SIST EN ISO 14956:2003
(Main)Air quality - Evaluation of the suitability of a measurement procedure by comparison with a required measurement uncertainty (ISO 14956:2002)
Air quality - Evaluation of the suitability of a measurement procedure by comparison with a required measurement uncertainty (ISO 14956:2002)
This International Standard specifies, for the field of air quality measurement procedures, the:
estimation of measurement uncertainty from actual or claimed values of all important performance characteristics of a method under stationary conditions; assessment of whether or not specified values for these performance characteristics comply with the required quality of a measured value at a stated measurand value; evaluation of the applicability of the measurement method based on laboratory performance and confirmatory field test; establishment of requirements on dynamic behaviour of instruments.
This International Standard is applicable to measurement procedures whose output is a defined time average.
Luftbeschaffenheit - Beurteilung der Eignung eines Messverfahrens durch Vergleich mit einer geforderten Messunsicherheit (ISO 14956:2002)
Diese Internationale Norm legt Verfahren fest
3 zur Schätzung der Messunsicherheit aufgrund des Einflusses von tatsächlichen oder vorgegebenen Werten aller wichtigen Verfahrenskenngrößen eines Messverfahrens unter stationären Bedingungen;
3 zur Bewertung, ob die geforderte Qualität des Messwertes mit den festgelegten Werten dieser Verfahrens-kenngrößen für einen bestimmten Wert der Messgröße eingehalten wird;
3 zur Überprüfung der Eignung des Messverfahrens aufgrund seiner Leistungsfähigkeit im Labor und deren Bestätigung in Feldtests;
3 zur Festlegung von Anforderungen an das dynamische Verhalten von Geräten.
Diese Internationale Norm gilt für Messverfahren, deren Messsignal ein festgelegter zeitlicher Mittelwert ist.
Qualité de l'air - Evaluation de l'aptitude a l'emploi d'une procédure de mesurage par comparaison avec une incertitude de mesure requise (ISO 14956:2002)
La présente Norme internationale spécifie, en ce qui concerne le domaine du mesurage de la qualité de l'air, les procédures pour estimer l'incertitude de mesure à partir des valeurs réelles ou déclarées de toutes les caractéristiques métrologiques importantes d'une méthode dans des conditions stables; évaluer si des valeurs spécifiées pour ces caractéristiques métrologiques sont conformes ou non à la qualité requise d'une valeur mesurée pour une valeur spécifiée du mesurande; évaluer l'applicabilité de la méthode de mesurage à partir de la performance en laboratoire et de l'essai de confirmation sur le terrain; établir les exigences en matière de comportement dynamique des instruments.
La présente Norme internationale s'applique aux méthodes de mesurage dont le résultat est une moyenne couvrant une période de temps donnée.
Kakovost zraka - Vrednotenje primernosti merilnega postopka s primerjavo z zahtevano merilno negotovostjo (ISO 14956:2002)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 14956:2003
01-marec-2003
Kakovost zraka - Vrednotenje primernosti merilnega postopka s primerjavo z
zahtevano merilno negotovostjo (ISO 14956:2002)
Air quality - Evaluation of the suitability of a measurement procedure by comparison with
a required measurement uncertainty (ISO 14956:2002)
Luftbeschaffenheit - Beurteilung der Eignung eines Messverfahrens durch Vergleich mit
einer geforderten Messunsicherheit (ISO 14956:2002)
Qualité de l'air - Evaluation de l'aptitude a l'emploi d'une procédure de mesurage par
comparaison avec une incertitude de mesure requise (ISO 14956:2002)
Ta slovenski standard je istoveten z: EN ISO 14956:2002
ICS:
13.040.01 Kakovost zraka na splošno Air quality in general
SIST EN ISO 14956:2003 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 14956:2003
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SIST EN ISO 14956:2003
EUROPEAN STANDARD
EN ISO 14956
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2002
ICS 13.040.01
English version
Air quality - Evaluation of the suitability of a measurement
procedure by comparison with a required measurement
uncertainty (ISO 14956:2002)
Qualité de l'air - Evaluation de l'aptitude à l'emploi d'une Luftbeschaffenheit - Beurteilung der Eignung eines
procédure de mesurage par comparaison avec une Messverfahrens durch Vergleich mit einer geforderten
incertitude de mesure requise (ISO 14956:2002) Messunsicherheit (ISO 14956:2002)
This European Standard was approved by CEN on 28 July 2002.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2002 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 14956:2002 E
worldwide for CEN national Members.
