Natural gas - Calculation of calorific values, density, relative density and Wobbe indices from composition (ISO 6976:2016)

ISO 6976:2016 specifies methods for the calculation of gross calorific value, net calorific value, density, relative density, gross Wobbe index and net Wobbe index of natural gases, natural gas substitutes and other combustible gaseous fuels, when the composition of the gas by mole fraction is known. The methods specified provide the means of calculating the properties of the gas mixture at commonly used reference conditions.
Mole fractions by definition sum to unity. Guidance on the achievement of this requirement by chromatographic analysis is available in ISO 6974‑1 and ISO 6974‑2.
The methods of calculation require values for various physical properties of the pure components; these values, together with associated uncertainties, are provided in tables and their sources are identified.
Methods are given for estimating the standard uncertainties of calculated properties.
The methods of calculation of the values of properties on either a molar, mass or volume basis are applicable to any natural gas, natural gas substitute or other combustible fuel that is normally gaseous, except that for properties on the volume basis the method is restricted to mixtures for which the compression factor at reference conditions is greater than 0,9.
Example calculations are given in Annex D for the recommended methods of calculation.
NOTE 1 The qualifiers "superior", "higher", "upper" and "total" are, for the purposes of this document, synonymous with "gross"; likewise, "inferior" and "lower" are synonymous with "net". The term "heating value" is synonymous with "calorific value"; "mass density" and "specific density" are synonymous with "density"; "specific gravity" is synonymous with "relative density"; "Wobbe number" is synonymous with "Wobbe index"; "compressibility factor" is synonymous with "compression factor". The dimensionless quantity molecular weight is numerically equal to the molar mass in kg·kmol−1.
NOTE 2 There are no explicit limits of composition to which the methods described in this document are applicable. However, the restriction of volume-basis calculations to mixtures with a compression factor greater than 0,9 at reference conditions sets implicit limits on composition.
NOTE 3 Because the mole fraction of any water present is not normally available from chromatographic analysis, it is common practice to calculate the physical properties on a dry gas basis and to allow for the effects of water vapour in a separate procedure. However, if the mole fraction of water vapour is known then the property calculations can be carried out completely in accordance with the procedures described herein. The effects of water vapour on calorific value, whether the latter is directly measured or calculated, are discussed in ISO/TR 29922.
NOTE 4 For aliphatic hydrocarbons of carbon number 7 or above, any isomer present is included with the normal isomer of the same carbon number.
NOTE 5 If the user's requirement includes the replacement of, for example, a C6+ or C7+ grouping of analytically unresolved components by a single pseudo-component, then it is the user's own task to set the mole fraction composition, and hence properties, of this pseudo-component so as to be fit for purpose in the particular application. Any so-called "spectator water" and "non-combustible hydrogen sulfide" are treated as pseudo-components by setting the appropriate enthalpy of combustion values to zero.

Erdgas - Berechnung von Brenn- und Heizwert, Dichte, relativer Dichte und Wobbeindex aus der Zusammensetzung (ISO 6976:2016)

Gaz naturel - Calcul des pouvoirs calorifiques, de la masse volumique, de la densité relative et des indices de Wobbe à partir de la composition (ISO 6976:2016)

