EN ISO 6142:2006

SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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.
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© 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.

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.
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
ISO 6142:2001(E)
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ii © ISO 2001 – All rights reserved

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
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

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)
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

ISO 6142:2001(E)
Figure 1 — Gravimetric method scheme for preparing calibration gas mixtures
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

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
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
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

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
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
ISO 6142:2001(E)
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 on the contribution to total uncertainty is necessary.
5.1.5 The combined uncertainty due to the contributions from the molar masses of the components, the weighing
results and the purity analyses is obtained by uncertainty propagation of equation (3) in 4.6. In this equation, the
targeted component quantities x are expressed as functions of a number of input quantities y , y , ., y ,i.e.
i 1 2 q
xf� (y ,y ,.,y )
ii 12 q
(4)
Here the input quantities y comprise the molar masses M , the parent gas masses m , and the mole fractions x .
i j A j,A
According to the rules of uncertainty propagation the standard uncertainties u(x)aregivenasfollows:
i
qq�1q
�� ����
��ff�f
22iii
ux()��u (y)�2 �u(y,y) (5)
ir rs
���
�� ����
��yy�y
�� ����
rrs
rr��11s�r�1
In this equation u(y ) are the standard uncertainties of the input quantities, and u(y ,y ) are the covariances
r r s
between different input quantities, should these be correlated.
Correlations can be made, for example, between different parent gas masses m , m , if these are determined as
A B
differences between the results of successive weighings. Correlations can also be made between the mole
fractions x , x of different components in the same parent gas, due to the fact that these mole fractions add up
j,A k,A
to unity. To avoid making such correlations, the primary input quantities can be considered instead. For example, in
the case of parent gas masses, these are the results of successive weighings, starting with the empty cylinder in
which the calibration gas mixture is to be contained (see informative annex C). In the case of the mole fractions of
parent gas components, the problem of correlation can be resolved by expressing the mole fraction of the main
component as the difference of the sum of the mole fractions of all the other components from unity (see 4.3),
which are generally uncorrelated. If, in this manner, the targeted component quantities x have been expressed as
i
functions of uncorrelated input quantities z , z , ., z ,i.e.
1 2 p
xg� (z ,z ,.,z ) (6)
ii 12 p
the combined uncertainties u(x ) are simply given by:
i
p
��
�g
22i
ux()��u (z) (7)
it�
��
�z
��
t
t�1
8 © ISO 2001 – All rights reserved

