How to Differentiate Calibration, Verification, and Validation?

with 4 Comments

I have received below comments in relation to last post about the differences between calibration, verification, and validation.

These are the comments:

  1. Can you give an example instrument with a specification that we can determine or differentiate calibration, verification, and validation?
  2. 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.

Differences Between Calibration, Verification, and Validation
Differences Between Calibration, Verification, and Validation

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.

CALIBRATION

Calibration of Differential Pressure Level Gauge

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.

Calibration Result:

Calibration Result of DPLG
Calibration Result of DPLG

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, based on the measurement uncertainty results, we are 95% confident that the exact value of 1519 is within the limits 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 corection 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 adjustment is not possible, add/subtract the correction factor to the final results. 

Same procedure applies to the pressure gauge.

VERIFICATION

Verification OF Differential Pressure Gauge
Verification Of Differential Pressure Gauge

From our simple definition, Verification is a process of “confirming” that a given specification is fulfilled.

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

Therefore,  

            the tolerance is 2000*0.025 = +/-50 mmWC,

Verification Result:

The error of the DPLG based on calibration result at 1500 is 19.

Based on the manufacturer specs, it is within the +/- 50 tolerance, therefore it is PASSED.

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.

VALIDATION

Liquid Oxygen Tank System
Liquid Oxygen Tank System

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 whole system, where all the instruments installed are functioning to its specification, resulting to 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:

  1. The DPLG will display the exact differential pressure at determined level based on the tank specifications. See in the photo below.
    Tank Specifications: Conversion of Differential Pressure to Level
    Tank Specifications: Conversion of Differential Pressure to Level
  2. The pressure should not exceed 35 bar at the acceptable level.
  3. The transmitter is transmitting the right information (level) for remote monitoring.

Validation Result:

The tank displays the exact level of  liquid oxygen with 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 a 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, of course 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.

Please feel free to comment and subscribe.

Edwin

4 Responses

  1. Alvin
    | Reply

    Hi S’Edwin,

    Good Day!
    Now I understand well their differences. I appreciate you sir. I hope my appreciation will boost your enthusiasm to post more about calibration or other topics and be as expert as you someday. Thanks a lot!

    Regards,
    Alvin

    • edwin
      | Reply

      Hi Mr. Alvin,

      You are welcome. I am happy to know that you understand it.Your appreciation will truly boost my motivation and enthusiasm to create more post that I believe can benefit us all.
      Thank you for your valuable comments. I also appreciate it.

      Any more comments or concern with regards to calibration, do not hesitate to message me.

      Best Regards,
      Edwin

  2. Siva
    | Reply

    Dear Edwin,

    I want to know ,,

    how to calculate uncertainity value (+or -) from Standard Reading and UUC Reading and Error..

    • edsponce
      | Reply

      Dear Siva,
      Thank you for reading my post.

      Calculating measurement uncertainty requires more steps and considerations of other errors. But for simplicity, I will just focus directly on your inquiry.

      For standard reading, you need to have the calibration certificate and look there the measurement uncertainty value.
      That value, you divide it by 2 (normal distribution), example, uncertainty from cal cert = 0.5 therefore, 0.5/2 = 0.25 psi

      For UUC reading, compute for the standard deviation of the repetitions you made then divide by Sqrt of 2 = 1.41 (for 2 repetitions)

      For example, trial 1 = 2.2 and trial 2= 2.3, using excel to compute for the standard deviation will lead to an answer of 0.07,type in excel, =STDEV(2.2,2.3)
      then 0.07/1.41= 0.05 psi

      And for the error, we will substitute the error for hysteresis error, for example, equal to 0.2, then 0.2/ sqrt of 3 for a rectangular distribution. Sqrt of 3 = 1.73,
      therefore, 0.2/1.73 = 0.115. psi

      Now we will combine all the uncertainties to one result using the Root Sum Square (RSS) method.

      It is easy to calculate with the help of an excel sheet. Just type below:
      =SQRT(SUMSQ(0.25,0.05,0.115)) = 0.28 psi

      Multiply 0.28 by 2 for a 95% confidence level which is equal to 0.56 psi
      +/- 0.56 >>> the final uncertainty value that you will use.

      This is just a simple example. Other sources of error are still needed to be determined and include in the calculation.
      I believe you have a background about uncertainty calculation in order to understand this.

      I hope this helps,
      Edwin

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