SIST EN ISO 6142:2006
(Main)Gas analysis - Preparation of calibration gas mixtures - Gravimetric method (ISO 6142:2001)
Gas analysis - Preparation of calibration gas mixtures - Gravimetric method (ISO 6142:2001)
Specifies the gravimetric method for the preparation of reference gas mixtures in cylinders and is intended for the preparation of calibration gas mixtures whose target accuracy is defined a priori. It is applicable only to mixtures of gaseous, or totally vaporized components which do not react with each other or with the cylinder walls. A procedure is given for the preparation strategy that is based upon preset uncertainty levels of the composition of the final mixture. Multi-component gas mixtures and multiple dilution mixtures are included in this standard and are considered to be special cases of single component weighings.
Gasanalyse - Herstellung von Prüfgasen - Wägeverfahren (ISO 6142:2001)
Diese Internationale Norm legt ein gravimetrisches Verfahren für die Herstellung von Kalibriergasgemischen
in Gasflaschen, von denen die Zielgenauigkeit der Zusammensetzung vorgegeben ist. Es ist nur auf
Gemische von gasförmigen oder vollständig verdampften Komponenten anwendbar, die weder miteinander
noch mit den Flaschenwänden reagieren. Es wird ein Herstellungsverfahren angegeben, das den
Anforderungen an die Zusammensetzung des Endgasgemisches innerhalb vorher festgelegter Unsicherheits-niveaus
zu liegen, genügt. Mehrkomponenten-Gasgemische (einschließlich Erdgas) und Mehrfachver-dünnungsgemische
sind in dieser Internationalen Norm ebenfalls enthalten und werden als Spezialfälle der
gravimetrischen Herstellungsmethode für Einzelkomponenten betrachtet.
Diese Internationale Norm beschreibt auch das Verfahren zur Verifizierung der Zusammensetzung von
gravimetrisch hergestellten Kalibriergasen. Unter der Voraussetzung, dass bei der Herstellung und
Validierung dieser gravimetrischen Gasgemische strenge und umfangreiche Qualitätssicherungs- und
Qualitätskontrollverfahren übernommen werden, können für eine große Palette von Gasgemischen
Kalibriergase höchster Genauigkeit im Vergleich zu anderen Herstellungsmethoden für solche Gase erhalten
werden.
Analyse des gaz - Préparation des mélanges de gaz pour étalonnage - Méthode gravimétrique (ISO 6142:2001)
L'ISO 6142:2001 spécifie une méthode gravimétrique de préparation des mélanges de gaz pour étalonnage dans des bouteilles dont l'exactitude de la composition a été prédéfinie. Elle est applicable uniquement aux mélanges de composants gazeux ou totalement vaporisés qui ne réagissent pas entre eux ou au contact des parois de bouteilles. Un mode opératoire pour une méthode de préparation est fourni avec un certain nombre d'exigences à respecter pour que la composition finale des mélanges gazeux se situe dans les niveaux d'incertitude préétablis. Les mélanges gazeux à plusieurs constituants (y compris le gaz naturel) et les mélanges à dilution multiple sont inclus dans l'ISO 6142:2001 et sont considérés comme des cas particuliers de la méthode de préparation gravimétrique à un seul constituant.
L'ISO 6142:2001 décrit également la méthode de vérification de la composition des gaz pour étalonnage préparés par voie gravimétrique. Sous réserve de l'adoption, au cours de la préparation et de la validation de ces mélanges gazeux gravimétriques, de procédures rigoureuses et exhaustives d'assurance et de contrôle de la qualité, des gaz pour étalonnage de haute exactitude peuvent être obtenus pour une large gamme de mélanges gazeux, en comparaison avec d'autres méthodes de préparation pour ces mêmes gaz.
Analiza plinov - Priprava kalibrirnih plinskih zmesi - Gravimetrijska metoda (ISO 6142:2001)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 6142:2006
01-november-2006
Analiza plinov - Priprava kalibrirnih plinskih zmesi - Gravimetrijska metoda (ISO
6142:2001)
Gas analysis - Preparation of calibration gas mixtures - Gravimetric method (ISO
6142:2001)
Gasanalyse - Herstellung von Prüfgasen - Wägeverfahren (ISO 6142:2001)
Analyse des gaz - Préparation des mélanges de gaz pour étalonnage - Méthode
gravimétrique (ISO 6142:2001)
Ta slovenski standard je istoveten z: EN ISO 6142:2006
ICS:
71.040.40 Kemijska analiza Chemical analysis
SIST EN ISO 6142:2006 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 6142:2006
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SIST EN ISO 6142:2006
EUROPEAN STANDARD
EN ISO 6142
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2006
ICS 71.040.40
English Version
Gas analysis - Preparation of calibration gas mixtures -
Gravimetric method (ISO 6142:2001)
Analyse des gaz - Préparation des mélanges de gaz pour Gasanalyse - Herstellung von Prüfgasen - Wägeverfahren
étalonnage - Méthode gravimétrique (ISO 6142:2001) (ISO 6142:2001)
This European Standard was approved by CEN on 21 July 2006.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,
Slovakia, Slovenia, 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
© 2006 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6142:2006: E
worldwide for CEN national Members.
