EN ISO 6976:2005

SLOVENSKI STANDARD
01-julij-2005
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Natural gas - Calculation of calorific values, density, relative density and Wobbe index
from composition (ISO 6976:1995 including Corrigendum 1:1997, Corrigendum 2:1997
and Corrigendum 3:1999)
Erdgas - Berechnung von Brenn- und Heizwert, Dichte, relativer Dichte und Wobbeindex
aus der Zusammensetzung (ISO 6976:1995 + Corrigendum 1:1997 + Corrigendum
2:1997 + Corrigendum 3:1999)
Gaz naturel - Calcul du pouvoir calorifique, de la masse volumique, de la densité relative
et de l'indice de Wobbe a partir de la composition (ISO 6976:1995, Corrigendum 1:1997,
Corrigendum 2:1997 et Corrigendum 3:1999 inclus)
Ta slovenski standard je istoveten z: EN ISO 6976:2005
ICS:
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 6976
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2005
ICS 75.060
English version
Natural gas - Calculation of calorific values, density, relative
density and Wobbe index from composition (ISO 6976:1995
including Corrigendum 1:1997, Corrigendum 2:1997 and
Corrigendum 3:1999)
Gaz naturel - Calcul du pouvoir calorifique, de la masse Erdgas - Berechnung von Brenn- und Heizwert, Dichte,
volumique, de la densité relative et de l'indice de Wobbe à relativer Dichte und Wobbeindex aus der
partir de la composition (ISO 6976:1995, Corrigendum Zusammensetzung (ISO 6976:1995 + Corigendum 1:1997
1:1997, Corrigendum 2:1997 et Corrigendum 3:1999 inclus) + Corigendum 2:1997 + Corigendum 3:1999)
This European Standard was approved by CEN on 17 April 2005.
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, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6976:2005: E
worldwide for CEN national Members.

Foreword
The text of ISO 6976:1995 has been prepared by Technical Committee ISO/TC 193 "Natural
gas” of the International Organization for Standardization (ISO) and has been taken over as EN
ISO 6976:2005 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 November 2005, and conflicting national
standards shall be withdrawn at the latest by November 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,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Endorsement notice
The text of ISO 6976:1995 has been approved by CEN as EN ISO 6976:2005 without any
modifications.
INTERNATIONAL
IS0
STANDARD
Second edition
1995-l Z-01
Corrected and reprinted
1996-02-01
Natural gas - Calculation of calorific
values, density, relative density and Wobbe
index from composition
Gaz na turel - Calcul du pouvoir calorifique, de la masse volumique, de la
densit relative et de I’indice de Wobbe P partir de la composition
Reference number
IS0 6976:1995(E)
IS0 6976:1995(E)
Contents
Page
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
............................................. ............ ......................
2 Definitions . .
...................................... .................. ................. 3
3 Principle .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Behaviour of ideal and real gases
........................ 3
4.1 Enthalpy of combustion .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.2 Calculation of compression factor
. . . . . . . . . . . . . . . . . . . . . . . . . . 4
5 Calculation of calorific value on a molar basis
5.1 Ideal gas . . .
........................................... ...................... ................. 4
5.2 Real gas
. . . . . . . . . . . . . . . . . . . . . . . . . . 4
6 Calculation of calorific value on a mass basis
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1 Ideal gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Real gas . . . 5
. . . . . . . . . . . . . . . . . . 5
7 Calculation of calorific value on a volumetric basis
............................. 5
7.1 Ideal gas . .
........................ ................. ......................................... 5
7.2 Real gas
. . . . . . . . 6
8 Calculation of relative density, density and Wobbe index
8.1 Ideal gas - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Real gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Accuracy . . 6
9 .
9.1 Precision . . . 6
9.2 Trueness . . . 8
................................. ............................. 9
9.3 Expression of results
IO Tables of recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IO
0 IS0 1995
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopyrng and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
II
0 IS0
IS0 6976: 1995(E)
Annexes
A Symbols and units . . . . . .*. . . . . . . . . . . . . . . 15
B Values of auxiliary constants, etc. . . 17
B.l Molar gas constant . . .
B.2 Critical constants and acentric factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B.3 Properties of dry air . . . . 17
B.4 Enthalpy of vaporization of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
C Conversion of volume fractions to mole fractions . . . . . . . . . . . . . . . . 20
D Examples of calculations . .
........ 21
D.l Calorific value on a molar basis (clause 5) . . . . . . . . . . . . . . . . . . . . . . . . . . 21
D.2 Calorific value on a mass basis (clause 6) . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
D.3 Calorific value on a volumetric basis (clause 7) . . . . . . . . . . . . . . . . . . 21
D.4 Relative density, density and Wobbe index (clause 8)
....... 22
D.5 Precision (clause 9) . . 23
E Behaviour of ideal and real gases . . 25
E.l Variation of ideal-gas enthalpy of combustion with
temperature . . . . 25
E.2 Corrections for non-ideality: volumetric effects .
E.3 Corrections for non-ideality: enthalpic effects
..................... 28
F Effects of water vapour on calorific value
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
F.l General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F.2 Excluded volume effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F.3 Latent heat (enthalpic) effect . . 31
F.4 Compression factor effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
G Summary, discussion and selection of the calorific value of
methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H Derivation of equations relating to precision .
H.l Methane by difference . 36
H.2 Methane by analysis . .
J Approximate conversion factors between reference states
. . 38
K Computer implementation of recommended methods . . . . . . . . . 40
L Calorific values on a molar basis for 60 “F reference
............................ ...........
temperature . . 43
iii
IS0 6976:1995(E) 0 IS0
IV! Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.
iv
0 IS0
IS0 6976:1995(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 6976 was prepared by Technical Committee
ISO/TC 193, Natural gas, Subcommittee SC I, Analysis of natural gas.
This second edition cancels and replaces the first edition
(IS0 6976:1983), of which it constitutes a technical revision.
Annexes A and B form an integral part of this International Standard. An-
nexes C, D, E, F, G, H, J, K, L and M are for information only.

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IS0 6976:1995(E)
INTERNATIONAL STANDARD 0 IS0
- Calculation of calorific values, density,
Natural gas
relative density and Wobbe index from composition
synonymous with “calorific value ”; “specific gravity” is
1 Scope
synonymous with “relative density ”; “Wobbe number” is
synonymous with “Wobbe index ”; “compressibility factor”
is synonymous with “compression factor ”.
This International Standard specifies methods for the
3 If the composition of the gas is known by volume frac-
calculation of the superior calorific value, inferior
tions these must be converted to mole fractions (see
calorific value, density, relative density and Wobbe
annex C). Note, however, that the derived mole fractions
index of dry natural gases, natural gas substitutes and
will have uncertainties greater than those of the original
other combustible gaseous fuels, when the compo-
volume fractions.
sition of the gas by mole fraction is known. The
4 For the purposes of this International Standard, the sum
methods provide a means of calculating the proper-
of the mole fractions used must be unity to the nearest
ties of the gas mixture at commonly used metric ref-
0,000 I, and all components with mole fractions greater
erence conditions.
than 0,000 05 must be accounted for.
The methods of calculation require values for various
5 For the calorific value calculated on a volumetric basis,
physical properties of the pure components; these
there are limitations on the amounts of components other
values are provided in tables and their sources are
than methane which may be present. It is impossible to be
definitive on this matter, but the following guidelines may
identified.
be useful:
Methods are given for estimating the precision of
N, should not be present in amounts
...