SIST EN ISO 14912:2025

SLOVENSKI STANDARD
01-september-2025
Nadomešča:
SIST EN ISO 14912:2006
SIST EN ISO 14912:2006/AC:2008
Analiza plinov - Pretvorba podatkov o sestavi plinskih zmesi (ISO 14912:2025)
Gas analysis - Conversion of gas mixture composition data (ISO 14912:2025)
Gasanalyse - Umrechnung von Zusammensetzungsangaben für Gasgemische (ISO
14912:2025)
Analyse des gaz - Conversion des données de composition de mélanges gazeux (ISO
14912:2025)
Ta slovenski standard je istoveten z: EN ISO 14912:2025
ICS:
71.040.40 Kemijska analiza Chemical analysis
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 14912
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2025
EUROPÄISCHE NORM
ICS 71.040.40 Supersedes EN ISO 14912:2006, EN ISO
14912:2006/AC:2007
English Version
Gas analysis - Conversion of gas mixture composition data
(ISO 14912:2025)
Analyse des gaz - Conversion des données de Gasanalyse - Umrechnung von
composition de mélanges gazeux (ISO 14912:2025) Zusammensetzungsangaben für Gasgemische (ISO
14912:2025)
This European Standard was approved by CEN on 8 May 2025.

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 CEN-CENELEC 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 CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 14912:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 14912:2025) has been prepared by Technical Committee ISO/TC 158 "Analysis
of gases" in collaboration with Technical Committee CEN/TC 238 “Test gases, test pressures, appliance
categories and gas appliance types” the secretariat of which is held by AFNOR.
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 November 2025, and conflicting national standards
shall be withdrawn at the latest by November 2025.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 14912:2006.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 14912:2025 has been approved by CEN as EN ISO 14912:2025 without any modification.

International
Standard
ISO 14912
Second edition
Gas analysis — Conversion of gas
2025-05
mixture composition data
Analyse des gaz — Conversion des données de composition de
mélanges gazeux
Reference number
ISO 14912:2025(en) © ISO 2025
ISO 14912:2025(en)
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 14912:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Quantities for the expression of gas mixture composition .2
3.2 Additional quantities involved in conversions of gas mixture composition .3
4 Symbols and units. 4
5 Basic Principles . 6
5.1 Expression of gas mixture composition.6
5.2 Conversion between different quantities .7
5.3 Conversion between different state conditions .9
6 Main procedures . 9
6.1 Conversion between different quantities of composition .9
6.1.1 Conversion of the content of single components .9
6.1.2 Conversion of complete compositions .10
6.2 Conversion to reference conditions .11
7 Practical implementation.12
7.1 Conversion between quantities of composition . 12
7.2 Conversion of single contents . . 13
7.3 Conversion of complete compositions . 13
7.4 Conversion between state conditions .14
7.5 Simple approximations applicable to conversion .14
7.5.1 Ideal mixture of ideal gases .14
7.5.2 Ideal mixture of real gases .14
7.5.3 Trace gas mixture . 15
8 Input quantities and their uncertainties .15
8.1 Pure gas data . 15
8.1.1 Molar mass . 15
8.1.2 Compression factor . 15
8.2 Gas mixture data .17
8.2.1 Molar mass .17
8.2.2 Compression factor .18
8.2.3 Mixing factor . 20
8.3 Rough uncertainty estimates .21
9 Conversion uncertainty .21
9.1 General considerations.21
9.2 Conversion of single contents . . 22
9.3 Conversion of complete compositions . 23
9.4 Variances and covariances of input composition data . 25
9.4.1 General procedure . 25
9.4.2 Correlation effects in complete composition data . 25
Annex A (informative) Assessment of state conditions .28
Annex B (informative) Summation relations for the expression of mixture properties .31
Annex C (informative) Mixture component data.32
Annex D (informative) Examples .38
Annex E (informative) Computer implementation of recommended methods.53
Bibliography .54

iii
ISO 14912:2025(en)
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 158, Analysis of gases, in collaboration with the
European Committee for Standardization (CEN) Technical Committee CEN/TC 238, Test gases, test pressures,
appliance categories and gas appliance types, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 14912:2003 and ISO 14912:2003/Cor.1:2006),
which has been technically revised.