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SIST EN ISO 6142:2006
EN ISO 6142:2006 (E)
Foreword
The text of ISO 6142:2001 has been prepared by Technical Committee ISO/TC 158 "Analysis
of gases” of the International Organization for Standardization (ISO) and has been taken over
as EN ISO 6142:2006 by Technical Committee CEN/SS N21 "Gaseous fuels and combustible
gas", the secretariat of which is held by CMC.
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 2007, and conflicting national
standards shall be withdrawn at the latest by February 2007.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Endorsement notice
The text of ISO 6142:2001 has been approved by CEN as EN ISO 6142:2006 without any
modifications.
2
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SIST EN ISO 6142:2006
INTERNATIONAL ISO
STANDARD 6142
Second edition
2001-04-01
Gas analysis — Preparation of calibration
gasmixtures—Gravimetric method
Analyse des gaz — Préparation des mélanges de gaz pour étalonnage —
Méthode gravimétrique
Reference number
ISO 6142:2001(E)
©
ISO 2001
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
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ii © ISO 2001 – All rights reserved
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
Contents Page
Foreword.iv
1 Scope .1
2 Normative references .1
3 Principle.1
4 Preparation of the mixture .2
5 Calculation of uncertainty.7
6 Verification of calibration gas mixture composition.9
7 Test report .10
Annex A (informative) Practical example.11
Annex B (informative) Guidelines for estimating filling pressures so as to avoid condensation of
condensable components in gas mixtures.22
Annex C (informative) Precautions to be taken when weighing, handling and filling cylinders .25
Annex D (informative) Derivation of the equation for calculating the calibration gas mixture
composition.29
Annex E (informative) Sources of error .31
Annex F (informative) Estimation of corrections and correction uncertainty .33
Annex G (informative) Computer implementation of recommended methods.35
Bibliography.36
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SIST EN ISO 6142:2006
ISO 6142:2001(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.
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.
International Standard ISO 6142 was prepared by Technical Committee ISO/TC 158, Analysis of gases, in
collaboration with ISO/TC 193, Natural gas.
This second edition cancels and replaces the first edition (ISO 6142:1981), which has been revised to update the
methods of preparation, estimation of the uncertainty and of validation of gravimetrically prepared calibration gases.
Annexes A to G of this International Standard are for information only.
iv © ISO 2001 – All rights reserved
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SIST EN ISO 6142:2006
INTERNATIONAL STANDARD ISO 6142:2001(E)
Gas analysis — Preparation of calibration gas mixtures —
Gravimetric method
1 Scope
This International Standard specifies a gravimetric method for the preparation of calibration gas mixtures in
cylinders of which the target accuracy of the composition has been pre-defined. It is applicable only to mixtures of
gaseous or totally vaporized components which do not react with each other or with the cylinder walls. A procedure
is given for a method of preparation based on requirements for the final gas mixture composition to be within pre-
set levels of uncertainty. Multi-component gas mixtures (including natural gas) and multiple dilution mixtures are
included in this International Standard and are considered to be special cases of the single component gravimetric
preparation method.
This International Standard also describes the procedure for verifying the composition of gravimetrically prepared
calibration gases. Provided rigorous and comprehensive quality assurance and quality control procedures are
adopted during the preparation and validation of these gravimetric gas mixtures, calibration gases of the highest
accuracy can be obtained for a wide range of gas mixtures, in comparison with other methods of preparing such
gases.
2 Normative references
The following normative documents contain 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 editions of the normative documents 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 6141, Gas analysis — Requirements for certificates for calibration gases and gas mixtures.
1)
ISO 6143:— , Gas analysis — Comparison methods for determining and checking the composition of calibration
gas mixtures.
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories.
IUPAC, Commission on atomic weights and isotopic abundances: Atomic Weights of the Elements, biennial review.
