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QSP for Uncertainty of Measurement

Laboratory Management System - ISO 17025

Clause Title5.4.6 Uncertainty of Measurement

DOCUMENT CONTROL BLOCK (Revision History)

Date Revision# DCR #Amendment

DetailSectionAffected

AmendmentApproved by

Signatures

Prepared By:

Name: ______________

Designation: ______________

Signature: ______________

Reviewed By:

Name: ______________

Designation: ______________

Signature: ______________

Approved By:

Name: ______________

Designation: ______________

Signature: ______________

1. Purpose:

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The purpose of this procedure is to guide Laboratory staff about uncertainty ofmeasurement.

2. Scope:

This procedure is applicable to Company Pvt. Ltd for the tests performed in the lab whereapplicable.

3. Responsibility:

Lab incharge and DMR are responsible for measurement of uncertainty of the test where itis applicable.

4. Terms & Definitions:

4.1 Uncertainty

Uncertainty is deviation from the true value and can only be expressed in the form of arange, e.g. +3, +10.6, +0.07, etc.

Published Definition:Uncertaintyis a quantification of the doubt about the measurement result.

4.2 Error Vs Uncertainty

Error expresses a problem and can only be expressed (in statistical terms) as eitherpositive or negative value, while uncertainty cannot be expressed in the form of a singlevalue.

Published Definition of Error:Error is the difference between the measured value and the true value of the thingbeing measured.

5. Sources of Uncertainty

5.1Test / Calibration Performer(s)

It is the uncertainty of the person performing the test / calibration, but as a persons errorcannot be calculated in the form of numerical values that be used in advanced combined

estimations, we determine Repeatability and Reproducibility through the test/ calibrationresults they achieve.

5.1.1.Uncertainty of Repeatability:It is the deviation found in results when an activity is repeatedly performed underthe same condition and by the same person.

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5.1.2.Uncertainty of Reproducibility:It is the deviation found in results when an activity is repeatedly performed on thesame type of sample under different conditions (in our case we suggest changingthe performer only and try to sustain the rest of the uncertainty sources conditionas much as possible).

6. Method used

It is uncertainty expressed in the test / calibration method (equipment manual, etc.) whensome assumptions have to be made due to limitation of features and/or resources.7. Accessories

The apparatus and accessories being used have uncertainty of their own. In most cases itcan be found in their manuals, on their label or their certificates. Experienced personnelwho are capable of performing test / calibration of equipment are also able to find itsuncertainty, if its not given in any other source.

8. Material, Sample and Chemical

Any material and/or sample being used has its own uncertainty, which is usually providedwith its certificate, literature or container. If its not provided, personnel can calculate it bycomparing it with a reference standard where uncertainty is already stated.

9. Types of Uncertainty

9.1 Type A

Value of Uncertainty achieved through statistical calculation. We use Type A forcalculation of two sources of uncertainty which are Repeatability and Reproducibility.

CALCULATING TYPE A:

Note: Applicable on most Physical Testing / Calibration Activities, Electronic Test /Calibration Activities have very low human error margin therefore reliance on TYPE B ishighly recommended.

9.1.1. Repeatability

a. Perform a Test / Calibration Activity at least 10 times and record all the results(final result)

b. Calculate Average of Results

Average =Sum of all Values / Total Number of Values (n)

c. Calculate Standard Deviation of Results

Standard Deviation =

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Square Root of [(Sum of Result Values - Average) 2 / n 1]

d. As we need the end result in Combined Uncertainty, we shall calculate itsRelative Uncertainty

Relative Uncertainty = Standard Deviation Calculated / Average

OPTIONAL NOTE: It is a common practice to multiply Relative Uncertaintywith 100 so that results can be expressed in the form of percentage whichmakes them easier for calculations, so the new formula would be;

Relative Uncertainty = (Standard Deviation Calculated / Average) x 100

9.1.2. Reproducibility

First two steps of repeatability are performed (up till Average estimation) by atleast three personnel (where possible) and their Averages are considered asResults. Through this result a New Average is calculated along with StandardDeviation. This standard deviation is referred as Uncertainty ofReproducibility.

After this relative uncertainty is estimated;

(Uncertainty of Reproducibility / New Average)

OPTIONAL NOTE: It is a common practice to multiply Relative Uncertainty

with 100 so that results can be expressed in the form of percentage whichmakes them easier for calculations, so the new formula would be;

Relative Uncertainty = (Uncertainty of Reproducibility / New Average) x100

9.2 Type B

Value of Uncertainty achieved from any other source e.g. manuals, certificates, etc.

CALCULATING TYPE B:

a. If the value has a confidence level of 95% divide it by 2 or if 99.7% divide it by3, before using it in calculation

b. In case its an equipment and no confidence level is given, divide the value bySquare Root of 3 (3)

c. In case uncertainty is provided in Percentage it shall be considered asRelative Uncertainty, other wise

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As in most cases of Type B, Average is not provided, the value (scale)provided with uncertainty shall be considered. So the formula in this case shallbe;

Relative Uncertainty (Type B) =(Uncertainty Provided / Value Provided) x 100

9.3 COMBINED UNCERTAINTY:

Combined Uncertainty = Square Root (Sum of Squares of RelativeUncertainties of all sources of uncertainties from both Type A and Type B)

Combined Uncertainty = Square Root of (a2 + b2 + c2 + ..)

Where a, b and c represent relative uncertainties.

9.4EXPANDED UNCERTAINTY :

The standard ISO 17025:2005 requires the labs to express their estimateduncertainties with certain confidence. This confidence is expressed through usingConfidence Levelwhich is more commonly known as kfactor.

Expanded Uncertainty =Combined Uncertainty x Confidence Level (k)

Whereas, Confidence Level, k

95%= 299.7% = 3

We recommend use ofK=2

9.5 UNCERTAINTY BUDGET

Foreach TEST / CALIBRATION METHOD youll have to compile a separate chart /table;

1 2 3 4 5 6

S/N Sources of Uncertainty Uncertainty Type A/BAverage or

ValueRelative

Uncertainty

Column 1:Serial Number for each sourceColumn 2:Describe the Source of Uncertainty; you need to give only those which

can be measured in terms of numerical valueColumn 3:Write Calculated or stated uncertaintyColumn 4:Name the TypeColumn 5:In case of Type A, give average; in case of Type B, give the value against

which the uncertainty is given. In case of Type B, if Relative Uncertainty isgiven, leave column 3 and 5

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Column 6:Give calculated Relative Uncertainty

Use the values in Column 6 for combined uncertainty as well as expandeduncertainty.

10. SUPPORTING DOCUMENTS

a. Calculating Type A

This sheet can be used for calculation of Uncertainty of Repeatability and Reproducibilityalong with their Relative Uncertainties. Lab Personnel are encouraged to perform thesesteps manually before using this sheet for verification of results.

b. Type B Uncertainty

This can be used for calculation of relative uncertainty of Type B uncertainties.

c. Uncertainty Finalization

This sheet can be used for calculation of Combined and Expanded Uncertainty based onthe requirement of the Person performing the test / calibration. The sheet providesexpanded uncertainty with k factor 2 and 3 both.

d. Uncertainty Budget

Form (HEC/LMS-FRM/TEC-14) to be used for formulation of Uncertainty Budget to beused for formulation of Uncertainty Budget.

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