L'ISO 6976:2016 décrit des méthodes pour le calcul des pouvoirs calorifiques supérieur et inférieur, de la masse volumique, de la densité relative, des indices de Wobbe supérieur et inférieur de gaz naturels, de substituts du gaz naturel et d'autres combustibles gazeux, lorsque la composition du gaz par fraction molaire est connue. Les méthodes spécifiées permettent de calculer les propriétés du mélange de gaz dans des conditions de référence généralement utilisées.
Pour les besoins de ce document, la somme des fractions molaires données doit correspondre à l'unité exacte. L'ISO 6974‑1 et l'ISO 6974‑2 donnent des préconisations pour satisfaire à cette exigence. Tous les constituants dont la fraction molaire est supérieure à 0,00005 doivent être comptabilisés.
Les méthodes de calcul exigent des valeurs pour différentes propriétés physiques des constituants purs; ces valeurs ainsi que les incertitudes associées sont consignées dans des tableaux et leurs sources sont identifiées.
Des méthodes sont données pour estimer les incertitudes des propriétés calculées.
Les méthodes de calcul des valeurs des propriétés, qu'elles reposent sur une base molaire, massique ou volumétrique, sont applicables à tout gaz naturel, substitut de gaz naturel ou à tout autre combustible habituellement gazeux, à l'exception que dans le cas des propriétés calculées sur une base volumétrique la méthode se limite aux mélanges dont le facteur de compression est supérieur à 0,9 dans les conditions de référence.
L'Annexe D donne des exemples de calcul pour les méthodes de calcul recommandées.
NOTE 1 Pour les besoins de la présente Norme internationale, les qualificatifs «supérieur», «élevé», «plus haut» et «total» sont synonymes de «brut»; de la même façon, «inférieur» et «bas» sont synonymes de «net». Les termes «masse volumique» et «masse spécifique» sont synonymes; le terme «poids spécifique» est synonyme de «densité relative»; le terme «nombre de Wobbe» est synonyme d'«indice de Wobbe»; le terme «facteur de compressibilité» est synonyme de «facteur de compression». Le poids moléculaire d'une grandeur sans dimension est numériquement équivalent à la masse molaire en kg·kmol−1.
NOTE 2 Pour les hydrocarbures aliphatiques dont le nombre d'atomes de carbone est supérieur ou égal à 7, il convient d'inclure à l'isomère linéaire comportant le même nombre d'atomes de carbone tout isomère présent à une fraction molaire supérieure à 0,000 05.
NOTE 3 Si l'utilisateur a pour exigence de remplacer, par exemple, un groupement C6+ ou C7+ de constituants non résolus de manière analytique par un pseudo-constituant unique, alors il doit se charger de définir lui-même la composition de la fraction molaire et, par conséquent, les propriétés de ce pseudo-constituant, afin d'en garantir l'adéquation avec l'application concernée. L'«eau inerte» et le «sulfure d'hydrogène non combustible» ainsi nommés peuvent également être traités en tant que pseudo-constituants, en attribuant une valeur nulle aux valeurs d'enthalpie de combustion appropriées.
NOTE 4 Il n'existe pas de limites de composition explicites auxquelles les méthodes décrites dans la présente Norme internationale sont applicables. Toutefois, des limites de composition sont implicitement établies par la limitation des calculs sur une base volumétrique aux mélanges dont le facteur de compression est supérieur à 0,9 dans les conditions de référence.
NOTE 5 Étant donné que la fraction molaire de toute forme d'eau présente ne

Zemeljski plin - Izračun kalorične vrednosti, gostote, relativne gostote in Wobbejevega indeksa iz sestave (ISO 6976:2016)

General Information

Status
Published
Publication Date
30-Aug-2016
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
31-Aug-2016
Completion Date
31-Aug-2016

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SLOVENSKI STANDARD
SIST EN ISO 6976:2017
01-marec-2017
1DGRPHãþD
SIST EN ISO 6976:2005
=HPHOMVNLSOLQ,]UDþXQNDORULþQHYUHGQRVWLJRVWRWHUHODWLYQHJRVWRWHLQ
:REEHMHYHJDLQGHNVDL]VHVWDYH ,62

Natural gas - Calculation of calorific values, density, relative density and Wobbe indices

from composition (ISO 6976:2016)

Erdgas - Berechnung von Brenn- und Heizwert, Dichte, relativer Dichte und Wobbeindex

aus der Zusammensetzung (ISO 6976:2016)

Gaz naturel - Calcul des pouvoirs calorifiques, de la masse volumique, de la densité

relative et des indices de Wobbe à partir de la composition (ISO 6976:2016)
Ta slovenski standard je istoveten z: EN ISO 6976:2016
ICS:
75.060 Zemeljski plin Natural gas
SIST EN ISO 6976:2017 en

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

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SIST EN ISO 6976:2017
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SIST EN ISO 6976:2017
EN ISO 6976
EUROPEAN STANDARD
NORME EUROPÉENNE
August 2016
EUROPÄISCHE NORM
ICS 75.060 Supersedes EN ISO 6976:2005
English Version
Natural gas - Calculation of calorific values, density,
relative density and Wobbe indices from composition (ISO
6976:2016)

Gaz naturel - Calcul des pouvoirs calorifiques, de la Erdgas - Berechnung von Brenn- und Heizwert, Dichte,

masse volumique, de la densité relative et des indices relativer Dichte und Wobbeindex aus der

de Wobbe à partir de la composition (ISO 6976:2016) Zusammensetzung (ISO 6976:2016)

This European Standard was approved by CEN on 24 August 2016.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6976:2016 E

worldwide for CEN national Members.
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SIST EN ISO 6976:2017
EN ISO 6976:2016 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 6976:2017
EN ISO 6976:2016 (E)
European foreword

This document (EN ISO 6976:2016) has been prepared by Technical Committee ISO/TC 193 “Natural

gas” in collaboration with Technical Committee CEN/TC 278 “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 February 2017, and conflicting national standards

shall be withdrawn at the latest by February 2017.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent

rights.
This document supersedes EN ISO 6976:2005.