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SIST EN ISO 14956:2003
EN ISO 14956:2002 (E)
CORRECTED 2002-10-02
Foreword
This document (EN ISO 14956:2002) has been prepared by Technical Committee ISO /TC
146 "Air quality" in collaboration with Technical Committee CEN/TC 264 "Air quality", the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication
of an identical text or by endorsement, at the latest by February 2003, and conflicting national
standards shall be withdrawn at the latest by February 2003.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the
United Kingdom.
Endorsement notice
The text of ISO 14956:2002 has been approved by CEN as EN ISO 14956:2002 without any
modifications.
2
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SIST EN ISO 14956:2003
INTERNATIONAL ISO
STANDARD 14956
First edition
2002-08-15
Air quality — Evaluation of the suitability of
a measurement procedure by comparison
with a required measurement uncertainty
Qualité de l'air — Évaluation de l'aptitude à l'emploi d'une procédure de
mesurage par comparaison avec une incertitude de mesure requise
Reference number
ISO 14956:2002(E)
©
ISO 2002
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
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ii © ISO 2002 – All rights reserved
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
Contents Page
Foreword . iv
Introduction. v
1 Scope. 1
2 Normative reference. 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms. 2
5 Principle . 4
6 Requirements . 7
6.1 Methods and materials . 7
6.2 Performance characteristics. 7
6.3 Required measurement quality. 7
7 Required performance related to dynamic conditions.7
7.1 General . 7
7.2 Response time. 7
8 Required performance related to stationary conditions.8
8.1 Analytical function, model function and variance function. 8
8.2 Identification of sources of uncertainty. 9
8.3 Assignment of sources of uncertainty to performance characteristics. 9
8.4 Definition and quantification of conditions of operation of the measurement system . 10
8.5 Quantification of the impact of selected performance characteristics as partial standard
uncertainties . 11
8.6 Estimation of the combined standard uncertainty . 14
8.7 Estimation of the expanded uncertainty. 14
8.8 Evaluation of compliance with the required measurement quality . 14
9 Field verification. 14
10 Report. 15
Annex A (informative) Default ranges of chemical interferents. 16
Annex B (normative) Coverage factors derived from effective degrees of freedom. 18
Annex C (informative) Example of an assessment of compliance of UV fluorescence method for SO
2
with requirements on ambient air quality. 19
Annex D (informative) Examples of field verification programmes. 24
Bibliography. 26
© ISO 2002 – All rights reserved iii
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 14956 was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 4, General aspects.
Annex B forms a normative part of this International Standard. Annexes A, C and D are for information only.
iv © ISO 2002 – All rights reserved
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
Introduction
A measuring task generally includes information on the required quality of the measurement result, which may be
quantified by the measurement uncertainty. The required quality may be specified, e.g. by legislation, by authorities
or the parties involved.
The quality of a measurement result strongly depends on the performance of the measuring method used. This
International Standard specifies the procedures to determine the measurement uncertainty of an individual
measurement result, using relevant performance characteristics of the measuring method, and to verify compliance
with the requirements of the measuring task.
A procedure for establishing the uncertainty of the time average of a series of single measurements will be given in
a separate International Standard [3].
© ISO 2002 – All rights reserved v
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SIST EN ISO 14956:2003
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SIST EN ISO 14956:2003
INTERNATIONAL STANDARD ISO 14956:2002(E)
Air quality — Evaluation of the suitability of a measurement
procedure by comparison with a required measurement
uncertainty
1 Scope
This International Standard specifies, for the field of air quality measurement procedures, the:
estimation of measurement uncertainty from actual or claimed values of all important performance
characteristics of a method under stationary conditions;
assessment of whether or not specified values for these performance characteristics comply with the required
quality of a measured value at a stated measurand value;
evaluation of the applicability of the measurement method based on laboratory performance and confirmatory
field test;
establishment of requirements on dynamic behaviour of instruments.
This International Standard is applicable to measurement procedures whose output is a defined time average.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent edition of the normative document indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 6879:1995, Air quality — Performance characteristics and related concepts for air quality measuring methods
3 Terms and definitions
For the purposes of this International Standard, the terms and definitions given in ISO 6879 and the following
apply.
3.1
dynamic condition
〈of operation〉 condition where the measurand value or/and the value of an influence quantity is time-dependent
3.2
performance requirement
requirement of the measurement, in terms of standard uncertainty and dynamic behaviour, against which the
suitability of the measurement system is being assessed
© ISO 2002 – All rights reserved 1
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
3.3
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation.
[GUM:1993, 2.3.1]
3.4
stationary condition
〈of operation〉 condition where the measurand value and the values of all influence quantities are constant.
3.5
uncertainty
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could
reasonably be attributed to the measurand.