The main changes are as follows:
— update of the molar mass data for mixture components in Annex C according to the 2019 to 2021 IUPAC/
CIAAW atomic mass data;
— update of the value of the gas constant according to the 2018 revision of the International System of
Units (SI);
— update of the bibliography and the corresponding references in the text;
— update of the information in Annex E on the computer programme CONVERT;
— correction of Formulae (37) and (39);
— recalculation of the examples in Annex D;
— addition of a table of molar mass data for the relevant elements from which the molar mass data for
mixture components were calculated;
— addition of information concerning data for synthetic air;
— editorial corrections.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO 14912:2025(en)
Introduction
The composition of a gas mixture is given by the identity of the mixture components and their content in
the mixture. For the purpose of expressing component contents, different quantities are in use, the most
common ones being mass concentration, amount fraction and volume fraction. This diversity is due to the
fact that in different applications, different quantities have decisive advantages. Therefore, procedures for
conversion between different quantities are needed.
As far as these quantities involve volumes, their value depends on the state conditions, i.e., pressure and
temperature, of the gas mixture. For these quantities, therefore, procedures for conversion between
different state conditions are needed.
As a crude approximation, all of the conversions referred to above can be performed on the basis of the ideal
gas law. In most cases, however, an accurate conversion shall take into account the real gas behaviour of the
components and of the entire gas mixture. These calculations use values of the compression factor (or the
density) of the components concerned and the entire gas mixture.
This document provides conversion procedures which fully account for real gas behaviour of pure gases
and gas mixtures. In addition to these, approximate procedures for practical applications are described,
designed for different levels of accuracy and available data. These procedures are based on approximate
calculations of a) pure gas compression factors using virial coefficients and b) mixture compression
factors using component data. Uncertainty estimates are given which account for the uncertainty due to
approximations in the conversion procedures and the uncertainty of the input data.
Recently, advanced compression factor calculations for pure gases and gas mixtures, based on multi-
parameter equations of state became publicly available (see e.g. NIST Reference Fluid and Transport
[17]
Properties Database (REFPROP)) and were even standardized (see e.g. ISO 20765-2). Concerning
accuracy and uncertainty, these tools clearly outperform the simple approach used in ISO 14912 (truncated
virial expansion, linear interpolation of virial coefficient data). However, for the intended use of ISO 14912,
the performance is sufficent and the simplicity is beneficial for many users.

v
International Standard ISO 14912:2025(en)
Gas analysis — Conversion of gas mixture composition data
1 Scope
This document defines the following quantities commonly used to express the composition of gas mixtures:
— amount fraction and concentration;
— mass fraction and concentration;
— volume fraction and concentration.
For these quantities of composition, this document specifies methods for:
— conversion between different quantities;
— conversion between different state conditions.
Conversion between different quantities means calculating the value of the content of a specified component
in terms of one of the quantities listed above from the value of the same content, at the same pressure and
temperature of the gas mixture, given in terms of another of these quantities. Conversion between different
state conditions means calculating the value of the content of a specified component, in terms of one of the
quantities listed above, under one set of state conditions from the value of the same quantity under another
set of state conditions, i.e., pressure and temperature, of the gas mixture. Gas mixture composition can be
converted simultaneously between different quantities of composition and different state conditions by
combination of the two types of conversion.
This document is applicable only to homogeneous and stable gas mixtures. Therefore, any state conditions
(pressure and temperature) considered need to be well outside the condensation region of the gas mixture.
In addition, volume concentrations can only be used if the component under consideration is completely
gaseous, and for the use of volume fractions, all components need to be completely gaseous. Further
restrictions of state conditions apply for approximations of compression factors using virial coefficients (see
Annex A).
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
NOTE Further information concerning the terms defined in 3.1 is given in 5.1.

ISO 14912:2025(en)
3.1 Quantities for the expression of gas mixture composition
3.1.1
amount fraction
amount-of-substance fraction
x
quotient of the amount of substance of a specified component and the sum of the amounts of substance of all
components of a gas mixture
Note 1 to entry: The amount fraction is independent of the pressure and the temperature of the gas mixture.
Note 2 to entry: In 5.1, this term is addressed mathematically and further discussed in context.
3.1.2
mass fraction
w
quotient of the mass of a specified component and the sum of the masses of all components of a gas mixture
Note 1 to entry: The mass fraction is independent of the pressure and the temperature of the gas mixture.
Note 2 to entry: In 5.1, this term is addressed mathematically and further discussed in context.