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, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 6976:2016 has been approved by CEN as EN ISO 6976:2016 without any modification.

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SIST EN ISO 6976:2017
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SIST EN ISO 6976:2017
INTERNATIONAL ISO
STANDARD 6976
Third edition
2016-08-15
Natural gas — Calculation of calorific
values, density, relative density and
Wobbe indices from composition
Gaz naturel — Calcul des pouvoirs calorifiques, de la masse
volumique, de la densité relative et des indices de Wobbe à partir de la
composition
Reference number
ISO 6976:2016(E)
ISO 2016
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SIST EN ISO 6976:2017
ISO 6976:2016(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2016, Published in Switzerland

All rights reserved. Unless otherwise specified, 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
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Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2016 – All rights reserved
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SIST EN ISO 6976:2017
ISO 6976:2016(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 2

3 Terms and definitions ..................................................................................................................................................................................... 2

4 Symbols and units ............................................................................................................................................................................................... 5

4.1 Quantities .................................................................................................................................................................................................... 5

4.2 Subscripts .................................................................................................................................................................................................... 6

4.3 Superscript ................................................................................................................................................................................................. 6

5 Principles ..................................................................................................................................................................................................................... 6

6 Behaviour of ideal and real gases ....................................................................................................................................................... 7

6.1 Enthalpy of combustion .................................................................................................................................................................. 7

6.2 Calculation of compression factor .......................................................................................................................................... 8

7 Calculation of calorific value on a molar basis ...................................................................................................................... 8

7.1 Gross calorific value ............................................................................................................................................................................ 8

7.2 Net calorific value ................................................................................................................................................................................. 9

8 Calculation of calorific value on a mass basis ........................................................................................................................ 9

8.1 Gross calorific value ............................................................................................................................................................................ 9

8.2 Net calorific value ..............................................................................................................................................................................10

9 Calculation of calorific value on a volume basis ...............................................................................................................10

9.1 Ideal-gas gross calorific value .................................................................................................................................................10

9.2 Ideal-gas net calorific value ......................................................................................................................................................10

9.3 Real-gas gross calorific value ...................................................................................................................................................11

9.4 Real-gas net calorific value ........................................................................................................................................................11

10 Calculation of associated properties ............................................................................................................................................11

10.1 Ideal-gas relative density ............................................................................................................................................................11

10.2 Ideal-gas density .................................................................................................................................................................................12

10.3 Ideal-gas gross Wobbe index ...................................................................................................................................................12

10.4 Ideal-gas net Wobbe index .........................................................................................................................................................12

10.5 Real-gas relative density ..............................................................................................................................................................13

10.6 Real-gas density ..................................................................................................................................................................................13

10.7 Real-gas gross Wobbe index .....................................................................................................................................................13

10.8 Real-gas net Wobbe index ..........................................................................................................................................................14

11 Uncertainty of calculation .......................................................................................................................................................................14

11.1 Principles ..................................................................................................................................................................................................14

11.2 Formulae for the analytical method ..................................................................................................................................15

11.3 Inputs for the analytical method ..........................................................................................................................................15

11.3.1 Composition and compositional uncertainties ...................................................................................15

11.3.2 Non-compositional inputs ....................................................................................................................................16

11.4 Expanded uncertainty ...................................................................................................................................................................17

11.5 Expression of results .......................................................................................................................................................................17

11.5.1 General...................................................................................................................................................................................17

11.5.2 Analytical method ........................................................................................................................................................17

11.5.3 Generic method ..............................................................................................................................................................17

11.5.4 Contingency method ........................................................................................................................................... .......17

11.6 Application of uncertainty .........................................................................................................................................................18

12 Tables of data ........................................................................................................................................................................................................19

Annex A (normative) Values of auxiliary constants ...........................................................................................................................25

© ISO 2016 – All rights reserved iii
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ISO 6976:2016(E)

Annex B (normative) Formulae for uncertainty calculations ..................................................................................................27

Annex C (informative) Conversion factors ...................................................................................................................................................32

Annex D (informative) Example calculations ...........................................................................................................................................34

Bibliography .............................................................................................................................................................................................................................57

iv © ISO 2016 – All rights reserved
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SIST EN ISO 6976:2017
ISO 6976:2016(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.