[VIM:1993, C.2.11]
4 Symbols and abbreviated terms
b sensitivity coefficient of c for influence quantity x at C = c
j j test
b maximum value of b
j, max j
C measurand
c measured value of the measurand
c value of the measurand at which the required measurement uncertainty is given
test
D( y ) drift of measured value on input quantity Y at C = c
i i test
f ( y ) analytical function; function of input quantities where the impact of influence quantities is excluded
i cal
I ratio of the change in measured value and the corresponding change of the interferent value x
j i
at C = c
test
i index of input quantities Y
j index of influence quantities X
k coverage factor
n total number of input quantities; last number
m total number of influence quantities
P percentage value
p index of the performance characteristic
p maximum number of performance characteristics considered
max
s[c(x )] standard deviation of c caused by x at C = c
j j test
s(x ) standard deviation of x at C = c
j j test
2 © ISO 2002 – All rights reserved
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
s ( y ) standard deviation of y due to the random part of instability
inst i i
s ( y ) repeatability standard deviation of input quantity Y at y
r i i i
s ( y ) reproducibility standard deviation of input quantity Y at y
R i i i
s yˆ standard deviation of experimentally determined calibration functions (bias due to calibration) of input
( )
i
quantity Y
i
t 97,5 percentile of the t-distribution
0,975
U combined expanded uncertainty of c at C = c expressed as a 95 % confidence interval
c test
U required expanded uncertainty of c at C = c expressed as a 95 % confidence interval
req test
u combined standard uncertainty of c at C = c
c test
u(b) standard uncertainty of b at C = c
j j test
u[c(x )] partial standard uncertainty of c due to the value x of influence quantity j at C = c
j j test
u(x ), u(∆ x ) standard uncertainty of the difference of x between measurement and corresponding calibration
j j j
u partial standard uncertainty of uncertainty source or group of sources of uncertainty represented by
p
performance characteristic p at C = c
test
ucˆˆy partial standard uncertainty of c due to uncertainty of the experimentally determined calibration functions
()
i
of input quantity Y at y corresponding to C = c
i i test
u [c( y )] partial standard uncertainty of c due to lack of fit of the calibration function of input quantity Y at y
fit i i i
corresponding to C = c
test
u [c ( y)] partial standard uncertainty of c due to the random part of instability of input quantity Y at y
inst i i i i
corresponding to C = c
test
u [c( y )] partial standard uncertainty of c due to repeatability of input quantity Y at y corresponding to C = c
r i i i test
u [c( y)] partial standard uncertainty of c due to reproducibility of input quantity Y at y corresponding to
R i i i
C = c
test
u maximum allowable standard uncertainty of the measured value at C = c
req test
u( y ) standard uncertainty of input quantity Y
i i
∂fyy, .,
( )
1 n
w weighting factor of input quantity Y ; first derivative
i i
∂y
i
X influence quantity
© ISO 2002 – All rights reserved 3
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
X jth influence quantity
j
x value of X
j j
x value of influence quantity X during calibration
j, cal j
x maximum value of influence quantity X
j, max j
x minimum value of influence quantity X
j, min j
Y input quantity
Y ith input quantity
i
y value of Y
i i
y lack of fit of input quantity Y at y corresponding to C = c
i, fit i i test
∆c(x ) systematic deviation of c due to x
j j
∆c(x) change in c caused by the maximum positive change of influence quantity X after calibration; take
j, p j
care to include the sign of the value
∆c(x) change in c caused by the maximum negative change of influence quantity X after calibration; take
j, n j
care to include the sign of the value
∆ x difference of x between measurement and corresponding calibration
j j
∆ x maximum positive difference of x between measurement and corresponding calibration
j, p j
∆ x maximum negative difference of x between measurement and corresponding calibration
j, n j
5 Principle
Performance characteristics indicate the deviation from a perfect measurement and therefore contribute to the
uncertainty of the measurement result. The combined impact of the performance characteristics on the
measurement result quantified by measurement uncertainty is taken as the criterion of suitability of a measurement
method rather than each of the performance characteristics.
The procedure for calculating measurement uncertainty as follows is based on the law on propagation of
uncertainty laid down in the GUM.
a) Define the measurand and determine the analytical function relating the measured value to the input
quantities. Take the quantity representing that part of the measurement system covered by calibration as a
single input quantity.
b) Identify all (major) sources of uncertainty (influence quantities) contributing to any of the input quantities or to
the measurand directly.
c) Determine the model function and the variance function. Retain major sources of uncertainty.
d) Use available performance characteristics of the measurement system.
