Calibration Guidelines for Weighing Instruments – Recent Developments

Dr. Klaus FritschManager ComplianceNSCLI Orlando 2014

1

Calibration of a Balance or a Scale

How do we put this into practice?

Regulations Affecting Scales & Balances - cGMP

“Automatic, mechanical or electronic equipment […]shall be routinely calibrated, inspected or checked according to a written program designed to assure proper performance.”

21 CFR part 211.68 (a), US GMP for Finished Pharmaceuticals

"Weighing shall be performed using a balance that is calibrated […] and meets the requirements defined for repeatability and accuracy ."

"Repeatability is satisfactory if two times the standard deviation of the weighed value, divided by the nominal value of the weight used, does not exceed 0.10%."

"The accuracy of a balance is satisfactory if its weighing value, when tested with a suitable weight(s) is within 0.10% of the test weight value."

USP 36–NF 31 Second Supplement, General Chapter 41 “Balances“

Regulations affecting Balances - USP

What does this mean for my balances?

4

“Measuring equipment shall be calibrated and/or verified at specified intervals … against measurement standards traceable to international or national measurement standards.”

ISO 9001:2008, 7.6 Control of Monitoring and Measuring Devices

Regulations affecting Scales and Balances - ISO

How do we put this into practice?

5

Calibration of Instruments

6

Calibration of Instruments

Calibration

"Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication."

International Vocabulary Of Basic And General Terms In Metrology (VIM) JCGM 200:2012, item 2.39

Large parts of the industry do not have a sound sci entific understanding of calibration.

7

Perception of Measurement Uncertainty

"When somebody hears the word 'uncertainty', then the reaction is

either panic or boredom."

Patrick Abbott, NISTNCSLI 2013

Existing Shortcomings of NAWI Calibration

� Incorrect perception of calibration- "A calibrate my balance on a daily basis – I put a weight on the pan and

take the reading."- "Calibration is important and required by regulation – so I better take 8 test

points instead of 4."

⇒Very frequently, measurement uncertainty is not conside red

� No international harmonization of calibration- Almost every company has its own calibration procedure- Almost every calibration lab has its own calibration procedure- There are a lot of guidance documents issued by NMIs or accreditation

bodies, but most of them only on a national level

⇒ (International) customers have no possibility to compare calibrationresults, as procedures, estimation of uncertainty and pr esentation ofthe data are very diverse

9

An Important International Calibration Guideline

� Acts in many (but not all) European countries as basis for accreditaton

� Currently under revision with the objective to remove ambiguities, improve the examples, include a chapter on minimum weight, etc.

� Revision 4.0 expected to be published by the end of the year 2014

10

SIM – Sistema Interamericano de Metrología

11

Transposition of cg-18 into a SIM Guideline

12

What about the US?

� No calibration guide- No procedures- Does not explicitly

describe all uncertaintycontributions

- Few formulas

� No calibration guide- Only procedures- No uncertainty

� Outdated

2013

13

Repeatability RP

Readability RD

Nonlinearity NL

Eccentricity EC

Sensitivity SE

Important Balance Contributions to Uncertainty

� Nominal property:

� Measurement properties:

The Perfect Weighing Instrument…

…Impaired by Readability, … (RD)

…Sensitivity Offset, … (SE)

…Nonlinearity, … (NL)

}Error of Indication

…Eccentricity, … (EC)

…and Repeatability (RP)

EURAMET: Determination of the "Error of Indication"

20

Error of Indication

}

Semi-micro balanceReadability 10 µgCapacity 220 g

� � � ������� usually is either the

nominal mass or the

conventional mass

21

1st Step: Determination of the "Error of Indication"

Error of Indication

Load

22

2nd Step: Uncertainty of the Error of Indication

Error of Indication

Load

23

EURAMET: Contributions to the Uncertainty of EOI

d0 / √12 Rounding error of no-load indication

dI / √12 Rounding error of indication at load

s Repeatability (standard deviation)

u(δIecc) Eccentricity (off-centre position)

u(δmc) Weight uncertainty (alternatively: max. permissible error)

u(δmB) Uncertainty due to air buoyancy

u(δmD) Uncertainty due to drift of weight value over time

u(δmconv) Uncertainty due to convection

Some of the contributions to the uncertainty of the Error of Indication (EOI) can be so small so that they might be neglected.

MEASUREMENTS

FROM BALANCE

FROM WEIGHTS

� Every indication of an electronic instrument is digitized, usually basedon a non-digitized (analogue) signal

� A rectangular distribution is assumed for the unrounded measurementvalue, which has a standard deviation of d/SQRT(12) 0.29 d

� The rectengular distribution is assumed as we have no knowledgeabout the exact (analogue) value, consequently we assume that everypossible unrounded result occurs with the same probability

� As every weighing consists of two readings, their individual roundingerrors are quadratically added, leading to 0.41 d

Rounding Error of a Digital Indication

50.000 mg 50.005 mg49.995 mg Unrounded value

Probability distribution

0.29d0.29d

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3rd Step: Expansion of the Standard Uncertainty