3Principle
Calibration gas mixtures are prepared by transferring parent gases (pure gases or gravimetrically prepared
mixtures of known composition) quantitatively from supply cylinders to the cylinder in which the calibration gas
mixture will be contained. The amount of gaseous component added from the parent gas is determined by
weighing after each successive addition.
1) To be published. (Revision of ISO 6143:1981)
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
The amount of parent gas added to the cylinder in which the calibration gas mixture will be contained is determined
by weighing either the supply cylinder or, alternatively, the cylinder in which the calibration gas mixture will be
contained, before and after each addition. The difference in these two weighings corresponds to the mass of the
gas added. The choice between these two weighing methods depends on which one represents the most suitable
procedure for preparing the specified mixture. For example, the addition of small amounts of a specified component
may best be performed by weighing a small, low-volume supply cylinder, before and after addition, on a highly
sensitive, low-capacity balance.
A single-step preparation method may be used where the amount of each gaseous component required is large
enough to accurately measure the mass of the cylinder, in which the calibration gas mixture will be contained, at
each addition within the required composition uncertainty of the final calibration gas mixture. Alternatively, a
multiple dilution method may be used to obtain a final mixture with acceptable uncertainty, particularly when low
concentrations of the minor components are required. In this method, “pre-mixtures” are gravimetrically prepared
and used as parent gases in one or more dilution steps.
The mass fraction of each component in the final calibration gas mixture is then given by the quotient of the mass
of that component to the total mass of the mixture.
The gravimetric method scheme for preparing calibration gas mixtures, based on pre-set requirements for
composition and the level of uncertainty, is given as a flow chart in Figure 1. The individual steps are explained in
more detail in clause 4 (reference is given to the subclause for each step in Figure 1). An example of the
gravimetric method scheme for preparing a calibration gas mixture following the Figure 1 flow chart is given in
annex A.
4 Preparation of the mixture
4.1 Mixture composition and uncertainty
The composition of the final gas mixture is, by the principle of the gravimetric method, defined by the mass of each
component. Gas composition is preferentially expressed as a mole fraction (mol/mol). If other quantities of
composition are required (for example mass concentration or volume fraction) then the applicable conditions
(pressure and temperature) shall be given and the additional uncertainty contributions shall be determined and
considered in the calculation of the uncertainty in the composition of the calibration gas. The uncertainty of the final
mixture composition is expressed as an expanded uncertainty, i.e. the combined standard uncertainty multiplied by
a coverage factor.
The molar masses of the components, and their uncertainties, needed for the conversion of mass fraction to mole
fraction, shall be derived using the most recent publication of the commission on atomic weights and isotopic
abundances of the International Union of Pure and Applied Chemistry (IUPAC).
4.2 Feasibility of obtaining the gas mixture
4.2.1 General
Gas mixtures potentially capable of reacting dangerously shall be excluded for safety reasons. These phenomena
shall be taken into account when considering the feasibility of preparing the required gas mixture, described in
4.2.2 to 4.2.4.
2 © ISO 2001 – All rights reserved
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
Figure 1 — Gravimetric method scheme for preparing calibration gas mixtures
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
4.2.2 Condensation of the vapour to either a liquid or a solid phase
When preparing, storing or handling gas mixtures which contain condensable components (see annex B), the
following measures shall be taken to prevent condensation because loss by condensation will change the gas
phase composition.
� During the preparation of the gas mixture, the filling pressure shall be set safely below the dew-point vapour
pressure of the final mixture at the filling temperature. To prevent condensation at intermediate stages, this
condition shall be fulfilled for every intermediate mixture as well. If condensation of an intermediate mixture
cannot be safely excluded, measures shall be taken to vaporize any possible condensate and to homogenize
the gas phase at an appropriate later stage.
� During the storage of the gas mixture, the storage temperature shall be set so as to maintain the filling
pressure safely below the dew-point vapour pressure of the mixture at that temperature.
� During the handling of the gas mixture, the same condition on the handling temperature applies. Furthermore,
to prevent condensation during mixture transfer, the transfer lines shall be heated if required.
In informative annex B, some guidance is given for estimating the maximum filling pressure for introducing
components of a gas mixture at which no condensation of the condensable components is expected to occur. An
example of this estimation is given in B.2 for a natural gas mixture.