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
e) Assign all (major) sources of uncertainty uniquely to performance characteristics. One performance
characteristic may cover several sources of uncertainty (e.g. reproducibility). Each major uncertainty source
shall not be assigned to more than one performance characteristic. If major sources of uncertainty are not
covered by available performance characteristics, their uncertainty shall be quantified separately.
f) Convert all uncertainty components (performance characteristics) to standard uncertainties of input and
influence quantities. Apply the weighting factor w derived from the analytical function or the sensitivity
i
coefficient b and the difference ∆ x between measurement and corresponding calibration for influence quantity
j j
x to calculate the corresponding standard uncertainty of the measured value.
j
g) Calculate the combined standard uncertainty and the expanded uncertainty taking correlation into account.
h) Judge the suitability of the measurement procedure by comparing the expanded uncertainty with the required
value.
i) Verify the expanded uncertainty in a field test.
j) Accept or reject fitness for use of the measurement procedure.
A flowchart for assessing fitness for use of the measurement procedure regarding the performance under
stationary conditions is given in Figure 1.
The dynamic response may contribute to measurement uncertainty. Performance requirements related to dynamic
conditions of operation are excluded from the uncertainty criterion. For the purpose of this International Standard, it
shall be demonstrated that the impact of the dynamic response on measurement uncertainty is negligible.
© ISO 2002 – All rights reserved 5
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
Figure 1 — Flowchart for assessing fitness for use of the measurement procedure
6 © ISO 2002 – All rights reserved
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
6 Requirements
6.1 Methods and materials
Ensure that the measurand is unambiguously defined.
Describe which steps of the measurement procedure (such as sampling, analysis, postprocessing and calibration)
and which materials (such as reference materials) are included in the procedure to estimate measurement
uncertainty.
If the output is continuous and the additional equipment to obtain a time-averaged value is not covered in the
evaluation, it shall not significantly contribute to measurement uncertainty.
6.2 Performance characteristics
Performance characteristics of the measurement system shall be available.
6.3 Required measurement quality
In order to apply this International Standard, the following information is required:
the required expanded uncertainty U , expressed as a 95 % confidence interval;
req
the test value c at which U is defined;
test req
the averaging time at which U is defined.
req
If the required measurement quality is given as a standard uncertainty, derive the expanded uncertainty by
multiplying by a coverage factor k = 2.
7 Required performance related to dynamic conditions
7.1 General
Dynamic performance characteristics are treated separately from those related to stationary conditions. It shall be
demonstrated that the impact of the dynamic response on measurement uncertainty is negligible.
7.2 Response time
Since the response time is finite, the measured value will be influenced by previous air samples, either by the
sampling process (e.g. residence, mixing, reversible adsorption) or by the measurement process (e.g. electronic
time constant, residence in detection cell). The actual impact depends on the time pattern of the measurand
(frequency and amplitude).
The following requirements apply:
response time shall be less than 25 % of the averaging time, since the impact is generally negligible if the
response time is less than 25 % of the averaging time;
under highly dynamic conditions, where measurand fluctuations higher than the test value, c , occur within
test
5 % of the averaging time, the response time shall be less than 10 % of the averaging time.
The response time applies to continuously measuring systems. For non-continuously measuring systems a similar
characteristic shall be considered, e.g. the residence time in the sampling train.
© ISO 2002 – All rights reserved 7
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
If the requirement is not met, the performance of the measurement procedure shall not be accepted.
8 Required performance related to stationary conditions
8.1 Analytical function, model function and variance function
The value c of a measurand is obtained from input quantities y applying a mathematical relationship called the
i
analytical function f [see Equation (1)]:
cf= (y , .,y ) (1)
1 n
Input quantities are variables and constants. The values of the variables are obtained from (imperfectly) calibrated
systems. As long as actual measurements resemble calibration in every respect, the measurand is solely a function
of the input quantities. The impact of influence quantities, e.g. temperature and sample matrix, is calibrated out.
Usually, actual measurement does not match calibration completely. If the measurement gives rise to additional
terms for influence quantities x , the general model function for the measured value c of the measurand shall be
j
applied [see Equation (2)].
m
cf=+(y , .,y ) b x−x (2)
()
1cnjal ∑jj,cal
j = 1
The size of the influence depends on the sensitivity b and the mismatch (x − x ). As influence quantities are not
j j j, cal
input quantities of the analytical function, their impact shall not be corrected for in the experiment.
The variance function is derived from the general model function by application of the law of propagation of
uncertainty in accordance with the GUM. Provided the input and influence quantities are uncorrelated, the variance
of c is given by Equation (3):
2
2
∂f
2
varcy=+varb varx−x+x−x varb (3)
() () ()()()
i j jj, cal jj, cal j
∑∑ ∑
∂Y
i
ij j
The squared combined standard uncertainty u derived from Equation (3) is a weighted sum of squared
c
uncertainties of input quantities and influence quantities [see Equation (4)]:
222 22
uw=+uy bu∆x (4)
( ) ( )
c ii j j
∑∑
ij
2 2
If the uncertainty in the experimentally determined sensitivity coefficients is not negligible, the term ∆x u (b )
j j
∑
shall be included in Equation (4).
