Natural gas - Measurement of properties - Calorific value and Wobbe index (ISO 15971:2008)

EN-ISO 15971 concerns the measurement of calorific value of natural gas and natural gas substitutes by non-separative methods, i.e. methods that do not involve the determination of the gas composition nor calculation from it. It describes the principles of operation of a variety of instruments in use for this purpose, and provides guidelines for the selection, evaluation, performance assessment, installation and operation of these. Calorific values can be expressed on a mass basis, a molar basis or, more commonly, a volume basis. The working range for superior calorific value of natural gas, on the volume basis, is usually between 30 MJ/m3 and 45 MJ/m3 at standard reference conditions (see ISO 13443). The corresponding range for the Wobbe index is usually between 40 MJ/m3 and 60 MJ/m3. This International Standard neither endorses nor disputes the claims of any commercial manufacturer for the performance of an instrument. Its central thesis is that fitness-for-purpose in any particular application (defined in terms of a set of specific operational requirements) can be assessed only by means of a well-designed programme of experimental tests. Guidelines are provided for the proper content of these tests.

Erdgas - Messung der Eigenschaften - Wärmewerte und Wobbe-Index (ISO 15971:2008)

Gaz naturel - Mesurage des propriétés - Pouvoir calorifique et indice de Wobbe (ISO 15971:2008)

L'ISO 15971:2008 concerne le mesurage du pouvoir calorifique du gaz naturel et de produits de substitution du gaz naturel par des méthodes non séparatives, c'est-à-dire des méthodes qui n'impliquent pas la détermination de la composition du gaz, ni le calcul à partir de celle-ci. Elle décrit les principes de fonctionnement d'une diversité d'instruments utilisés à cet effet, et elle fournit des lignes directrices pour leur sélection, évaluation, évaluation de performance, installation et fonctionnement.
Les pouvoirs calorifiques peuvent être exprimés sur une base massique, molaire ou, plus communément, volumique. La plage de travail pour le pouvoir calorifique supérieur du gaz naturel, sur une base volumique, est généralement comprise entre 30 MJ/m3 et 45 MJ/m3 dans les conditions de référence standard (voir l'ISO 13443). La plage correspondante de l'indice de Wobbe est généralement comprise entre 40 MJ/m3 et 60 MJ/m3.
L'ISO 15971:2008 ne cautionne ni ne conteste les revendications d'un quelconque constructeur commercial pour la performance d'un instrument. Sa thèse centrale est que l'adaptation à un usage dans une quelconque application particulière (définie en termes d'un ensemble d'exigences opérationnelles spécifiques) ne peut être évaluée qu'au moyen d'un programme bien conçu d'essais expérimentaux. Des lignes directrices sont fournies pour le contenu correct de ces essais.

Zemeljski plin - Merjenje njegovih lastnosti - Kalorična vrednost in Wobbejev indeks (ISO 15971:2008)

Standard EN-ISO 15971 obravnava na merjenje kalorične vrednosti zemeljskega plina in nadomestkov zemeljskega plina z neseparacijskimi metodami, ki ne vključujejo določevanja sestave plina ali izračuna, ki iz tega izhaja. Opisuje načela delovanja različnih instrumentov, ki se uporabljajo v ta namen, in zagotavlja smernice za njihovo izbiro, vrednotenje, oceno učinka, namestitev in delovanje. Kalorične vrednosti se lahko izrazijo na podlagi mase, števila molov ali, najpogosteje, prostornine. Delovno območje za boljšo kalorično vrednost zemeljskega plina na podlagi prostornine je pri standardnih referenčnih pogojih med 30 in 45 MJ/m3 (glej ISO 13443). Ustrezen razpon za Wobbejev indeks je običajno med 40 in 60 MJ/m3. Ta mednarodni standard niti ne podpira in niti ne ugovarja trditvam katerega koli proizvajalca o zmogljivosti inštrumentov. Njegova osrednja teza je, da je primernost za uporabo za kateri koli namen (opredeljen z vidika nabora posebnih obratovalnih zahtev) lahko ocenjena le z dobro zasnovanim programom poskusnih preskusov. Na voljo so smernice za pravilno izvajanje teh preskusov.

