I have received the below comments in relation to the last post about the differences between calibration, verification, and validation.
These are the comments:
- Can you give an example instrument with a specification that we can determine or differentiate calibration, verification, and validation?
- When is verification, validation or calibration is the most appropriate for the intended M&TE?
I presented here a concrete example to differentiate these three terms in a direct and hopefully clear way.
Also, through these examples, it will show if your intention to use these terms regarding your M&TE (Measurement and Test Equipment) is appropriate.
Let us start!
As an example to, we will perform a calibration, verification, and validation in a Liquide Oxygen (LOX) Tank. The tank has Differential Pressure Level Gauge (DPLG) and a pressure gauge to determine its level and actual pressure together with Pressure Safety Valve and Pressure Transmitter.
But in this example, I will focus only on the DPLG and pressure gauge for simplicity.
By definition, calibration is the “comparison” of the unknown reading of a UUC to a known reading of the Reference Standard.
During calibration, our objective is to determine how accurate the Differential Pressure Level Gauge by comparing its output reading and computing the error.
Buy determining the error, we can see how far (or close) the value of the UUC to the value of the reference standard, thus, we can also determine the correction value.
For DPLG, based on the photo above, the standard reads at 1519 mmWC while the UUC reads 1500 mmWC.
Error = Measured value – true value
Error = 1500-1519 = -19 mmWC
Therefore, the Correction Factor is +19 mmWC
Moreover, calibration is incomplete without the measurement uncertainty results because this will inform us the exact location (boundary) of the true value. When we know the location of the true value, then a more accurate estimate is determined with higher confidence. Based on the measurement uncertainty results in the above example, we are 95% confident that the exact value of 1519 is within the interval or boundary of +/- 38.5 (between 1480.5 – 1557.5).
Take note that during calibration, we follow a standard procedure to determine how accurate or close (or far) the result is to the true value.
Now, because of calibration, we have determined the error and correction factor. We can use this to improve the accuracy of our measurement results by performing adjustments to return the UUC to its most accurate reading, or if an adjustment is not possible, add/subtract the correction factor to the final results.
The same procedure applies to the pressure gauge.
From our simple definition, Verification is a process of “confirming” that a given specification is fulfilled.
How to verify after calibration? After the process of calibration, immediately comes verification to determine if the results are within the set tolerance.
During verification, our objective is to determine if the DPLG has an acceptable output reading based on a specification or user requirement. For simplicity, we will use the manufacturer specs regarding tolerance to determine a pass or a fail status.
We will verify if the UUC is within the limits defined by the manufacturer.
The accuracy of the DPLG based on manufacturer specs is 2.5% of Full Scale (FS) (see photo)
The full scale =2000 mmWC
the tolerance limit is 2000*0.025 = +/-50 mmWC,
The error of the DPLG based on calibration result at 1500 is 19.
Based on the manufacturer specs, it is within the +/- 50 tolerance limit, therefore it is PASSED.
The same process of verification with the pressure gauge is applied.
Take note that during verification, we follow a specification to determine the acceptance criteria.
Because of verification, we confirmed that the DPLG is within manufacturer specification and therefore, re-calibration and/or adjustment is not needed.
As we continue in this stage, we define again validation as “ensuring” the acceptability of the implemented measurement process.
Which is in this example, the suitability of the tank to be used as a storage and monitoring for Liquid Oxygen (L0X)
During validation, we will consider the entire system, where all the instruments installed are functioning to its specification, resulting in an output based on the intended purpose.
Let us assume that the intended purpose is to display the exact level of liquid oxygen with an acceptable pressure range.
The objective of the validation is to determine that:
- The DPLG will display the exact differential pressure at a determined level based on the tank specifications. See in the photo below.
- The pressure should not exceed 35 bar.
- The transmitter is transmitting the right information (level) for remote monitoring.
The tank displays the exact level of liquid oxygen with an acceptable range of pressure as per calibration and verification results with the Tank Specifications.
Therefore, the whole system (tank) is suitable for storing and monitoring liquid oxygen (LOX).
Take note, that prior to validation, we performed calibration and verification in order to support our objective above.
Through calibration, we have determined the instrument accuracy, while through verification, we have confirmed that it is within manufacturer tolerance/specifications.
Validation results will conclude if the use of the LOX tank is fit for its purpose (storing and monitoring) and therefore leads to the approval or rejection decision by the concerned individuals.
This is just an example, in actual, more test and verification is performed during the actual validation process which includes a leak test, the safety valve, the transmitter, and others.
Final results of validation will be documented through a validation report with signatures of approval for ‘fit to be used‘ on its purpose
Check out in this link the whole post about the differences between calibration, verification, and validation.
I hope this makes all clear.
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