3.1.3
volume fraction
φ
quotient of the volume of a specified component and the sum of the volumes of all components of a gas
mixture before mixing, all volumes referring to the pressure and the temperature of the gas mixture
Note 1 to entry: The volume fraction is a function of the pressure and the temperature of the gas mixture. Therefore,
the pressure and the temperature have to be specified.
Note 2 to entry: The volume fraction can only be used for state conditions where all mixture components are
completely gaseous.
Note 3 to entry: The (equivalent) definition in ISO 80000-9 is obtained by expressing the volume of the components as
the product of the amount of substance and the molar volume of the components and then dividing the numerator and
denominator of the defining quotient by the amount of substance of the gas mixture.
Note 4 to entry: In 5.1, this term is addressed mathematically and further discussed in context.
3.1.4
amount concentration
amount-of-substance concentration
c
quotient of the amount of substance of a specified component and the volume of a gas mixture
Note 1 to entry: The amount concentration is a function of the pressure and the temperature of the gas mixture.
Therefore, the pressure and the temperature have to be specified.
Note 2 to entry: In 5.1, this term is addressed mathematically and further discussed in context.
3.1.5
mass concentration
γ
quotient of the mass of a specified component and the volume of a gas mixture
Note 1 to entry: The mass concentration is a function of the pressure and the temperature of the gas mixture.
Therefore, the pressure and the temperature have to be specified.
Note 2 to entry: In 5.1, this term is addressed mathematically and further discussed in context.

ISO 14912:2025(en)
3.1.6
volume concentration
σ
quotient of the volume of a specified component before mixing and the volume of a gas mixture, both volumes
referring to the same pressure and the same temperature
Note 1 to entry: The volume concentration is a function of the pressure and the temperature of the gas mixture.
Therefore, the pressure and the temperature have to be specified.
Note 2 to entry: The volume concentration can only be used at state conditions where the specified component is
completely gaseous.
Note 3 to entry: The volume fraction (3.1.3) and volume concentration (3.1.6) take the same value if, at the same state
conditions, the sum of the component volumes before mixing and the volume of the mixture are equal. However,
because the mixing of two or more gases at the same state conditions is usually accompanied by a slight contraction or,
less frequently, a slight expansion, this is not generally the case.
Note 4 to entry: In 5.1, this term is addressed mathematically and further discussed in context.
3.2 Additional quantities involved in conversions of gas mixture composition
3.2.1
compression factor
compressibility factor
Z
quotient of the volume of an arbitrary amount of gas at specified pressure and temperature and the volume
of the same amount of an ideal gas at the same state conditions
Note 1 to entry: This definition is applicable to pure gases and to gas mixtures, therefore the term “gas” is used as a
general term to be understood to cover pure gases as well as gas mixtures.
Note 2 to entry: By definition, the compression factor of an ideal gas is 1. At room temperature and atmospheric
pressure, for many gases the compression factor differs only moderately from 1.
Note 3 to entry: The compression factor is a function of pressure, temperature and the amount fractions of the
components.
3.2.2
mixing factor
f
quotient of the volume of an arbitrary amount of a gas mixture at specified pressure and temperature and
the sum of the volumes of all mixture components, before mixing, at the same state conditions
Note 1 to entry: If the component volumes are strictly additive, i.e., if the sum of the component volumes before mixing
is the same as the volume after mixing, the mixing factor is 1. At room temperature and atmospheric pressure, for
many gas mixtures the mixing factor differs only slightly from 1.
3.2.3
density
ρ
quotient of the mass of an arbitrary amount of gas and its volume at specified pressure and temperature
Note 1 to entry: This definition is applicable to pure gases and to gas mixtures, therefore the term “gas” is used as a
general term to be understood to cover pure gases and gas mixtures.
3.2.4
molar volume
V
mol
quotient of the volume of an arbitrary amount of gas at specified pressure and temperature and its amount
of substance
Note 1 to entry: This definition is applicable to pure gases and to gas mixtures, therefore the term “gas” is used as a
general term to be understood to cover pure gases and gas mixtures.

ISO 14912:2025(en)
Note 2 to entry: The amount of substance of a mixture is given by the sum of the amounts of substance of the
components.
Note 3 to entry: In ISO 80000-9 the symbol V is used for the molar volume.
m
Note 4 to entry: In ISO 80000-9 molar volume is restricted to pure substances.