General Information

Status
Published
Public Enquiry End Date
01-Dec-2013
Publication Date
01-Apr-2014
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
20-Mar-2014
Due Date
25-May-2014
Completion Date
02-Apr-2014

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SLOVENSKI STANDARD
SIST EN ISO 15971:2014
01-maj-2014
=HPHOMVNLSOLQ0HUMHQMHQMHJRYLKODVWQRVWL.DORULþQDYUHGQRVWLQ:REEHMHY
LQGHNV ,62
Natural gas - Measurement of properties - Calorific value and Wobbe index (ISO
15971:2008)
Erdgas - Messung der Eigenschaften - Wärmewerte und Wobbe-Index (ISO 15971:2008)

Gaz naturel - Mesurage des propriétés - Pouvoir calorifique et indice de Wobbe (ISO

15971:2008)
Ta slovenski standard je istoveten z: EN ISO 15971:2014
ICS:
75.060 Zemeljski plin Natural gas
75.180.30 Oprema za merjenje Volumetric equipment and
prostornine in merjenje measurements
SIST EN ISO 15971:2014 en

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

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SIST EN ISO 15971:2014
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SIST EN ISO 15971:2014
EUROPEAN STANDARD
EN ISO 15971
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2014
ICS 75.060
English Version
Natural gas - Measurement of properties - Calorific value and
Wobbe index (ISO 15971:2008)

Gaz naturel - Mesurage des propriétés - Pouvoir calorifique Erdgas - Messung der Eigenschaften - Wärmewerte und

et indice de Wobbe (ISO 15971:2008) Wobbe-Index (ISO 15971:2008)
This European Standard was approved by CEN on 16 February 2014.

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

© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 15971:2014 E

worldwide for CEN national Members.
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SIST EN ISO 15971:2014
EN ISO 15971:2014 (E)
Contents Page

Foreword ..............................................................................................................................................................3

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SIST EN ISO 15971:2014
EN ISO 15971:2014 (E)
Foreword

The text of ISO 15971:2008 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 15971:2014.

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 September 2014, and conflicting national standards shall be

withdrawn at the latest by September 2014.

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.

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 15971:2008 has been approved by CEN as EN ISO 15971:2014 without any modification.

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SIST EN ISO 15971:2014
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SIST EN ISO 15971:2014
INTERNATIONAL ISO
STANDARD 15971
First edition
2008-12-15
Natural gas — Measurement of
properties — Calorific value and Wobbe
index
Gaz naturel — Mesurage des propriétés — Pouvoir calorifique et indice
de Wobbe
Reference number
ISO 15971:2008(E)
ISO 2008
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
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Published in Switzerland
ii © ISO 2008 – All rights reserved
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
Contents Page

Foreword............................................................................................................................................................ iv

Introduction ........................................................................................................................................................ v

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

2 Normative references ........................................................................................................................... 1

3 Terms and definitions........................................................................................................................... 1

3.1 Calorific value and Wobbe index ........................................................................................................ 1

3.2 Water content of gas ............................................................................................................................ 2

3.3 Performance classification .................................................................................................................. 2

3.4 Terms from metrology.......................................................................................................................... 3

4 Principles of measurement.................................................................................................................. 4

4.1 Introduction ........................................................................................................................................... 4

4.2 Direct combustion calorimetry............................................................................................................ 5

4.3 Indirect methods................................................................................................................................... 5

4.4 Inferential methods............................................................................................................................... 6

5 Performance assessment and acceptance tests............................................................................... 7

5.1 Performance assessment for instrument selection.......................................................................... 7

5.2 Factory and site acceptance tests .................................................................................................... 20

6 Sampling and installation guidelines ............................................................................................... 21

6.1 Sampling.............................................................................................................................................. 21

6.2 Installation guidelines ........................................................................................................................ 22

7 Calibration ........................................................................................................................................... 25

7.1 Calibration procedures....................................................................................................................... 25

7.2 Calibration gases................................................................................................................................ 26

8 Verification .......................................................................................................................................... 27

8.1 Verification procedures...................................................................................................................... 27

8.2 Verification gases ............................................................................................................................... 28

9 Maintenance ........................................................................................................................................ 29

9.1 Preventive maintenance..................................................................................................................... 29

9.2 Corrective maintenance ..................................................................................................................... 29

10 Quality control..................................................................................................................................... 29

10.1 General................................................................................................................................................. 29

10.2 Environmental parameters and ancillary equipment...................................................................... 31

10.3 Instrumental factors ........................................................................................................................... 32

Annex A (normative) Symbols and units....................................................................................................... 33

Annex B (informative) Examples of type-approval and technical specifications...................................... 34

Annex C (informative) Class 0 mass-basis calorimetry ............................................................................... 36

Annex D (informative) Direct combustion calorimetry................................................................................. 40

Annex E (informative) Stoichiometric combustion devices......................................................................... 43

Annex F (informative) Effect of non-alkane gases on stoichiometric combustion devices..................... 47

Annex G (informative) Measurement of Wobbe index.................................................................................. 48

Bibliography ..................................................................................................................................................... 49

© ISO 2008 – All rights reserved iii
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SIST EN ISO 15971:2014
ISO 15971:2008(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 2.

