Let me ask you below questions:
- As a Calibration Technician and Equipment Engineer, do you know how to interpret a calibration certificate?
- How to use a calibration certificate during measurements and preventive maintenance?
- If you receive a newly delivered calibration certificate, what would you do?
- As a Calibration Technician and Equipment Engineer, what are the important parts of a calibration certificate that we need to fully understand?
- What are the requirements of a calibration certificate based on ISO 17025?
- Regarding SAFETY concern, 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 that 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 of us if not all are aware of what a calibration certificate is. But I am not sure if all who uses a measuring instrument with calibration certificate knows how to use one.
A calibration certificate is a record of the results of calibration performed on a specific Instrument or group of instruments. This also called a calibration report.
Usually, an accredited calibration certificate have below information:
- issued by an accredited calibration laboratory
- It has a traceability information about the standards used
- the estimated uncertainty of measurements result is reflected
- It has all the elements that I listed below. It complies with the requirements of ISO 17025:2005 under clause 5.10, reporting the results ( clause 7.8 in the 2017 edition).
- 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.
This is to differentiate it to other non-accredited calibration records where there is no measurement uncertainty results or no traceability.
These kinds of certificates are just a verification result/record.
Mostly what I observe in some users (quality personnel, safety officer, or other technicians) is that they only use the calibration certificate as an evidence that the measuring instruments that they are using are calibrated.
Only a show part for the auditors. (“Hey Mr. Auditor, our Instruments are calibrated, we have here the calibration certificate, we have complied with your requirements”).
And some technicians, they 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 requirements for calibration certificates 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 the address of the customer.
- Identification of the calibration method.
- A description of the condition and identification of item calibrated.
- The date of the receipt of the calibrated item and the date of the calibration.
- Calibration results and the unit of measurement. ( see below table-under correction or error)
- The names, functions, and signature of persons authorizing calibration report.
- A statement to the effect that the results relate only to the item calibrated.
- Deviation from the calibration method, and information on specific calibration conditions such as environmental conditions (an example is a limited calibration).
- Where relevant, a statement of compliance with requirements and specification.
- The result of the estimated measurement uncertainty of the equipment ( see below sample tables)
- Where appropriate and needed opinion and interpretations.
- the environmental conditions under which the calibrations were made that have an influence on the measurement results;
- evidence that the measurements are traceable to SI units of measurements.
- A statement specifying that the calibration certificate shall not be reproduced except in full, without the written approval of the laboratory. ( This is usually seen at the bottom of the certificate page.)
- Additional information which may be required by a specific method, customers or groups of customers.
Above list should be considered 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.
What do you do once you have received a calibration certificate?
Definitely, we review them. But what needs to review? 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, they should contain the above list or ensure that the above requirements are followed. 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 concern 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 the instruments that are newly calibrated have passed the calibration and is safe or acceptable to be used. Below is a sample statement.
There are some cases where an “Out of Tolerance” results will be encountered upon receipt by the external calibration lab.
By determining this in the first place will prompt you to apply the necessary corrective actions. (please see below info on how to check the results or data under as found/as left)
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.
Most are assured 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 use the instrument, the technician or simply the operator.
But if you are a technician or an engineer directly using the instruments, in addition to the above requirements, you should 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, it is commonly called as 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 great effect on our measurement process. Most of the calibration results have this what we call a Correction Factor.
To understand better, below is an example:
Correction or Error
Based on the figure above, as we observe, there is the column of a correction.
When we are measuring a temperature, for example at 0 setting 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 correction a correction factor, this is added or subtracted in the actual measurement that you have in order to compensate for the error.
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 obtain are the most accurate one.
The most technician does not use this correction factor simply 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 a 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” the instrument was calibrated.
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.
But if you want to use a correction, a simple formula is used: Just subtract the ‘Nominal Value’ from the ‘As left’ value. ( True value-Actual Value)
Example: 1.00 – 1.01 = – 0.01, which means that the correction is -0.01
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.
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 value, usually an estimate to quantify the doubt that exists in a certain measurement 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 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 +/-0.007960mV. The smaller the uncertainty, the better the accuracy.
With this result or statement, it is clear that uncertainty and error are NOT the same.
Where to use this uncertainty results? In creating a specification in terms of determining the tolerance of a certain instrument, we can use the uncertainty results as a basis in addition to the Manufacturers specification.
If we are also performing a 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 A source of uncertainty computation.
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 regarding the principles of calibration through a training on calibration awareness.
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