NOTE 1 The intrinsic uncertainty u(y ) of a measured input quantity originate from “natural” fluctuations of the signal (“noise”)
i
and calibration. Sources of calibration uncertainty are lack of fit, uncertainty of reference materials and the uncertainty of the
calibration function due to a limited number of calibration points.
NOTE 2 Automated measuring systems (AMS) determine the measurand directly. Ideally, the measurand is the only input
quantity. However, the transmission efficiency of the sampling line is an extra input quantity, if transmission is not covered by
calibration. Several manual procedures consist of absorbing the analyte in a liquid followed by analysis of the solution in the
laboratory. As the chemical calibration is performed on solutions, the concentration of the analyte in the solution c′ is an input
quantity. The other input quantities are the volume of solution V , the collection efficiency f and the volume of air V . Each
sol col air
of the weighting factors w is directly obtained as a first derivative of the analytical function c = c′ V / ( V f ).
i sol air col
8 © ISO 2002 – All rights reserved
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
Experimentally determined contributions to measurement uncertainty are given by, or are derived from,
performance characteristics. Performance characteristics express directly or indirectly an effect on measurement
uncertainty.
Bias characteristics (e.g. lack of fit and trueness) and dispersion characteristics (e.g. repeatability and
reproducibil
...
SLOVENSKI STANDARD
SIST EN ISO 14956:2003
01-marec-2003
Kakovost zraka - Vrednotenje primernosti merilnega postopka s primerjavo z
zahtevano merilno negotovostjo (ISO 14956:2002)
Air quality - Evaluation of the suitability of a measurement procedure by comparison with
a required measurement uncertainty (ISO 14956:2002)
Luftbeschaffenheit - Beurteilung der Eignung eines Messverfahrens durch Vergleich mit
einer geforderten Messunsicherheit (ISO 14956:2002)
Qualité de l'air - Evaluation de l'aptitude a l'emploi d'une procédure de mesurage par
comparaison avec une incertitude de mesure requise (ISO 14956:2002)
Ta slovenski standard je istoveten z: EN ISO 14956:2002
ICS:
13.040.01 Kakovost zraka na splošno Air quality in general
SIST EN ISO 14956:2003 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST EN ISO 14956:2003
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SIST EN ISO 14956:2003
EUROPEAN STANDARD
EN ISO 14956
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2002
ICS 13.040.01
English version
Air quality - Evaluation of the suitability of a measurement
procedure by comparison with a required measurement
uncertainty (ISO 14956:2002)
Qualité de l'air - Evaluation de l'aptitude à l'emploi d'une Luftbeschaffenheit - Beurteilung der Eignung eines
procédure de mesurage par comparaison avec une Messverfahrens durch Vergleich mit einer geforderten
incertitude de mesure requise (ISO 14956:2002) Messunsicherheit (ISO 14956:2002)
This European Standard was approved by CEN on 28 July 2002.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2002 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 14956:2002 E
worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 14956:2003
EN ISO 14956:2002 (E)
CORRECTED 2002-10-02
Foreword
This document (EN ISO 14956:2002) has been prepared by Technical Committee ISO /TC
146 "Air quality" in collaboration with Technical Committee CEN/TC 264 "Air quality", the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication
of an identical text or by endorsement, at the latest by February 2003, and conflicting national
standards shall be withdrawn at the latest by February 2003.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the
United Kingdom.
Endorsement notice
The text of ISO 14956:2002 has been approved by CEN as EN ISO 14956:2002 without any
modifications.