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 documents 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).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on 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 the following URL: www.iso.org/iso/foreword.html.

The committee responsible for this document is ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis

of natural gas.

This third edition cancels and replaces the second edition (ISO 6976:1995), which has been technically

revised.
© ISO 2016 – All rights reserved v
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SIST EN ISO 6976:2017
ISO 6976:2016(E)
Introduction

Both international and intra-national custody transfer of natural gas usually require precise

determination of both the quantity and the quality of the gas to be traded. This document specifies

methods for the calculation of key properties that describe gas quality, namely gross and net calorific

value, density, relative density, and gross and net Wobbe index. The methods provide the means

of calculating these properties and their uncertainties for any natural gas, natural gas substitute or

similar combustible gaseous fuel of known composition at commonly used reference conditions.

Values of the various properties calculated in accordance with this document will, in general, differ

only by very small amounts from those calculated using the second (1995) edition of this document. In

this context, it is recognized that:

a) adoption of the revisions detailed in this document will not be without cost, since instrumental

software will need updating;

b) recorded energy content and hence billed energy will, in consequence of these revisions, change by

small amounts;

c) unintended impacts could occur if the revisions are implemented uncritically; for instance, if the

revisions are implemented at input points to a pipeline system but not at exit points, then a costly

accountancy imbalance may result;

d) commercial, contractual, regulatory and legislative obligations will need to be taken into account.

For these reasons, and depending upon the user’s application, it may be appropriate to undertake an

impact assessment in order to determine an agreed timing and procedure for implementation of the

provisions of this document.
vi © ISO 2016 – All rights reserved
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SIST EN ISO 6976:2017
INTERNATIONAL STANDARD ISO 6976:2016(E)
Natural gas — Calculation of calorific values, density,
relative density and Wobbe indices from composition
1 Scope

This document specifies methods for the calculation of gross calorific value, net calorific value, density,

relative density, gross Wobbe index and net Wobbe index of natural gases, natural gas substitutes

and other combustible gaseous fuels, when the composition of the gas by mole fraction is known. The

methods specified provide the means of calculating the properties of the gas mixture at commonly

used reference conditions.

Mole fractions by definition sum to unity. Guidance on the achievement of this requirement by

chromatographic analysis is available in ISO 6974-1 and ISO 6974-2.

The methods of calculation require values for various physical properties of the pure components; these

values, together with associated uncertainties, are provided in tables and their sources are identified.

Methods are given for estimating the standard uncertainties of calculated properties.

The methods of calculation of the values of properties on either a molar, mass or volume basis are

applicable to any natural gas, natural gas substitute or other combustible fuel that is normally gaseous,

except that for properties on the volume basis the method is restricted to mixtures for which the

compression factor at reference conditions is greater than 0,9.

Example calculations are given in Annex D for the recommended methods of calculation.

NOTE 1 The qualifiers “superior”, “higher”, “upper” and “total” are, for the purposes of this document,

synonymous with “gross”; likewise, “inferior” and “lower” are synonymous with “net”. The term “heating value”

is synonymous with “calorific value”; “mass density” and “specific density” are synonymous with “density”;

“specific gravity” is synonymous with “relative density”; “Wobbe number” is synonymous with “Wobbe index”;

“compressibility factor” is synonymous with “compression factor”. The dimensionless quantity molecular weight

is numerically equal to the molar mass in kg·kmol .

NOTE 2 There are no explicit limits of composition to which the methods described in this document are

applicable. However, the restriction of volume-basis calculations to mixtures with a compression factor greater

than 0,9 at reference conditions sets implicit limits on composition.

NOTE 3 Because the mole fraction of any water present is not normally available from chromatographic

analysis, it is common practice to calculate the physical properties on a dry gas basis and to allow for the effects

of water vapour in a separate procedure. However, if the mole fraction of water vapour is known then the

property calculations can be carried out completely in accordance with the procedures described herein. The

effects of water vapour on calorific value, whether the latter is directly measured or calculated, are discussed in

ISO/TR 29922.

NOTE 4 For aliphatic hydrocarbons of carbon number 7 or above, any isomer present is included with the

normal isomer of the same carbon number.