4.2.3 Reactions between mixture components
Before preparing a gas mixture, it is necessary to consider possible chemical reactions between the components of
the mixture. The method cannot be used to prepare mixtures
� containing potentially interactive substances (e.g. hydrochloric acid and ammonia),
� producing other possible dangerous reactions including explosions (e.g. mixtures containing flammable gases
and oxygen),
� producing strong exothermic polymerizations (e.g. hydrogen cyanide), and
� which can decompose (e.g. acetylene).
Exceptionally this method can be used for substances undergoing dimerization, such as NO to N O ,which is a
2 2 4
reversible reaction.
A comprehensive compilation of reactive combinations is not available. Therefore, chemical expertise is necessary
to assess the stability of a gas mixture.
For dangerous reactions and dangerous combinations, to be excluded for safety reasons, some information can be
found in regulations on dangerous goods and in gas supplier handbooks.
4.2.4 Reactions with container materials
Before preparing a gas mixture, it is necessary to consider possible chemical reactions of mixture components with
materials of a high-pressure cylinder, its valve and the transfer system. Special consideration shall be given to the
attack by corrosive gases with metals and possible reactions with elastomers and greases used, for example, in the
valve seat and seals. Such reactions should be prevented by using only materials that are inert to all components
of the mixture. If this is not possible, measures shall be taken to minimize corrosive attack on the materials with
which the gases make contact so as to prevent any significant effect on mixture composition and any danger in
storage and use.
Information on the compatibility of gases with container materials is given in gas sampling guidelines, corrosion
tables and gas supplier handbooks.
4 © ISO 2001 – All rights reserved
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
4.3 Purity analysis of primary gas standards
The accuracy achievable by the gravimetric method will depend significantly on the purity of the parent gases used
for the preparation of the calibration gas mixture. Impurities in the parent gases are often one of the most critical
contributors to the uncertainty of the final mixture composition. The uncertainty contributions depend on the amount
of impurities present in the pure, parent gases and upon the accuracy with which these impurities have been
measured. In many cases the purity of the major component (matrix gas) is of most importance. This is especially
true when the mole fraction of the minor component is low and is likely to be an impurity in the major component. It
is also important to evaluate critical impurities that may react with the minor component (e.g. oxygen present in
pure nitrogen will react with NO to form NO ). The result of purity analysis of parent gases shall be incorporated
2
into a purity table containing the mole (or mass) fractions of all components with accompanying uncertainties
derived from analysis.
Generally, impurities in a nominally “pure” parent gas are established by analysis and the mole fraction of the major
component is conventionally determined by difference such that
N
xx� 1� (1)
pure � i
i�1
where
x is the mole fraction of impurity i, determined by analysis;
i
N is the number of impurities likely to be present in the final mixture;
x is themolefraction “purity” of the “pure” parent gas.
pure
When an impurity, likely to be present in the “pure” parent gas, is not detectable by the analytical method used, the
mole fraction of the expected impurity shall be set equal to half of the value of the detection limit of the analytical
method. The uncertainty of the determination of this mole fraction is based upon a rectangular distribution between
zero and the value of the detection limit of the analytical method. In this way, the gravimetric method assumes that
there is an equal likelihood that the impurity may be present in the “pure” parent gas at a level up to its value of the
detection limit. Hence, the content of an undetected impurity forms a rectangular distribution from which its
standard uncertainty is defined as half the value of the detection limit divided by 3 .
4.4 Choice of preparation procedure
When choosing a suitable preparation procedure, a number of considerations shall be made to ensure the most
appropriate method is used. The following is a list of parameters which shall be considered:
� pressure at which the gases are available and possibility of condensation (see annex B);
� maximum filling pressure of the cylinder to be used;
� established composition of each parent gas mixture used;
� filling method, i.e. direct method, multiple dilution, transfer method (use of small cylinder separately weighed
on a low-capacity, high-resolution balance);
� characteristics of the type of balance to be used with its determined performance specifications;
� requirements for the preparation tolerance.
First calculate the value of the masses desired, or target masses m , of each component i, using equation (2).
i
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
xM
ii
mm�� (2)
i f
N
xM
� jj
j�1
where
x is the mole fraction of component i;
i
x is the mole fraction of component j;
j
M is the molar mass of component i;
i
M is the molar mass of component j;
j
N is the number of components in the final mixture;
m is the mass of final mixture.
f
After the target masses have been calculated, a preparation procedure is selected and the uncertainties associated
with the preparation process are calculated. If the calculated uncertainty for that procedure proves to be
unacceptable, another procedure shall be adopted. It may be necessary to perform an iterative process to select a
procedure with acceptable uncertainty.