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SIST EN ISO 14956:2003
INTERNATIONAL ISO
STANDARD 14956
First edition
2002-08-15
Air quality — Evaluation of the suitability of
a measurement procedure by comparison
with a required measurement uncertainty
Qualité de l'air — Évaluation de l'aptitude à l'emploi d'une procédure de
mesurage par comparaison avec une incertitude de mesure requise
Reference number
ISO 14956:2002(E)
©
ISO 2002
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SIST EN ISO 14956:2003
ISO 14956:2002(E)
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© ISO 2002
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SIST EN ISO 14956:2003
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Contents Page
Foreword . iv
Introduction. v
1 Scope. 1
2 Normative reference. 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms. 2
5 Principle . 4
6 Requirements . 7
6.1 Methods and materials . 7
6.2 Performance characteristics. 7
6.3 Required measurement quality. 7
7 Required performance related to dynamic conditions.7
7.1 General . 7
7.2 Response time. 7
8 Required performance related to stationary conditions.8
8.1 Analytical function, model function and variance function. 8
8.2 Identification of sources of uncertainty. 9
8.3 Assignment of sources of uncertainty to performance characteristics. 9
8.4 Definition and quantification of conditions of operation of the measurement system . 10
8.5 Quantification of the impact of selected performance characteristics as partial standard
uncertainties . 11
8.6 Estimation of the combined standard uncertainty . 14
8.7 Estimation of the expanded uncertainty. 14
8.8 Evaluation of compliance with the required measurement quality . 14
9 Field verification. 14
10 Report. 15
Annex A (informative) Default ranges of chemical interferents. 16
Annex B (normative) Coverage factors derived from effective degrees of freedom. 18
Annex C (informative) Example of an assessment of compliance of UV fluorescence method for SO
2
with requirements on ambient air quality. 19
Annex D (informative) Examples of field verification programmes. 24
Bibliography. 26
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SIST EN ISO 14956:2003
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 14956 was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 4, General aspects.
Annex B forms a normative part of this International Standard. Annexes A, C and D are for information only.
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SIST EN ISO 14956:2003
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Introduction
A measuring task generally includes information on the required quality of the measurement result, which may be
quantified by the measurement uncertainty. The required quality may be specified, e.g. by legislation, by authorities
or the parties involved.
The quality of a measurement result strongly depends on the performance of the measuring method used. This
International Standard specifies the procedures to determine the measurement uncertainty of an individual
measurement result, using relevant performance characteristics of the measuring method, and to verify compliance
with the requirements of the measuring task.
A procedure for establishing the uncertainty of the time average of a series of single measurements will be given in
a separate International Standard [3].
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SIST EN ISO 14956:2003
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SIST EN ISO 14956:2003
INTERNATIONAL STANDARD ISO 14956:2002(E)
Air quality — Evaluation of the suitability of a measurement
procedure by comparison with a required measurement
uncertainty
1 Scope
This International Standard specifies, for the field of air quality measurement procedures, the:
estimation of measurement uncertainty from actual or claimed values of all important performance
characteristics of a method under stationary conditions;
assessment of whether or not specified values for these performance characteristics comply with the required
quality of a measured value at a stated measurand value;
evaluation of the applicability of the measurement method based on laboratory performance and confirmatory
field test;
establishment of requirements on dynamic behaviour of instruments.
This International Standard is applicable to measurement procedures whose output is a defined time average.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent edition of the normative document indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 6879:1995, Air quality — Performance characteristics and related concepts for air quality measuring methods
3 Terms and definitions
For the purposes of this International Standard, the terms and definitions given in ISO 6879 and the following
apply.
3.1
dynamic condition
〈of operation〉 condition where the measurand value or/and the value of an influence quantity is time-dependent
3.2
performance requirement
requirement of the measurement, in terms of standard uncertainty and dynamic behaviour, against which the
suitability of the measurement system is being assessed
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3.3
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation.
[GUM:1993, 2.3.1]
3.4
stationary condition
〈of operation〉 condition where the measurand value and the values of all influence quantities are constant.
3.5
uncertainty
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could
reasonably be attributed to the measurand.
[VIM:1993, C.2.11]
4 Symbols and abbreviated terms
b sensitivity coefficient of c for influence quantity x at C = c
j j test
b maximum value of b
j, max j
C measurand
c measured value of the measurand
c value of the measurand at which the required measurement uncertainty is given
test
D( y ) drift of measured value on input quantity Y at C = c
i i test
f ( y ) analytical function; function of input quantities where the impact of influence quantities is excluded
i cal
I ratio of the change in measured value and the corresponding change of the interferent value x
j i
at C = c
test
i index of input quantities Y
j index of influence quantities X
k coverage factor
n total number of input quantities; last number
m total number of influence quantities
P percentage value
p index of the performance characteristic
p maximum number of performance characteristics considered
max
s[c(x )] standard deviation of c caused by x at C = c
j j test
