page).
How to Use the Calibration Certificate during the Measurement Process and/or Preventive Maintenance?
We need to determine and understand the calibration certificate on this part as an engineer or technician simply because it has a significant effect on our measurement process. Most of the calibration results have what we call a Correction Factor.
To understand better, below is an example:
Correction or Error
What is a Correction Factor and How to Calculate Correction Factor in calibration results?
When we are measuring temperature, for example at 0 settings using our newly calibrated thermometer, we notice that at zero points; it has a correction of + 0.035 (where the error is -0.035).
Therefore, every time that we make a zero-point measurement, our actual value should be added with a +0.035. The same thing with the other test points.
We call this value a Correction Factor (CF). It is a value that we either add or subtract in the actual measurement in order to compensate for the error. Correction Factor is the opposite of Error
It is simply the difference of STD value and the UUC results. To calculate correction factor, just subtract the ‘UUC reading’ from the ‘Nominal Value’ (STD-UUC).
Below is an example based on the table above.
…… Nominal Value = 10.00
…… UUC reading = 9.924
…… Therefore: 10.00 – 9.924 = 0.076, which means that the correction is +0.076
This is important if we are targeting a smaller error or implementing a strict tolerance. In this way, we will ensure that the results we get are the most accurate ones.
The most technician does not use this correction factor because they do not review the certificate they received or does not know its purpose (luckily if they have a wider tolerance in which correction does not matter).
They simply use the instruments and knowing that it is already perfect because it is calibrated.
The correction factor is very important in Measuring Instruments or machines that can be adjusted physically or electronically.
An example of this is during preventive maintenance, once we determine the correction factor, we can input this on the calibration part or program of the equipment and compensate or correct the errors.
As found/As Left
If you observe the figure above, it has the ‘as found’ and ‘as left’ column. This is the “before and after” we calibrated the instrument.
It has no correction but as long as the measurement value lies within the limit, your measurement is acceptable. The ”As Left” column will be different just in case an adjustment was done if ever the “As Found” value is outside the limit.
As an engineer or quality personnel, one should check if ever the “as found” to be within the upper and lower limit. Just in case it is out of this limit, it means that your instrument is “out of tolerance” before it was calibrated.
By being aware of this information, you can make a proper adjustment and perform corrective action and assessment to scope up regarding the affected units or parts where these instruments are used.
To understand more regarding data interpretation, visit my other post >> Differences Between Accuracy, Error, Tolerance, and Uncertainty in a Calibration Results
Uncertainty Results and Traceability
One of the ways to determine that we have a traceable calibration is to check that the calibration results have a computed or estimated measurement uncertainty result.
This will ensure traceability because, during estimation or computation, the accredited laboratory’s BMC (Best Measurement Capability) is included which they also acquire from a higher laboratory standard.
What is measurement uncertainty? Measurement uncertainty is a computed range of value, usually an estimate to quantify the doubt that exists in a certain measurement result of a specific instrument.
It is the combined effect of the most valid sources of error with a confidence level of 95%.
For example on the measured voltage above, 1 mV is the true value where the measured or actual value is 1.01 and the uncertainty result is 0.007960 or (1.01+/-0.007960)mV at a confidence level of 95%
This means that we are 95% sure that the measured value lies within the limits of 1.00204 to 1.01796 (+/-0.007960) mV. The smaller the uncertainty, the better the accuracy.
With this result or statement, clearly, uncertainty and error are NOT the same.
Where to use this uncertainty results?
In creating a specification to determine the tolerance of a certain instrument, we can use the uncertainty results as a basis besides the Manufacturers specification.
Uncertainty results are also used during the assessment of compliance to specifications, included with the tolerance to determine a pass or failed results.
If we are also performing calibration and estimating the uncertainty of measurements (this is another topic), we will use the uncertainty results as part of our uncertainty budget, a type-B source of uncertainty computation.
To see the presentation regarding the use of actual uncertainty results, visit my other post
>> Differences Between Accuracy, Error, Tolerance, and Uncertainty in a Calibration Results
>> 8 Ways How You Can Use the Measurement Uncertainty Reported in a Calibration Certificate
We have determined what is a calibration certificate and its importance in our industry and in our measurement process. The need to review a calibration certificate and what are the necessary checkpoints which are based on ISO 17025 requirements are discussed. And finally, the important requirements to consider for technical personnel in the calibration certificate when measuring or performing an equipment preventive maintenance are also presented.
In using a calibration certificate, one must also be familiar with the principles of calibration through training on calibration awareness.
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