NOTE 5 If the user’s requirement includes the replacement of, for example, a C6+ or C7+ grouping of

analytically unresolved components by a single pseudo-component, then it is the user’s own task to set the mole

fraction composition, and hence properties, of this pseudo-component so as to be fit for purpose in the particular

application. Any so-called “spectator water” and “non-combustible hydrogen sulfide” are treated as pseudo-

components by setting the appropriate enthalpy of combustion values to zero.
© ISO 2016 – All rights reserved 1
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SIST EN ISO 6976:2017
ISO 6976:2016(E)
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 6974-1, Natural gas — Determination of composition and associated uncertainty by gas

chromatography — Part 1: General guidelines and calculation of composition

ISO 6974-2, Natural gas — Determination of composition and associated uncertainty by gas

chromatography — Part 2: Uncertainty calculations
ISO 14912:2003, Gas analysis — Conversion of gas mixture composition data
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
gross calorific value

amount of heat that would be released by the complete combustion with oxygen of a specified quantity

of gas, in such a way that the pressure, p , at which the reaction takes place remains constant, and all the

products of combustion are returned to the same specified temperature, t , as that of the reactants, all

of these products being in the gaseous state except for water, which is condensed to the liquid state at t

Note 1 to entry: Where the quantity of gas is specified on a molar basis, the gross calorific value is designated as

(Hc) (t ,p ); on a mass basis, the gross calorific value is designated as (Hm) (t ,p ). Where the quantity of gas is

G 1 1 G 1 1

specified on a volume basis, the gross calorific value is designated as (Hv) (t ,p ;t ,p ), where t and p are the

G 1 1 2 2 2 2
gas volume (metering) reference conditions (see Figure 1).
3.2
net calorific value

amount of heat that would be released by the complete combustion with oxygen of a specified quantity

of gas, in such a way that the pressure, p , at which the reaction takes place remains constant, and all

the products of combustion are returned to the same specified temperature, t , as that of the reactants,

all of these products being in the gaseous state

Note 1 to entry: On molar, mass and volume bases, the net calorific value is designated respectively as

(Hc) (t ,p ), (Hm) (t ,p ) and (Hv) (t ,p ;t ,p ).
N 1 1 N 1 1 N 1 1 2 2
3.3
density

mass of a gas sample divided by its volume at specified conditions of pressure and temperature

3.4
relative density

density of a gas divided by the density of dry air of reference composition at the same specified

conditions of pressure and temperature

Note 1 to entry: The term ideal relative density applies when both gas and air are considered as gases that obey

the ideal gas law (3.8). The term real relative density applies when both gas and air are considered as real fluids

(3.9). For the fixed reference composition of dry air, see ISO/TR 29922.
2 © ISO 2016 – All rights reserved
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SIST EN ISO 6976:2017
ISO 6976:2016(E)
3.5
gross Wobbe index

volume-basis gross calorific value, at specified reference conditions, divided by the square root of the

relative density at the same specified metering reference conditions

Note 1 to entry: In common usage, and in the absence of any other qualifier, the term Wobbe index is taken to

mean the quantity that is here identified as gross Wobbe index.
3.6
net Wobbe index

volume-basis net calorific value, at specified reference conditions, divided by the square root of the

relative density at the same specified metering reference conditions
3.7
enthalpy of transformation

amount of heat release that accompanies the change in condition (transformation) of a substance or

system from one (initial) condition to another (final) condition

Note 1 to entry: A positive heat release is represented by thermodynamic convention as a numerically equal

negative increment of enthalpy.

Note 2 to entry: In the context of this document, the following can be identified:

— enthalpy of combustion: the initial condition is that of an unburned stoichiometric mixture of reactants and

the final condition is that of the products of combustion at the same pressure and temperature;

— standard enthalpy of vaporization: the initial condition is that of a substance in the liquid state at saturation

and the final condition is that of the same substance in the hypothetical state of the ideal gas at the same

temperature;

— enthalpy (or enthalpic) difference: the initial condition is that of a gas or gas mixture at temperature T and

the final condition is that of the same gas or gas mixture at the same pressure but at a different temperature T ;

— enthalpy (or enthalpic) correction (residual enthalpy): the initial condition is that of a gas or gas mixture

in the hypothetical state of an ideal gas and the final condition is that of the same gas or gas mixture at the

same pressure and temperature in the state of the real gas.
3.8
ideal gas
gas that obeys the ideal gas law
Note 1 to entry: The ideal gas law can be expressed as
p · V = R · T
where
p is the absolute pressure;
T is the thermodynamic temperature;
V is the volume occupied by one mole of ideal gas (ideal molar volume);
R is the gas constant in coherent units.
3.9
real gas
gas that deviates from volumetric ideality
Note 1 to entry: No real gas obeys the
...

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