When To Calibrate?
Calibration is a requirement before using any measuring instrument that has an effect on the quality of products or services provided.
The main purpose (in a technical sense) why we need to perform calibration are:
To determine the exact values of the output parameters of our instruments- Accuracy.
To determine or calculate the measurement uncertainty which has an effect to the validity of measurement results.
To calculate the correction factors to be used in our measurement process to remove or compensate the encountered errors.
In addition to this, calibration is needed if:
You need a calibration report to determine the accuracy of your Instruments to make corrections to the measured value.
Requirement of a customer, standard, process or a regulatory requirement to ensure confidence of the instruments to be used in the measurement process.
To established metrological traceability of standards used.
If equipments has been mishandled or overloaded or give questionable results.
ISO 9001 requirements (see Clause 7.1.5.2), which states that Any instrument that is used to verify the conformity of products or needs confidence that the instrument is providing a valid results, then it should be calibrated.
Calibration provides you with the measurement results where you can calculate the error and the measurement uncertainties. This in return will inform you how accurate your instruments are just by looking at the errors encountered. The smaller the errors the more accurate your instruments.
By having the calculated errors at hand, you can determine the necessary corrections in order to perform the necessary adjustment during the measurement process. The adjustment is either system adjustment (automated adjustment) or through the use of a ‘correction factor’. With the use of correction factors or automated adjustment (for some instruments), we can now make our measuring instrument more accurate.
When To Verify?
Our main goal in verification is to confirm if our measuring instrument’s measured value (DUT output) is within the specified acceptance criteria or tolerance limit which leads us to a ‘pass’ or ‘fail’ decision.
Verification can be performed before or after a calibration takes place. This is one part of quality control in order to determine the before and after the status of your DUT. See #6 below.
The following are some reasons why we perform verification:
To ensure that a measuring instrument in use is measuring accurately while it is still within the calibration interval (calibration due date). This will fall under the process of ‘Intermediate Check’.
After installation or reinstallation of a new instrument to a measuring system.
Important change in the measuring instruments such as transferring to a new location or update of a new software.
You are performing a ‘Test’ to confirm if it will pass or fail that will lead to a decision.
If you want to determine the accuracy at a specific range only.
As part of quality control where we check our instrument every time it is returned back from a 3rd party calibration. A requirement under clause 6.4.4 of ISO 17025:2017 which stated that: The laboratory shall verify that equipment conforms to specified requirements before being placed or returned into service.
In relation to a calibration method, we need to confirm if it is suitable with the existing reference standards to apply with the calibration procedure before introducing it for use. This is a requirement under clause 7.2.1.5 of ISO 17025:2017 Standards, which states that: The laboratory shall verify that it can properly perform methods before introducing them by ensuring that it can achieve the required performance.
If the measuring instrument has been mishandled or overloaded or give questionable results.
During the receipt of a newly calibrated DUT or after using the Reference Standard onsite – also an Intermediate check.
Also, one good reason to perform verification is to determine if the measuring instrument we use is still fit for its intended purpose. If the intended purpose is to monitor temperatures at a specific range, then we should verify that range if it is within the tolerable limit of the set temperature, not necessarily the whole range.
Then what comes next after Verification?.
Depending on your results, below are some dispositions after verification, these are:
to recalibrate,
to adjust, either automatically, mechanically or through the use of a correction factor
extend/lessen usability or calibration interval,
scrap or make obsolete.
Both (Calibrate and Verify)
Both calibration and verification are used to maintain fitness for purpose of the monitoring and measurement resources in every industry.
If you are implementing a calibration lab whether an in-house calibration lab or an external calibration lab, these two processes mostly come together.
Some examples are:
After every calibration that is performed, a verification always follows in order to determine the acceptability of results and the assurance that the calibrated instruments are in confidence and fit to use.
Before calibration, we perform verification first to determine if the DUT for calibration is functioning well and still in tolerance, this is what we call the “as found” status. If the DUT is still in tolerance, then no adjustment is needed, we continue to calibrate as per the calibration procedure and a final calibration report will be provided.
Both calibration and verification are also applicable during the review of results in a calibration report. Applicable in the sense that we review first the calibration result before we perform verification using the provided tolerance limit. This is done by comparing the “error” results with the tolerance limit of the DUT that is reported in a calibration certificate. See the example below:
This is also the part where we need to learn how to interpret a calibration certificate. To check any remarks and to determine any failed or out of tolerance results.
As we can see, the ‘verification process’ needs to meet the requirements of the ‘calibration process’ in order to have a valid decision during verification.
One good example that we always verify and calibrate is the weighing scale. We use a standard weight in performing calibration and verification.
First, we verify the scale by putting the standard weights on its platform, in this process, we should know the expected value and the tolerance limit.
If the measured output of the scale is within the tolerance limit, it has “passed” our verification then calibration is not needed. But if it fails, we need to adjust and then perform calibration.
We need to review again the measurement results after calibration and use the results to perform another verification to ensure that it is fit to use.
Every time the scale has “passed” our verification, we can continue to use it. But at the time the weighing scale is out of tolerance, then it is time to send it for calibration or perform a calibration.
Conclusion
Now to answer the question, “what needs to be performed first, to verify or to calibrate”?
The answer to this question is to “calibrate first before verification can take place”. One main reason is, we need first the calibration data (measured value and/or error value) before we can verify the results using the specifications of the measuring instrument.
Also, it is a best practice to perform verification after calibration in order to determine if the calibration performed is acceptable. If the results are not acceptable, then we need to adjust and recalibrate the Instrument.
Another way to determine that calibration takes place first is through the calibration certificate. If we observe the calibration results (presented above), we need to determine first the error by subtracting the STD and DUT results, then afterward, we compare the error (or the measured value) with the tolerance limit or MPE.
When it comes to choosing whether to calibrate or verify, ask this question, “what is my goal in doing this measurement, is it to determine the accuracy (error or correction) or to determine if it is still suitable for its intended purpose?“
If your goal is to determine the ‘accuracy’ only of the DUT for the purpose of correcting or removing errors, then you need to perform calibration only.
But if your goal is to determine if an instrument is ‘within its specification and fit to use’, then perform only verification using a calibrated standard.
If your answer is both, then you need to perform calibration and verification.