The main task of technical committees is to prepare International Standards. 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 document may be the subject of patent

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

ISO 15971 was prepared by Technical Committee ISO/TC 193, Natural gas.
iv © ISO 2008 – All rights reserved
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
Introduction

The amount of energy delivered by a flowing natural gas is often determined as the product of the volume

delivered and the calorific value per unit volume of the gas. It is, therefore, important to have available

standardized methods of determining the calorific value. In many cases, it is possible to calculate the calorific

value of natural gas, with sufficient accuracy, given the composition (see ISO 6976). However, it is also

possible, and sometimes a preferred alternative, to measure calorific value using any one of several

techniques that do not require a compositional analysis. The methods currently in use, and the many factors

that it is necessary to address in the selection, evaluation, performance assessment, installation and operation

of a suitable instrument, are detailed herein. The measurement of the Wobbe index, a property closely related

to calorific value, is discussed briefly in an informative annex, but is not considered in detail in the normative

parts of this International Standard.
© ISO 2008 – All rights reserved v
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SIST EN ISO 15971:2014
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SIST EN ISO 15971:2014
INTERNATIONAL STANDARD ISO 15971:2008(E)
Natural gas — Measurement of properties — Calorific value and
Wobbe index
1 Scope

This International Standard concerns the measurement of calorific value of natural gas and natural gas

substitutes by non-separative methods, i.e. methods that do not involve the determination of the gas

composition nor calculation from it. It describes the principles of operation of a variety of instruments in use for

this purpose, and provides guidelines for the selection, evaluation, performance assessment, installation and

operation of these.

Calorific values can be expressed on a mass basis, a molar basis or, more commonly, a volume basis. The

working range for superior calorific value of natural gas, on the volume basis, is usually between 30 MJ/m

and 45 MJ/m at standard reference conditions (see ISO 13443). The corresponding range for the Wobbe

3 3
index is usually between 40 MJ/m and 60 MJ/m .

This International Standard neither endorses nor disputes the claims of any commercial manufacturer for the

performance of an instrument. Its central thesis is that fitness-for-purpose in any particular application (defined

in terms of a set of specific operational requirements) can be assessed only by means of a well-designed

programme of experimental tests. Guidelines are provided for the proper content of these tests.

2 Normative references

The following referenced documents are indispensable for the application 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 6976:1995, Natural gas — Calculation of calorific values, density, relative density and Wobbe index from

composition
ISO 14532: 2001, Natural gas — Vocabulary
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 Calorific value and Wobbe index
3.1.1
superior calorific value

amount of heat that would be released by the complete combustion in air of a specified quantity of gas (on a

molar, mass or volume basis), 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 formed by combustion, which is

condensed to the liquid state at T
See ISO 6976.
© ISO 2008 – All rights reserved 1
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
3.1.2
inferior calorific value

amount of heat that would be released by the complete combustion in air of a specified quantity of gas (on a

molar, mass or volume basis), 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
See ISO 6976.
3.1.3
Wobbe index

superior calorific value on a volumetric basis at specified reference conditions, divided by the square root of

the relative density at the same specified metering reference conditions
See ISO 6976.
3.1.4
standard reference conditions

temperature, T = 288,15 K, and (absolute) pressure, p = 101,325 kPa, for the real dry gas

See ISO 13443.

NOTE Standard reference (or base) conditions of temperature, pressure and humidity (state of saturation) are

defined for use only in natural gas and similar applications. For the calorific value on a volumetric basis, these conditions

apply to both the metering and combustion of the gas. In the expression of physical quantities throughout this International

Standard, these standard reference conditions as defined in ISO 13443 are taken to apply.

3.2 Water content of gas
3.2.1
saturated gas

natural gas which, at the specified conditions of temperature and pressure, is at its water dew-point

3.2.2
dry gas

natural gas which does not contain water vapour at a mole fraction greater than 0,000 05

See ISO 6976.
3.2.3
partially saturated or wet gas

natural gas which contains an amount of water vapour between that of the saturated gas and that of the dry

gas, at the specified conditions of temperature and pressure
3.3 Performance classification

NOTE The following classification scheme is adopted in order to categorize the uncertainties associated with

measurement of calorific value. The attached notes are explanatory, not parts of the definitions. The values given refer to

an expanded uncertainty with a coverage factor of 2.
3.3.1
class 0

performance with which uncertainty limits of no greater than ± 0,1 % in calorific value may be associated