3.2.5
virial coefficients
coefficients in the expansion of the compression factor in terms of powers of a quantity of state
Note 1 to entry: In practice, only two virial expansions are used, where the quantity of state is either the pressure, p,
or the inverse molar volume, 1/V , as given in Formulae (1) and (2).
mol
BT() CT()
ZV(),T =+1 + + . (1)
mol
V
mol V
mol
′ ′
Zp(),TB=+1 ()Tp+CT()p + . (2)
3.2.5.1
second molar-volume virial coefficient
B
coefficient of 1/V in the expression of the compression factor as a series of inverse powers of the molar
mol
volume, V
mol
3.2.5.2
third molar-volume virial coefficient
C
coefficient of 1/V in the expression of the compression factor as a series of inverse powers of the molar
mol
volume, V
mol
3.2.5.3
second pressure virial coefficient
B′
coefficient of p in the expression of the compression factor as a series of powers of the pressure p
3.2.5.4
third pressure virial coefficient
C′
coefficient of p in the expression of the compression factor as a series of powers of the pressure p
4 Symbols and units
Symbol Quantity SI unit
α = p/(RT) mol/m
B second molar-volume virial coefficient m /mol
B′ second pressure virial coefficient 1/Pa
γ mass concentration kg/m
c amount concentration mol/m
6 2
C third molar-volume virial coefficient m /mol
C′ third pressure virial coefficient 1/Pa
D dilution factor 1
f mixing factor 1
3 3
φ volume fraction m /m
m mass kg
ISO 14912:2025(en)
Symbol Quantity SI unit
M molar mass kg/mol
n amount of substance mol
N number of gas mixture components 1
p pressure Pa
p saturation vapour pressure Pa
vap
p dew pressure Pa
dew
R gas constant J/(mol⋅K)
(numerical value 8,314 462 618 153 24)
ρ density kg/m
3 3
σ volume concentration m /m
t Celsius temperature °C
T thermodynamic temperature K
V volume m
V molar volume m /mol
mol
w mass fraction kg/kg
W mass of a gas cylinder kg
x amount fraction mol/mol
X reference value of state conditions (X = p, T) same as for X
ref
X critical property of a pure gas (X = p, T, V, Z) same as for X
crit
X pseudo-critical property of a gas mixture (X = p, T) same as for X
pscrit
Z compression factor 1
Symbol Index
i, j, k for gas mixture components (i, j, k = 1, …, N)
S (sample of) gas mixture
In addition to the symbols specified above, the following symbols are used to denote objects of generic
mathematical expressions.
Symbol Quantity
F mathematical function expressing a conversion
I input quantity of composition
O output quantity of composition
Ω conversion factor
K,L,P,Q,Y general variables or quantities
∂F/∂P partial derivative (sensitivity coefficient)
r(P,Q) correlation coefficient of quantities P,Q
R correlation matrix
u(P) standard uncertainty of quantity P
u (P) variance of quantity P
u(P,Q) covariance of quantities P,Q
U variance-covariance matrix
v(P) relative standard uncertainty of quantity P
v(P) = u(P)/|P|
ISO 14912:2025(en)
5 Basic Principles
5.1 Expression of gas mixture composition
The generic term for the amount of a component in a mixture is “content”. This term is intended for use only
in a purely descriptive or qualitative sense.
Quantitative statements require the expression of content as a value (the product of a number and a unit) of
a “quantity of composition”.
For the purposes of this document, six quantities of composition, subdivided into two distinct conceptual
families, called fractions and concentrations, are defined in 3.1. The terms “fraction” and “concentration”
are themselves incomplete, and shall not be used in quantitative statements of content without further
qualification by one of the modifiers “amount”, “mass” or “volume”.
In quantitative expressions of gas mixture composition, the applicable quantity, for example the amount
fraction or the mass concentration, shall be used in conjunction with the name or the chemical formula of
the component.
EXAMPLE 1 The hydrogen content in a hydrogen/nitrogen mixture, expressed as amount fraction, is x(H ) = 0,1
mol/mol.
EXAMPLE 2 The sulfur dioxide content in air at 101,325 kPa and 25 °C, expressed as mass concentration, is
γ(SO ) = 1 mg/m .