Error of Indication

Load

� The standard uncertainty u is expanded by multiplying it with thecoverage factor k (sometimes also called expansion factor)

� The coverage factor k is chosen so that the expanded uncertaintyU = k·u corresponds to a coverage probability of approximately95%

� In other words: The true value (which is unknown) shall be with a probability of 95% within ± U around the measured value

� The coverage factor is calculated; for some – but not all – casesthe coverage factor is 2 (normal distribution and sufficientreliability)

� A coverage factor larger than 2 (e.g. 2.05, 2.13,…) is anticipatedfor calibrations where the number of repeated weighings of therepeatability test is smaller than 10 (application of the Welsh-Satterthwaite Formula)

Expansion of the Standard Uncertainty

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EURAMET: Uncertainty of the "Error of Indication"

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"Accredited" Part and Beyond

� Now, the calibration is accomplished

But:

� The calibration data only show the performance of the device AT CALIBRATION (not yet any information on the performance IN USE, i.e. the uncertainty of a weighing result)

� The performance of the instrument was only assessed for specificpoints but information on the performance for any load is not yetevaluated

Therefore:

� Interpretation of the calibration results is of utmost importance

� This is outside the "accredited part" of the calibration certificate

EURAMET/cg-18 is one of the very few calibration gu ides that give concrete advice on the estimation of uncertain ty in use.

29

4th Step: Interpolation of Error of Indication

Red linear curve characterizesapproximated error of indication

Error of Indication

Load

30

5th Step: Interpolation of Uncertainty

Blue band characterizes approximation of the standard uncertainty of the error

Error of Indication

Load

31

Uncertainty of a Weighing Result

�� � Uncertainty of the error

��� � ������ Variation of temperature (change of characteristic)

��� � ������ Variation of air density

��� � ����� Long term drift

��� � ������ Tare function (nonlinearity of instrument)

�� � ����

12�� �

12� !� � ����

� ���"" �� � �� �

�#��� � ������ ����

� ������ ���� � �����

���� � ������ $��

Uncertainty at calibration

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7th Step: Combine Error and Uncertainty in Use

Red linear curve characterizesapproximated error of indication

Error of Indication / Global Uncertainty

Load

Green linear curve characterizes the"global uncertainty"

Blue linear curve characterizes the(expanded) uncertainty of a weighing result

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0 1 2 3 4 5 6 7 8 9 10

Global (absolute) Measurement Uncertainty [mg]

Unc

erta

inty

U [m

g]

U [g] = U 0 + Constant x Weight

By applying the global measurement uncertainty, a c orrection of the error is not required anymore.

MaxLoad [g]

Behavior of the Global Measurement Uncertainty

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0 1 2 3 4 5 6 7 8 9 10Unc

erta

inty

U [m

g]

MaxLoad [g]

Example of a Calibration Certificate

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Relative Measurement Uncertainty [%] (= Absolute measurement uncertainty / weight)

Relative measurement uncertainty increases as sample mass decreases

Unc

erta

inty

U [m

g or

%]

For small sample weights, the relative uncertainty can become so high that the weighing results may not be truste d!

MaxLoad [g]

Behavior of the Global Measurement Uncertainty

Global (absolute) Measurement Uncertainty [mg]

U [g] = U 0 + Constant x Weight

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Relative Measurement Uncertainty [%]

Unc

erta

inty

U [m

g or

%]

Weighing accuracy [%]

MaxLoad [g]

When weighing below the minimum weight, the measure ment un-certainty is larger than the accuracy required: →→→→ Inaccurate results

Accuracy limit: Minimum sample weight

The Accuracy Limit = The Minimum Weight

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0 1 2 3 4 5 6 7 8 9 10

Relative Measurement Uncertainty [%]

Unc

erta

inty

U [g

or

%]

Weighing accuracy [%]

MaxLoad [g]Accuracy limit: Minimum sample weight

MaxWeight [g]When weighing above the minimum weight, the measure ment un-certainty is smaller than the accuracy required: →→→→ Accurate results

The Accuracy Limit = The Minimum Weight

Minimum Weight Increases with Higher Accuracy

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1%

Minimum weight foran accuracy of 1%

More stringent accuracy = Increased minimum sample weight

Relative Measurement Uncertainty [%]

Weighing accuracy 1%

Minimum weight foran accuracy of 0.1%

0.1%Weighing accuracy 0.1%

Weight [g]

Minimum Weight from Calibration Certificate

� EURAMET/cg-18 is currently the most widespread calibration guideworldwide

� Besides of uncertainty at calibration, the guide also provides concreteguidance for the uncertainty of a weighing result (one of the very fewguides touching that topic)

� By applying the "global uncertainty" (i.e. the combination of error anduncertainty of a weighing result), a correction of the error is not necessary anymore

� Revision includes annex about "minimum weight"

� The concepts of uncertainty of a weighing result and minimum weightprovide higher value to the customer as it supports practicalinterpretation of calibration results

� Cg-18 is seen as a good candidate for striving international harmonization of NAWI calibration

Conclusions

Calibration Guidelines for Weighing Instruments – Recent Developments

Dr. Klaus FritschManager ComplianceNSCLI Orlando 2014