Traceability and Uncertainty The National Physical Laboratory is operated on behalf of the DTI by NPL Management Limited, a wholly owned subsidiary of Serco Group plc Measurement today is more valuable than ever. We depend on measurement for almost everything – from time keeping to weather forecasts, from DIY work at home to heavy-duty manufacturing, industrial research and medical science. Since measurement plays such a fundamental part in our lives, it is important that the accuracy of the measurement is fit for purpose, i.e. it fully meets the requirements of the application. Every measurement is inexact and therefore requires a statement of uncertainty to quantify that inexactness. The uncertainty of a measurement is the doubt that exists about the result of any measurement. One way of ensuring that your measurements are accurate is by tracing them back to national standards. This method of guaranteeing a measurement’s accuracy through an unbroken chain of reference is called traceability. What is the difference between ACCURACY and PRECISION? The difference between accuracy and precision is illustrated below by 4 different archers… each with varying degree of ability. The bull’s-eye in the target represents the true value of a measurement. Accuracy is a qualitative term relating the mean of the measurements to the true value, while precision is representative of the spread of these measurements. Even when we are precise and accurate, there will still be some uncertainty in our measurements. When the uncertainty of a measurement is evaluated and stated, then the fitness of purpose for a particular application can be properly understood. Accurate but imprecise American Indian’s 3 attempts are near the bull’s-eye, but were not near each other. Accurate and precise Olympic archer hits the bull’s-eye 3 times! Accurate measurement enables us to: Maintain quality control during production processes Comply with and enforce laws and regulations Undertake research and development Calibrate instruments and achieve traceability to a national measurement standard Develop, maintain and compare national and international measurement standards Precise but inaccurate Robin Hood’s Merry Man missed the bull’s-eye but the 3 attempts were near each other. Inaccurate and imprecise (unrepeatable) Stone age man missed the bull’s-eye and the 3 attempts were not near each other. In the United Kingdom, the National Measurement System (NMS) is in place to enable measurements to be traced back to their national standards. As the UK’s national standards laboratory, NPL is at the pinnacle of this system guaranteeing the accuracy of physical measurments for the nation and abroad. What is uncertainty? No measurement is ever guaranteed to be perfect. Uncertainty of measurement is the doubt that exists about the result of any measurement. By quantifying the possible spread of measurements, we can say how confident we are about the result. Expressing uncertainty A measurement result is only complete when accompanied by a statement of its uncertainty. A statement of uncertainty is required in order to decide if the result is adequate for its intended purpose and consistent with other similar results. It does not matter how accurate a measuring instrument is considered to be, the measurements made will always be subject to a certain amount of uncertainty. In order to express the uncertainty of a measurement, we need to evaluate as accurately as possible the errors associated with that particular measurement. For example – we might say that a particular stick is 200 centimetres long, plus or minus 1 centimetre, at a 95% confidence level. This is written: This means we are 95% sure that the length of the stick is between 199 centimetres and 201 centimetres. It should be possible to demonstrate an unbroken chain of comparisons that ends at a national standards body such as NPL. This demonstrable linkage to national standards with known accuracy is called ‘traceability’. National standards laboratories such as NPL also routinely undertake international comparisons in order to establish worldwide consensus on the accepted value of fundamental measurement units. Representatives of seventeen nations signed the Convention of the Metre (Convention du Mètre) on 20th May 1875 in Paris. This diplomatic treaty provided the foundations for the establishment of the Système International d’Unités (International System of Units, international abbreviation SI) in 1960. Since then, national standards laboratories have cooperated in the development of measurement standards that are traceable to the SI. Any organisation can achieve traceability to national standards through the correct use of an appropriate traceable standard from NPL. Successful measurement depends on the following: Accurate instruments Traceability to national standards An understanding of uncertainty Application of good measurement practice 200 cm ±1 cm at a level of confidence of 95% Inferior measuring equipment There are many factors that can cause inaccuracy: Environmental effects Poor measuring techniques Who is who in the measurement world? International Committee for Weights and Measures (CIPM – Comité International des Poids et Mesures) the world’s highest authority in the field of measurement science. International Bureau of Weights and Measures (BIPM – Bureau International des Poids et Mesures) co-ordinating body for international metrology, based in Sèvres, France. National Physical Laboratory (NPL) is the UK’s national standards laboratory, a world-leading centre in the development and application of highly accurate measurement technology and material science. Traceability Traceability is a method of ensuring that a measurement (even with its uncertainties) is an accurate representation of what it is trying to measure. What is traceability to national standards? The simple and basic concept behind calibration is that measuring equipment should be tested against a standard of higher accuracy. National standard accurate to Calibration laboratory Company ‘master’ item Company production equipment Produced product 0,002% 0,01% 0,07% 1% 10% Why does uncertainty matter? Calculating and expressing uncertainty is important to anybody wishing to make good quality measurements. It is also crucial where uncertainty can influence a pass or failure in a particular test, and must therefore be reported on a calibration certificate. If you have a measurement-related scientific question contact us on: telephone 020 8943 6880 e-mail: [email protected] or visit our website which has lots of measurement-related information at: www.npl.co.uk “ “ When you can measure what you are speaking about and express it in numbers you know something about it; but when you can not express it in numbers your knowledge is of a meagre and unsatisfactory kind. Lord Kelvin, British Scientist (1824 – 1907) There are established rules for the evaluation of uncertainty. [More information can be found in NPL’s Good Practice Guide (011) ‘A Beginner’s Guide to Uncertainty of Measurement’.] Of course, we must all make every effort to ‘control’ the uncertainty in our measurements. This is done by regular inspection and calibration of our instruments, careful calculation, good record-keeping. Don’t confuse the terms Error and Uncertainty Error is the difference between the measured value and the true value of the object being measured. Uncertainty is the quantification of the doubt about the measurement result.