Gas mixture composition may either relate to the preparation of gas mixtures or to the analysis of gas
mixtures. In the first case, the composition expresses the formulation of a prepared mixture. Here the
components are the parent gases that were mixed. These can be technically pure gases or specified gas
mixtures. In the second case, the composition expresses the results of analysis. Here, the components are
the analytes (i.e., the distinct chemical substances typically determined quantitatively) and the matrix (i.e.,
the complementary gas).
Fractions are often used in the expression of results of gas mixture preparation. If a gas mixture consists of
N components, 1, 2, ., N, and if the amounts of these components in the mixture are quantified by amount of
substance, n , n , ., n , the amount fraction x of any component i is given by Formula (3):
1 2 N i
n
i
x = (3)
i
n
∑ k
k
If the amounts of the mixture components are quantified by mass, m , m , ., m , the mass fraction w of any
1 2 N i
component i is given by Formula (4):
m
i
w = (4)
i
m
∑ k
k
If the amounts of the mixture components are quantified by volume, V , V , ., V , the volume fraction φ of
1 2 N i
any component i is given by Formula (5):
V
i
ϕ = (5)
i
V
∑ k
k
Concentrations are often used to express the results of chemical analysis of a mixture. If the amount of a
specified component, i, found in the analysed sample is quantified by amount of substance, n , and if V is
i S
ISO 14912:2025(en)
the sample volume at specified pressure and temperature, the amount concentration (amount-of-substance
concentration) c is given by Formula (6):
i
n
i
c = (6)
i
V
S
If the amount of the component is quantified by mass, m , the mass concentration γ is given by Formula (7):
i i
m
i
γ = (7)
i
V
S
If the amount of the component is quantified by volume, V , the volume concentration σ is given by
i i
Formula (8):
V
i
σ = (8)
i
V
S
In all the above expressions, it shall be noted that the sample volume depends on pressure and temperature.
In the expression for the volume concentration, the volume of the component also depends on pressure and
temperature. For both volumes, the state conditions shall be the same.
The quantities of composition exhibit different behaviour concerning the dependence on pressure and
temperature, as follows:
— amount concentration and mass concentration depend strongly on state conditions,
— volume fraction and volume concentration depend weakly on state conditions,
— amount fraction and mass fraction are strictly independent of state conditions.
Given the application to homogeneous gas mixtures only (see Scope), the state conditions (pressure and
temperature) shall be such that the mixture is completely gaseous. In addition,
a) the volume concentration is only applicable if the state conditions are such that the individual component
under consideration, at those state conditions, is completely gaseous;
b) the volume fraction is only applicable if the state conditions are such that all components, at those state
conditions, are completely gaseous.
The expression “completely gaseous” means one homogeneous gas phase, i.e., for an individual component,
that the pressure is well below the saturation vapour pressure at the given temperature or that the
temperature is well above the critical temperature. For a mixture, the equivalent condition is that the
pressure is well below the dew pressure at the given temperature or that the temperature is well above
the cricondentherm. Thus, the state conditions are to be well outside the relevant condensation regions.
Methods for assessing whether, at specified state conditions, gas mixtures and their components are
completely gaseous are described in Annex A.
NOTE For gas mixtures the applicable term is “cricondentherm”: maximum temperature at which condensation
can occur (see ISO 7504).
5.2 Conversion between different quantities
The conversion between different quantities of composition uses the basic relations between the following
quantities, which apply both to pure gases and to gas mixtures:
— amount of substance, n;
— mass, m;
— volume, V.
ISO 14912:2025(en)
The relation between amount of substance and mass is given by Formula (9):
m
n= (9)
M
where M is the molar mass of the gas or gas mixture.
The molar masses of pure gases are calculated from the molar masses of the elements (see 8.1.1). The molar
masses of gas mixtures are calculated from the composition and the molar masses of the components (see 8.2.1).
The relation between amount of substance and volume is given by Formula (10), which is the general
equation of state for real gases.
pV
n= (10)
ZRT
where
p is the pressure of the gas or gas mixture;
T is the temperature of the gas or gas mixture;
Z is the compression factor of the gas or gas mixture;
R is the gas constant.
The relation between mass and volume is given by Formula (11):
m = ρV (11)
where ρ is the density of the gas or gas mixture.
The three quantities M, Z and ρ can be related by combining Formulae (9), (10) and (11) to give Formula (12):
Mp
ρZ = (12)
RT
Therefore, only two of these quantities are independent. In this document, the conversion between quantities
of composition is based on known values of M and Z.