These considerations result in a preparation procedure whereby a filling sequence consisting of several stages is
selected in which gases are transferred into a cylinder in which the calibration gas mixture will be contained and
subsequently weighed. Each stage has its own associated uncertainty and when combined, remain within the
required level of uncertainty. This procedure shall be used in the subsequent preparation.
4.5 Preparation of the mixture
Precautions to be taken when weighing, handling and filling cylinders are given for information in annex C.
To achieve the intended composition of the mixture, a tool is required. Normally the parameters used in targeting
this composition are pressure and/or mass. When pressure is used for targeting this composition, temperature
effects, resulting from the pressurization and the compressibility of the introduced components, is of importance. In
particular, non-ideal behaviour of certain components makes it difficult to establish a simple relationship between
added pressure and added mass. However, the compression factor, which quantifies these deviations from ideal
behaviour, is a function of pressure, temperature and composition and can be calculated and used to predict the
required pressure.
A more direct way of targeting the desired masses is by use of a balance on which the cylinder is placed to observe
the difference in mass which occurs during transfer.
6 © ISO 2001 – All rights reserved
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
4.6 Calculation of the mixture composition
The mole fractions of the components in the final mixture are calculated using equation (3):
P
��xm�
iA, A
�
��n
A�1
��
xM�
� iA, i
��
��
i�1
x � (3)
i
P
��m
A
�
n
��
A�1
��
xM�
� iA, i
��
��
i�1
where
x is the mole fraction of the component i in the final mixture, i = 1,., n;
i
P is the total number of the parent gases;
n is the total number of the components in the final mixture;
m is the mass of the parent gas A determined by weighing, A = 1,., P;
A
M is the molar mass of the component i, i =1,…, n;
i
x is the mole fraction of the component i, i =1,…, n, in parent gas A, A = 1,., P.
i,A
A method for deriving this formula is given for information in annex D.
5 Calculation of uncertainty
5.1.1 The uncertainty in the values of the mole or mass fractions of the components in a gravimetrically prepared
calibration gas mixture indicates the dispersion of values which can reasonably be attributed to these fractions.
The procedure for evaluating the uncertainty may be summarized in 5.1.2 to 5.1.7.
5.1.2 Identify the steps taken in the preparation procedure. Following equation (3) in 4.6, three categories can be
identified that will influence the uncertainty:
� the uncertainty in the weighing of the parent gases;
� the uncertainty in the purity of the parent gases;
� the uncertainty in molar masses.
NOTE The parent gases may themselves be gravimetrically prepared mixtures.
5.1.3 For each step in the gravimetric preparation procedure, a list shall be made of all sources of uncertainty,
i.e., a list of all factors that may influence the resulting composition. A list of possible error sources is given for
information in annex E. Some of these uncertainty contributions, for example the standard deviation in the repeated
weighings, can be determined by repeated measurements (type A evaluation). For a well-characterized
2
measurement under statistical control, a combined or pooled estimate of variance s (or a pooled experimental
p
standard deviation s ) that characterizes the measurement may be available. In such cases, when the value of the
p
measurand q is determined from n independent observations, the experimental variance of the arithmetic mean q
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SIST EN ISO 6142:2006
ISO 6142:2001(E)
2
2
of the mean observations is more closely estimated bysn than bysn and the standard uncertainty more
p q
closely estimated by us� n . For uncertainty contributions that cannot be estimated by repeated
p
measurements (type B evaluation), a realistic evaluation should be made to estimate this contribution. This applies,
for example, to adsorption/desorption effects and thermal effects on the cylinder that influence the balance.
Variations in some parameters can be decreased by monitoring and/or controlling these and then calculating the
appropriate correction factors. For example, the uncertainty of the buoyancy effect may be decreased by accurately
monitoring the ambient pressure, humidity and temperature conditions and using these to calculate the density of
air at the time of weighing. Each significant uncertainty contribution shall be evaluated as a standard uncertainty,
i.e. as a single standard deviation.
NOTE More details about type A and type B evaluations of standard uncertainty are given in the Guide to expression of
[17]
uncertainty in measurement .
5.1.4 For each contribution to the total uncertainty, decide which ones merit evaluation (significant contributions)
and which ones can be neglected (insignificant contributions). As the total certainty is the sum of squared
contributions, a contribution equalling less than 1/10 of the largest contribution can safely be neglected.
NOTE This method cannot always be applicable to the purity analysis of the parent gases, as some insignificant impurities
can be critical to the mixture under preparation (for example, some impurities can react with the minor component). In such
cases, an evaluation of the influence of the parent gas purity o
...
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