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Traceability andUncertainty
T h e N a t i o n a l P h y s i c a l La b o r a to r y i s o p e r a te d o n b e h a l f o f t h e DT I b y N P L M a n a g e m e n t L i m i te d , a w h o l l y o w n e d s u b s i d i a r y o f S e r c o G r o u p p l c
Measurement today is more valuable than ever. We depend on measurement for almost everything – from timekeeping to weather forecasts, from DIY work at home to heavy-duty manufacturing, industrial research andmedical science. Since measurement plays such a fundamental part in our lives, it is important that the accuracyof the measurement is fit for purpose, i.e. it fully meets the requirements of the application. Every measurementis inexact and therefore requires a statement of uncertainty to quantify that inexactness. The uncertainty ofa measurement is the doubt that exists about the result of any measurement. One way of ensuring that yourmeasurements are accurate is by tracing them back to national standards. This method of guaranteeing ameasurement’s accuracy through an unbroken chain of reference is called traceability.
WWhhaatt iiss tthhee ddiiffffeerreennccee bbeettwweeeennAACCCCUURRAACCYY aanndd PPRREECCIISSIIOONN??The difference between accuracy and precision is illustratedbelow by 4 different archers… each with varying degree ofability. The bull’s-eye in the target represents the true value ofa measurement.
Accuracy is a qualitative term relating the mean of the measurementsto the true value, while precision is representative of the spread ofthese measurements. Even when we are precise and accurate, therewill still be some uncertainty in our measurements. When the uncertaintyof a measurement is evaluated and stated, then the fitness ofpurpose for a particular application can be properly understood.
AAccccuurraattee bbuutt iimmpprreecciissee
AAmmeerriiccaann IInnddiiaann’’ss 33 aatttteemmppttss aarreenneeaarr tthhee bbuullll’’ss--eeyyee,, bbuutt wweerree nnoottnneeaarr eeaacchh ootthheerr..
AAccccuurraattee aanndd pprreecciissee
OOllyymmppiicc aarrcchheerr hhiittss tthheebbuullll’’ss--eeyyee 33 ttiimmeess!!
Accurate measurement enables us to:�Maintain quality control during production processes
�Comply with and enforce laws and regulations
�Undertake research and development
�Calibrate instruments and achieve traceabilityto a national measurement standard
�Develop, maintain and compare national andinternational measurement standards
PPrreecciissee bbuutt iinnaaccccuurraattee
RRoobbiinn HHoooodd’’ss MMeerrrryy MMaann mmiisssseeddtthhee bbuullll’’ss--eeyyee bbuutt tthhee 33 aatttteemmppttsswweerree nneeaarr eeaacchh ootthheerr..
IInnaaccccuurraattee aanndd iimmpprreecciissee ((uunnrreeppeeaattaabbllee))
SSttoonnee aaggee mmaann mmiisssseedd tthhee bbuullll’’ss--eeyyeeaanndd tthhee 33 aatttteemmppttss wweerree nnoott nneeaarreeaacchh ootthheerr..
In the United Kingdom, the NNaattiioonnaall MMeeaassuurreemmeenntt SSyysstteemm (NMS)is in place to enable measurements to be traced back to theirnational standards. As the UK’s national standards laboratory,
NPL is at the pinnacle of this system guaranteeing the accuracyof physical measurments for the nation and abroad.
What is uncertainty?No measurement is ever guaranteed to be perfect. Uncertainty ofmeasurement is the doubt that exists about the result of any
measurement. By quantifying the possible spread of measurements,we can say how confident we are about the result.
Expressing uncertainty
A measurement result is only complete when accompanied bya statement of its uncertainty. A statement of uncertainty isrequired in order to decide if the result is adequate for its
intended purpose and consistent with other similar results.
It does not matter how accurate a measuring instrument is consideredto be, the measurements made will always be subject to a certain amountof uunncceerrttaaiinnttyy..
In order to express the uunncceerrttaaiinnttyy of a measurement, we need toevaluate as accurately as possible the errors associated with that
particular measurement.
For example – we might say that a particular stick is 200 centimetres long,plus or minus 1 centimetre, at a 95% confidence level. This is written:
This means we are 95% sure that the length of the stick is between199 centimetres and 201 centimetres.
It should be possible to demonstrate an unbroken chain ofcomparisons that ends at a national standards body such as
NPL. This demonstrable linkage to national standards withknown accuracy is called ‘‘ttrraacceeaabbiilliittyy’’..