In addition to the formulas above, conversions between fractions and concentrations require relations
between the amount of a gas mixture and the amounts of its components.

ISO 14912:2025(en)
If a gas mixture sample S consists of N components, 1, 2, ., N, the amount of substance n of the gas mixture
S
sample is given by the sum of the amounts of substance of the components, as in Formula (13):
nn= (13)
S ∑ k
k
Analogously, the mass m of the gas mixture sample is given by the sum of the component masses, as in
S
Formula (14):
mm= (14)
S ∑ k
k
While amounts of substance and masses of mixture components are strictly additive, for component volumes
additivity is only an approximation (though usually very good). The relation between the volume V of the
S
gas mixture sample and the component volumes at identical state conditions is as follows in Formula (15):
Vf= V (15)
SS∑ k
k
where f is the mixing factor of the gas mixture S.
S
In the majority of applications, the mixing factor is assumed to be unity (see 8.2.3).
5.3 Conversion between different state conditions
If gas mixture composition is expressed in terms of quantities which depend on pressure and temperature,
reference conditions are necessary for comparison purposes. Therefore, procedures for converting any such
mixture composition from given state conditions to specified reference conditions are required. The basic
relation involved in these conversions is the relation between the volume V(p,T) of a sample of a gas or gas
mixture at given state conditions (p,T) and the volume V(p ,T ) of the same sample at specified reference
ref ref
conditions (p ,T ), using Formula (16).
ref ref
Vp(),T pTZp(),T
ref
= (16)
Vp ,T pT Zp ,T
() ()
refref refref ref
6 Main procedures
6.1 Conversion between different quantities of composition
6.1.1 Conversion of the content of single components
Conversion between different quantities of composition refers to specified state conditions (pressure p,
temperature T) of the gas mixture under consideration. Conversion is performed by multiplication with a
conversion factor which in most cases is a quotient, composed of one or several quantities of the component
i under consideration and the gas mixture S. For example, amount concentrations c are converted into mass
i
fractions w as follows in Formula (17):
i
RTZ
S
w = Mc (17)
ii i
pM
S
where
M and M are the molar masses of component i and mixture S;
i S
Z is the compression factor of mixture S at the specified state conditions of p and T.
S
Depending on which two of the three dependent quantities molar mass, M, compression factor, Z, and density,
ρ, are selected, the conversion factors can be expressed differently. In this document, conversion factors are

ISO 14912:2025(en)
always expressed in terms of M and Z. To utilize density data of pure gases and gas mixtures, these would be
converted into compression factor data according to Formula (12).
In Clause 8, the determination of the input quantities required for conversions is addressed.
Table 1 specifies a complete set of conversion factors, expressed in terms of molar masses and compression
factors of the component and the mixture under consideration. To obtain a desired quantity, take a given
quantity and multiply it by the corresponding factor in Table 1.
Table 1 — Conversion factors between quantities of composition
Desired Given Given Given Given Given Given
quantity quantity quantity quantity quantity quantity quantity
x φ w c σ γ
i i i i i i
Z M Z Z
Z
S S S S
S
x 1
i
fZ M Z αM
α
S i i i i
fZ fMZ fZ
fZ
S i SS i S i
S i
φ 1 f
i S
Z ZM αM
α
S S i i
M ZM ZM ZM Z
i
S i S i S i S
w 1
i
M fMZ αM MZ αM
S
SS i S S i S
α α αM α 1
S
c 1
i
Z fZ Z M
ZM
S S i i i
S i
Z MZ Z
1 Z
i S i i
i
σ 1
i
Z f ZM αM
α
S S S i i
αM αM αM αM
i i i
S
γ M 1
i i
Z fZ Z Z
S S i i
S
x is the amount fraction of component i M is the molar mass of component i
i i
φ is the volume fraction of component i Z is the compression factor of component i
i i
w is the mass fraction of component i M is the molar mass of gas mixture S
i S
c is the amount concentration of component i Z is the compression factor of gas mixture S
i S
σ is the volume concentration of component i f is the mixing factor of gas mixture S
i S
γ is the mass concentration of component i α is the abbreviation of the quotient p/(RT)
i
NOTE All quantities refer to the specified state conditions (pressure p, temperature T) of the gas mixture under consideration.
6.1.2 Conversion of complete compositions
If the composition for a gas mixture is completely known, that is, if the content of all components of the
mixture (including the m
...