s(x ) standard deviation of x at C = c
j j test
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s ( y ) standard deviation of y due to the random part of instability
inst i i
s ( y ) repeatability standard deviation of input quantity Y at y
r i i i
s ( y ) reproducibility standard deviation of input quantity Y at y
R i i i
s yˆ standard deviation of experimentally determined calibration functions (bias due to calibration) of input
( )
i
quantity Y
i
t 97,5 percentile of the t-distribution
0,975
U combined expanded uncertainty of c at C = c expressed as a 95 % confidence interval
c test
U required expanded uncertainty of c at C = c expressed as a 95 % confidence interval
req test
u combined standard uncertainty of c at C = c
c test
u(b) standard uncertainty of b at C = c
j j test
u[c(x )] partial standard uncertainty of c due to the value x of influence quantity j at C = c
j j test
u(x ), u(∆ x ) standard uncertainty of the difference of x between measurement and corresponding calibration
j j j
u partial standard uncertainty of uncertainty source or group of sources of uncertainty represented by
p
performance characteristic p at C = c
test
ucˆˆy partial standard uncertainty of c due to uncertainty of the experimentally determined calibration functions
()
i
of input quantity Y at y corresponding to C = c
i i test
u [c( y )] partial standard uncertainty of c due to lack of fit of the calibration function of input quantity Y at y
fit i i i
corresponding to C = c
test
u [c ( y)] partial standard uncertainty of c due to the random part of instability of input quantity Y at y
inst i i i i
corresponding to C = c
test
u [c( y )] partial standard uncertainty of c due to repeatability of input quantity Y at y corresponding to C = c
r i i i test
u [c( y)] partial standard uncertainty of c due to reproducibility of input quantity Y at y corresponding to
R i i i
C = c
test
u maximum allowable standard uncertainty of the measured value at C = c
req test
u( y ) standard uncertainty of input quantity Y
i i
∂fyy, .,
( )
1 n
w weighting factor of input quantity Y ; first derivative
i i
∂y
i
X influence quantity
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X jth influence quantity
j
x value of X
j j
x value of influence quantity X during calibration
j, cal j
x maximum value of influence quantity X
j, max j
x minimum value of influence quantity X
j, min j
Y input quantity
Y ith input quantity
i
y value of Y
i i
y lack of fit of input quantity Y at y corresponding to C = c
i, fit i i test
∆c(x ) systematic deviation of c due to x
j j
∆c(x) change in c caused by the maximum positive change of influence quantity X after calibration; take
j, p j
care to include the sign of the value
∆c(x) change in c caused by the maximum negative change of influence quantity X after calibration; take
j, n j
care to include the sign of the value
∆ x difference of x between measurement and corresponding calibration
j j
∆ x maximum positive difference of x between measurement and corresponding calibration
j, p j
∆ x maximum negative difference of x between measurement and corresponding calibration
j, n j
5 Principle
Performance characteristics indicate the deviation from a perfect measurement and therefore contribute to the
uncertainty of the measurement result. The combined impact of the performance characteristics on the
measurement result quantified by measurement uncertainty is taken as the criterion of suitability of a measurement
method rather than each of the performance characteristics.
The procedure for calculating measurement uncertainty as follows is based on the law on propagation of
uncertainty laid down in the GUM.
a) Define the measurand and determine the analytical function relating the measured value to the input
quantities. Take the quantity representing that part of the measurement system covered by calibration as a
single input quantity.
b) Identify all (major) sources of uncertainty (influence quantities) contributing to any of the input quantities or to
the measurand directly.
c) Determine the model function and the variance function. Retain major sources of uncertainty.
d) Use available performance characteristics of the measurement system.
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e) Assign all (major) sources of uncertainty uniquely to performance characteristics. One performance
characteristic may cover several sources of uncertainty (e.g. reproducibility). Each major uncertainty source
shall not be assigned to more than one performance characteristic. If major sources of uncertainty are not
covered by available performance characteristics, their uncertainty shall be quantified separately.
f) Convert all uncertainty components (performance characteristics) to standard uncertainties of input and
influence quantities. Apply the weighting factor w derived from the analytical function or the sensitivity
i
coefficient b and the difference ∆ x between measurement and corresponding calibration for influence quantity
j j
x to calculate the corresponding standard uncertainty of the measured value.
j
g) Calculate the combined standard uncertainty and the expanded uncertainty taking correlation into account.
h) Judge the suitability of the measurement procedure by comparing the expanded uncertainty with the required
value.
i) Verify the expanded uncertainty in a field test.
j) Accept or reject fitness for use of the measurement procedure.
A flowchart for assessing fitness for use of the measurement procedure regarding the performance under
stationary conditions is given in Figure 1.
The dynamic response may contribute to measurement uncertainty. Performance requirements related to dynamic
conditions of operation are excluded from the uncertainty criterion. For the purpose of this International Standard, it
shall be demonstrated that the impact of the dynamic response on measurement uncertainty is negligible.
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SIST EN ISO 14956:2003
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Figure 1 — Flowchart for assessing fitness for use of the measurement procedure
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6 Requirements
6.1 Methods and materials
Ensure that the measurand is unambiguously defined.
Describe which steps of the measurement procedure (such as sampling, analysis, postprocessing and calibration)
and which materials (such as reference materials) are included in the procedure to estimate measurement
uncertainty.