ISO 6142:2001(E)
or by:
pp
ux()����c u()z � u ()x (8)
ii t ti
����
tt��11
where c , the sensitivity coefficient is given by:
i
�g
i
cu��,(x)c�u(z)
itiit
�z
t
5.1.6 The total uncertainty is given by the result of this calculation, combined with the contributions from all other
sources of significant error. Simple methods for evaluating uncertainty contributions based on estimates of
maximum errors are described in annex F.
NOTE The total uncertainty is only applicable to stand-alone applications of single analyte contents. In any joint application
of several analyte contents or the complete composition, covariances have to be taken into account, whose estimation is
beyond the scope of this International Standard.
5.1.7 In order to obtain the expanded uncertainty, the combined standard uncertainty is multiplied by the
coverage factor, k.
NOTE 1 A coverage factor, k, is typically in the range from 2 to 3.
NOTE 2 Within ISO TC 158, a coverage factor of k = 2 has been agreed, unless specific reasons necessitate another choice.
NOTE 3 For a normal distribution, a coverage factor of k = 2 corresponds to a coverage probability of approximately 95 %.
More information on the estimation of corrections and correction uncertainty is given for information in annex F. A
computer program implementing the recommended methods for calculating gas mixture composition as well as
uncertainty is given for information in annex G.
6 Verification of calibration gas mixture composition
6.1 Objectives
The objective of verifying the composition of a calibration gas mixture is to check that the composition, calculated
from the gravimetric process, is consistent with measurements made on the mixture by independent means, for
example by an analytical comparison method. This verification acts to highlight significant errors in the preparation
process of the individual gas mixture. Moreover, further checks may be required over a period of time to
demonstrate the stability of specific mixtures.
Verification of the composition of a gas mixture may be achieved by:
a) establishing consistency between prepared mixtures and appropriate traceable standards (see note);
b) establishing consistency between several nominally similar prepared mixtures;
c) monitoring continuing production of validated mixtures using a suitable statistical process control method.
NOTE A traceable standard refers to a mixture of appropriate metrological quality that is traceable, through an unbroken
chain of comparisons with stated uncertainties, to a national or an International Standard.
When seeking to verify a prepared mixture and confirm its composition, the mixture components may all be in a
range where suitable traceable standards are readily available, so that consistency may easily be demonstrated.
However, often the reason for relying on gravimetric preparation is that one or more components are outside the
range where traceable standards already exist, so verification shall be performed by other methods, such as
ISO 6142:2001(E)
demonstrating internal consistency of prepared mixtures and the capability of the process in suitable ranges where
traceable standards are available.
In practice, one of the two cases in 6.2 and 6.3 can be used for verification of the composition of a freshly prepared
calibration gas mixture.
6.2 Traceable standards available for direct comparison with the mixture
Each single gravimetrically prepared calibration gas mixture should be verified with traceable standards following
the procedure described in 6.1 of ISO 6143:—.
6.3 Traceable standards not available for direct comparison with the mixture
When the approach described in 6.2 cannot be used, the following subsequent steps should be used for the
verification of the prepared calibration gas mixture.
a) Prepare at least five calibration gas mixtures with the required mole or mass fraction lying within the expected
linear range of the analytical method. Prepare these mixtures independently, i.e. no two mixtures should be
made from the same parent gas mixture. Verify that they are consistent with each other, following the
procedure described in 6.2 of ISO 6143:—.
b) Verify the preparation procedure. Use the same preparation procedure as in a) to prepare a “check” gas
mixture consisting of components having traceable standards available. Verify the composition of this check
mixture using the procedure given in 6.1 of ISO 6143:—. If the value of the composition obtained analytically is
consistent with the gravimetric value, then this provides some evidence that the preparation procedure is
suitable. It is preferable to use a “check” component which is similar chemically to the component of interest.
c) In cases where the composition of the component of interest in a multi-component mixture falls outside the
range of traceable standards available or when no traceable standards exist for that component, comparison
with gas mixture(s) prepared by another standardized method may be required for the verification of that
[19]
component (e.g. dynamic volumetric methods ISO 6145 ).
7 Test report
The test report shall be prepared in accordance with the general requirements of ISO/IEC 17025. Requirements on
the contents of certificates for calibration gases are specified in ISO 6141.
The following information shall be given in the test report:
a) a reference to this International Standard, i.e. ISO 6142;
b) the preparation procedure;
c) a purity table for all the parent gases;
d) the masses of gases transferred at each stage in the preparation procedure;
e) a list of the contributions to the composition uncertainty;
f) the details of all of the verification procedures;
g) the final composition, including the expanded uncertainty.
10 © ISO 2001 – All rights reserved

ISO 6142:2001(E)
Annex A
(informative)
Practical example
A.1 Introduction
In order to facilitate understanding of this International Standard, especially the calculation of the composition and
the calculation of uncertainty, an example is given. The magnitude of the uncertainties and the instrumentation
presented should not be regarded as representative for a typical preparation process. The result of the analysis will
depend on the equipment available. The example follows the flow chart in Figure 1.
A.2 Starting parameters
�3
Mixture: 1 � 10 mol/mol carbon monoxide in nitrogen
desired expanded relative uncertainty: 0,5 %, k =2
Desired total pressure: 150 � 10 Pa (150 bar)
�3 3
Cylinder: 5 � 10 m aluminium
�2
Component purities: carbon monoxide 99,9 � 10 mol/mol
�2
nitrogen 99,999 � 10 mol/mol
Balance: mechanical, capacity 10 kg
Readability of balance: 1 mg
Weighing uncertainty for a 5 l cylinder: � 4 mg per weighing (pooled experimental standard deviation s )
p
Number of weighings: 3
A.3 Evaluation of mixture feasibility
The components of this mixture are not reactive with each other. Furthermore, it is known from previous experience
that mixtures containing considerably lower concentrations of carbon monoxide in nitrogen are stable in aluminium
cylinders. Thus, there is no risk of reaction between the components nor reaction between the components and the
cylinder. If the stability is not known, tests should be performed beforehand. It is known, for example, that mixtures
with carbon monoxide prepared in steel cylinders can be unstable.
There is no risk of condensation of carbon monoxide. The mixture is feasible and there should be no problem its
stability.
A.4 Choice of preparation procedure
A calculation should first be performed, to establish whether the chosen gas mixture can be prepared directly or
whether multi-stage dilutions or pre-mixtures are required.
ISO 6142:2001(E)
The masses targeted are calculated as follows:
xp��V �M
iifcyl
m � (A.1)
i
RT��Z
f
where
m is the mass, expressed in grams, of component i in the mixture;
i
x is the intended mole fraction of component i;
i
p is the final fill pressure, expressed in pascals, of the mixture;
f
V is the volume, expressed in cubic metres, of the cylinder;
cyl
M is the molar mass, expressed in grams per mole, of component i;
i
R is the gas constant (8,314 51 J/mol�K);
T is the temperature, expressed in kelvin;
Z is the compression factor of the mixture at T and p .
f f
In this example
�3 �2
x =1 � 10 mol/mol x = 99,9 � 10 mol/mol
CO N
p = 150 � 10 Pa M = 28,013 48 g/mol
f N
�3 3
V =5 � 10 m m =0,86g
cyl CO
M = 28,010 4 g/mol m =858,6 g
CO N
T = 294 K
Z =1,0
f
The weighing technique used is well-characterized and under statistical control. The pooled estimate of standard
deviation s is 4 mg; the standard uncertainty isus� n ; in this case, it is equal to 2,3 mg. The estimated mass
p p
of carbon monoxide is 860 mg. A standard uncertainty of 2,3 mg makes a contribution of 0,27 % relative to the final
uncertainty. The requested expanded uncertainty compared to this uncertainty contribution is already so close to
the limit that it should be minimized.
Consequently, it would be best to perform a multi-step dilution or to weigh the carbon monoxide in a separate
smaller container on a low-capacity balance. In this example, the first method is evaluated and a pre-mixture is
prepared.
In order to limit the uncertainty contribution of weighing the carbon monoxide to at most 0,05 %, a mass of at least
8 g of carbon monoxide should be weighed.
For the pre-mixture, a mass of 8,5 g of carbon monoxide is chosen. If the total pressure of the mixture is
150 � 10 Pa, the maximum amount of N � CO should be 850 g.
�2
The approximated mole fraction of the pre-mixture of CO (x )is1 � 10 mol/mol.
pm
�3
This pre-mixture should then be diluted by a factor 10 to reach the final mole fraction of 1 � 10 mol/mol.
12 © ISO 2001 – All rights reserved