National standards laboratories such asNPL also routinely undertake international
comparisons in order to establish worldwideconsensus on the accepted value of
fundamental measurement units.
Representatives of seventeen nationssigned the CCoonnvveennttiioonn ooff tthhee MMeettrree(Convention du Mètre) on 20th May1875 in Paris. This diplomatic treatyprovided the foundations for the
establishment of the SSyyssttèèmmeeIInntteerrnnaattiioonnaall dd’’UUnniittééss (InternationalSystem of Units, international abbreviationSI) in 1960. Since then, national
standards laboratories havecooperated in the developmentof measurement standards thatare traceable to the SI.
AAnnyy oorrggaanniissaattiioonn ccaann aacchhiieevvee ttrraacceeaabbiilliittyyttoo nnaattiioonnaall ssttaannddaarrddss tthhrroouugghh tthhee ccoorrrreecctt uussee ooff aann
aapppprroopprriiaattee ttrraacceeaabbllee ssttaannddaarrdd ffrroomm NNPPLL..
Successful measurementdepends on the following:
�AAccccuurraattee instruments
�TTrraacceeaabbiilliittyy to national standards
�An understanding of uunncceerrttaaiinnttyy
�Application of ggoooodd mmeeaassuurreemmeenntt pprraaccttiiccee
220000 ccmm ±±11 ccmm aatt aa lleevveell ooff ccoonnffiiddeennccee ooff 9955%%
IInnffeerriioorrmmeeaassuurriinnggeeqquuiippmmeenntt
There are manyfactors that cancause iinnaaccccuurraaccyy::
EEnnvviirroonnmmeennttaalleeffffeeccttss
PPoooorrmmeeaassuurriinnggtteecchhnniiqquueess
Who is who in the measurement world?IInntteerrnnaattiioonnaall CCoommmmiitttteeee ffoorr WWeeiigghhttss aanndd MMeeaassuurreess(CIPM – Comité International des Poids et Mesures) the world’s highest authority in thefield of measurement science.
IInntteerrnnaattiioonnaall BBuurreeaauu ooff WWeeiigghhttss aanndd MMeeaassuurreess(BIPM – Bureau International des Poids et Mesures) co-ordinating body for internationalmetrology, based in Sèvres, France.
NNaattiioonnaall PPhhyyssiiccaall LLaabboorraattoorryy(NPL) is the UK’s national standards laboratory, a world-leading centre in the developmentand application of highly accurate measurement technology and material science.
TraceabilityTTrraacceeaabbiilliittyy is a method of ensuring that a measurement(even with its uncertainties) is an accurate representationof what it is trying to measure.
What is traceability to national standards?The simple and basic concept behind calibration is thatmeasuring equipment should be tested against a standardof higher accuracy.
NNaattiioonnaall ssttaannddaarrdd aaccccuurraattee ttoo
CCaalliibbrraattiioonn llaabboorraattoorryy
CCoommppaannyy ‘‘mmaasstteerr’’ iitteemm
CCoommppaannyy pprroodduuccttiioonn eeqquuiippmmeenntt
PPrroodduucceedd pprroodduucctt
00,,000022%%
00,,0011%%
00,,0077%%
11%%
1100%%
Why does uncertainty matter?Calculating and expressing uunncceerrttaaiinnttyy is important to
anybody wishing to make good quality measurements.
It is also crucial where uunncceerrttaaiinnttyy can influencea pass or failure in a particular test, and musttherefore be reported on a calibration certificate.
If you have a measurement-related scientific question contact us on: telephone 020 8943 6880 e-mail: [email protected] or visit our website which has lots of measurement-related information at: www.npl.co.uk
“
“
WWhheenn yyoouu ccaannmmeeaassuurree wwhhaatt yyoouu aarree
ssppeeaakkiinngg aabboouutt aanndd eexxpprreessss iittiinn nnuummbbeerrss yyoouu kknnooww ssoommeetthhiinnggaabboouutt iitt;; bbuutt wwhheenn yyoouu ccaann nnoott
eexxpprreessss iitt iinn nnuummbbeerrss yyoouurrkknnoowwlleeddggee iiss ooff aa mmeeaaggrree aanndd
uunnssaattiissffaaccttoorryy kkiinndd..
LLoorrdd KKeellvviinn,, BBrriittiisshh SScciieennttiisstt((11882244 –– 11990077))
There are established rules for the evaluation ofuunncceerrttaaiinnttyy. [More information can be found in NPL’sGood Practice Guide (011) ‘A Beginner’s Guide toUncertainty of Measurement’.] Of course, we must allmake every effort to ‘control’ the uncertainty in ourmeasurements. This is done by regular inspection andcalibration of our instruments, careful calculation,
good record-keeping.
DDoonn’’tt ccoonnffuussee tthheetteerrmmss EErrrroorr
aanndd UUnncceerrttaaiinnttyy
EErrrroorr is the difference between themeasured value and the true value of
the object being measured.
UUnncceerrttaaiinnttyy is the quantificationof the doubt about the
measurement result.
Related Documents