If the output is continuous and the additional equipment to obtain a time-averaged value is not covered in the
evaluation, it shall not significantly contribute to measurement uncertainty.
6.2 Performance characteristics
Performance characteristics of the measurement system shall be available.
6.3 Required measurement quality
In order to apply this International Standard, the following information is required:
the required expanded uncertainty U , expressed as a 95 % confidence interval;
req
the test value c at which U is defined;
test req
the averaging time at which U is defined.
req
If the required measurement quality is given as a standard uncertainty, derive the expanded uncertainty by
multiplying by a coverage factor k = 2.
7 Required performance related to dynamic conditions
7.1 General
Dynamic performance characteristics are treated separately from those related to stationary conditions. It shall be
demonstrated that the impact of the dynamic response on measurement uncertainty is negligible.
7.2 Response time
Since the response time is finite, the measured value will be influenced by previous air samples, either by the
sampling process (e.g. residence, mixing, reversible adsorption) or by the measurement process (e.g. electronic
time constant, residence in detection cell). The actual impact depends on the time pattern of the measurand
(frequency and amplitude).
The following requirements apply:
response time shall be less than 25 % of the averaging time, since the impact is generally negligible if the
response time is less than 25 % of the averaging time;
under highly dynamic conditions, where measurand fluctuations higher than the test value, c , occur within
test
5 % of the averaging time, the response time shall be less than 10 % of the averaging time.
The response time applies to continuously measuring systems. For non-continuously measuring systems a similar
characteristic shall be considered, e.g. the residence time in the sampling train.
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If the requirement is not met, the performance of the measurement procedure shall not be accepted.
8 Required performance related to stationary conditions
8.1 Analytical function, model function and variance function
The value c of a measurand is obtained from input quantities y applying a mathematical relationship called the
i
analytical function f [see Equation (1)]:
cf= (y , .,y ) (1)
1 n
Input quantities are variables and constants. The values of the variables are obtained from (imperfectly) calibrated
systems. As long as actual measurements resemble calibration in every respect, the measurand is solely a function
of the input quantities. The impact of influence quantities, e.g. temperature and sample matrix, is calibrated out.
Usually, actual measurement does not match calibration completely. If the measurement gives rise to additional
terms for influence quantities x , the general model function for the measured value c of the measurand shall be
j
applied [see Equation (2)].
m
cf=+(y , .,y ) b x−x (2)
()
1cnjal ∑jj,cal
j = 1
The size of the influence depends on the sensitivity b and the mismatch (x − x ). As influence quantities are not
j j j, cal
input quantities of the analytical function, their impact shall not be corrected for in the experiment.
The variance function is derived from the general model function by application of the law of propagation of
uncertainty in accordance with the GUM. Provided the input and influence quantities are uncorrelated, the variance
of c is given by Equation (3):
2
2
∂f
2
varcy=+varb varx−x+x−x varb (3)
() () ()()()
i j jj, cal jj, cal j
∑∑ ∑
∂Y
i
ij j
The squared combined standard uncertainty u derived from Equation (3) is a weighted sum of squared
c
uncertainties of input quantities and influence quantities [see Equation (4)]:
222 22
uw=+uy bu∆x (4)
( ) ( )
c ii j j
∑∑
ij
2 2
If the uncertainty in the experimentally determined sensitivity coefficients is not negligible, the term ∆x u (b )
j j
∑
shall be included in Equation (4).
NOTE 1 The intrinsic uncertainty u(y ) of a measured input quantity originate from “natural” fluctuations of the signal (“noise”)
i
and calibration. Sources of calibration uncertainty are lack of fit, uncertainty of reference materials and the uncertainty of the
calibration function due to a limited number of calibration points.
NOTE 2 Automated measuring systems (AMS) determine the measurand directly. Ideally, the measurand is the only input
quantity. However, the transmission efficiency of the sampling line is an extra input quantity, if transmission is not covered by
calibration. Several manual procedures consist of absorbing the analyte in a liquid followed by analysis of the solution in the
laboratory. As the chemical calibration is performed on solutions, the concentration of the analyte in the solution c′ is an input
quantity. The other input quantities are the volume of solution V , the collection efficiency f and the volume of air V . Each
sol col air
of the weighting factors w is directly obtained as a first derivative of the analytical function c = c′ V / ( V f ).
i sol air col
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Experimentally determined contributions to measurement uncertainty are given by, or are derived from,
performance characteristics. Performance characteristics express directly or indirectly an effect on measurement
uncertainty.
Bias characteristics (e.g. lack of fit and trueness) and dispersion characteristics (e
...
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