ncsli 2014 cg-18
TRANSCRIPT
Calibration Guidelines for Weighing Instruments – Recent Developments
Dr. Klaus FritschManager ComplianceNSCLI Orlando 2014
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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?
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“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?
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Calibration of Instruments
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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.
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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
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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
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SIM – Sistema Interamericano de Metrología
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Transposition of cg-18 into a SIM Guideline
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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
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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"
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Error of Indication
}
Semi-micro balanceReadability 10 µgCapacity 220 g
� � � ������� usually is either the
nominal mass or the
conventional mass
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1st Step: Determination of the "Error of Indication"
Error of Indication
Load
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2nd Step: Uncertainty of the Error of Indication
Error of Indication
Load
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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.
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4th Step: Interpolation of Error of Indication
Red linear curve characterizesapproximated error of indication
Error of Indication
Load
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5th Step: Interpolation of Uncertainty
Blue band characterizes approximation of the standard uncertainty of the error
Error of Indication
Load
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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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0 1 2 3 4 5 6 7 8 9 10
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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0 1 2 3 4 5 6 7 8 9 10
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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0 1 2 3 4 5 6 7 8 9 10
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