ISO 6142:2001(E)
The masses targeted can be calculated using equation (A.1) assuming a 10 % dilution.
�2 �2
x =10 � 10 mol/mol x =90 � 10 mol/mol
pm N2
p = 150 � 10 Pa M = 28,013 48 g/mol
f N2
�3 3
V =5 � 10 m m = 85,9 g
cyl pm
M = 28,013 44 g/mol m = 773,5 g
mix N2
T = 294 K
Z =1,0
f
A.5 Evaluation of sources of uncertainty
A.5.1 General
The sources of uncertainty are identified in three categories, as described in A.5.2 to A.5.4.
A.5.2 Uncertainty in the weighings (category 1)
A.5.2.1 Balance (u )
m
The uncertainty given in A.2 has been determined by repeated weighing of a cylinder which has undergone a
simulated filling process. The uncertainty thus incorporates the following components: resolution of balance, drift,
incorrect zero point, effect of location of the cylinder on the pan, typical changes (not those which are occasionally
large ones) in mass due to handling and connection of cylinder and adsorption phenomena occurring when the
cylinder is at constant temperature. This evaluation gives a pooled estimate of standard uncertainty in the mass
determination of a component of s = 4 mg; the standard uncertainty isus� n, in this case 2,3 mg.
p p
A.5.2.2 Weights (u )
w
The substitution method is used in weighing and to avoid linearity and adjustment errors, it is expedient to add
auxiliary weights of known mass to keep the weighing difference between the mixture and reference cylinder small.
The weights used in this example are made of stainless steel and calibrated against national standards and comply
with OIML class E2. The calibration certificate for these weights shows an associated expanded uncertainty on the
1 g mass piece (which is the smallest mass piece used and has the largest relative uncertainty). Usually this is
reported as 1/3 of the maximum allowable error.
The maximum allowable error is 0,06 mg. Thus the expanded uncertainty, U , is 0,02 mg (coverage factor k=2).
w
For each added mass piece the expanded uncertainty can be calculated in a similar way. The density of the mass
pieces is listed in the certificate as (7 850 � 50) kg/m . This uncertainty is rather high. However, the nominal value
of the mass pieces, the calibrated correction and uncertainty in the correction are normally listed in terms of the
conventional mass. This means that the mass pieces are equal to the masses of reference weights with a density
3 3 3
of 8 000 kg/m when weighing with an air density of 1,2 kg/m . In this case, the uncertainty is 0,002 kg/m (k=1).
A.5.2.3 Buoyancy effects (u ), (u )
B exp
A buoyancy corr
...