NOTE Performance of this quality can currently be achieved only by instruments in which all operations are carried

out in strict accordance with the best metrological practices and in which all relevant physical measurements are directly

traceable to primary metrological standards. Typically, such an instrument is custom-built and installed in a purpose-built,

environmentally controlled specialist laboratory; a specially trained and identified operator is likely required. Instruments of

this type are sometimes known as “reference calorimeters” and all, to date, make measurements discontinuously on

discrete samples of gas.
2 © ISO 2008 – All rights reserved
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
3.3.2
class 1

performance with which uncertainty limits of no greater than ± 0,1 MJ/m on a volume-basis calorific value

(approximately 0,25 %) may be associated

NOTE This is the lowest level of measurement uncertainty currently available for any form of commercial instrument

used in routine field (i.e. non-laboratory) operation. Even for the few types of instrument that are intrinsically capable of

this performance, it is unlikely to be achieved unless installation is in accordance with both the manufacturer's instructions

and the principles described in this International Standard, and operation is in accordance with the calibration, verification,

maintenance and quality control procedures described in this International Standard.

3.3.3
class 2

performance with which uncertainty limits of no greater than ± 0,2 MJ/m on a volume-basis calorific value

(approximately 0,5 %) may be associated
3.3.4
class 3

performance with which uncertainty limits of no greater than ± 0,5 MJ/m on a volume-basis calorific value

(approximately 1,0 %) may be associated
3.4 Terms from metrology

NOTE The following definitions, including the Notes attached to them (except the Note to 3.4.6), are all taken from

ISO 14111, where additional explanatory details are given.
3.4.1
accuracy

closeness of agreement between a measurement result and the true value of the measurand

NOTE The term “accuracy”, when applied to a set of measurement results, describes a combination of random

components and a common systematic error or bias component.
3.4.2
trueness

closeness of agreement between the average value obtained from a large series of measurement results and

the true value of the measurand
NOTE The measure of trueness is usually expressed in terms of bias.
3.4.3
bias

difference between the expectation of the measurement results and an accepted reference value

3.4.4
precision

closeness of agreement between independent measurement results obtained under prescribed conditions

NOTE Precision depends only on the distribution of random errors and does not relate to the true value.

3.4.5
repeatability

precision under conditions where independent measurement results are obtained with the same method on

identical measuring objects in the same laboratory by the same operator within short intervals of time

NOTE Repeatability is expressed quantitatively based on the standard deviation of the results.

© ISO 2008 – All rights reserved 3
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SIST EN ISO 15971:2014
ISO 15971:2008(E)
3.4.6
uncertainty

estimate attached to a measurement result which characterizes the range of values within which the true

value is asserted to lie

NOTE An alternative, but equivalent, definition taken from Reference [1] is as follows: parameter, associated with the

result of a measurement, that characterizes the dispersion of the values that can reasonably be attributed to the

measurand.
3.4.7
calibration

set of operations that establish, under specified conditions, the relationship between values of quantities

indicated by a measuring instrument or measuring system, or values represented by a material measure or a

reference material, and the corresponding values realized by standards
3.4.8
verification

confirmation by examination and provision of objective evidence that specified requirements have been

fulfilled
4 Principles of measurement
4.1 Introduction

Instruments capable of class 0 performance (hereafter, for brevity, called class 0 calorimeters) have been

established in a few specialist laboratories; but since they are, inevitably, labour-intensive, spot-test

instruments, not commercially available and not suitable for field operation, details of their installation,

operation and maintenance are beyond the scope of the main part of this International Standard.

Nevertheless, measurements made using calorimeters of this type can have an important part to play in the

“everyday” determination of calorific value, mainly as one possible accredited means for the provision of

certified calibration gases (certified gaseous reference materials) having traceability to international

metrological standards (see 7.2). They may also be used for research purposes and the resolution of disputes.

The principles upon which typical class 0 calorimeters operate, together with details of many of the other

relevant factors, are given in Annex C. All class 0 calorimeters so far devised have, as their primary

determination, the mass-basis calorific value. To be useful for most routine applications, it is necessary to

convert this by some secondary means to the volume-basis value. In order to achieve a volume-basis calorific

value with an uncertainty of ± 0,1 %, it is usual to dedicate a density meter of sufficient accuracy for use with

instruments of this type.

Instruments capable of class 1, class 2 or class 3 performance usually measure calorific value on the volume

basis. They are normally designed for continuous, unattended operation in the field, producing an essentially

continuous record of calorific value. Except for process gas chromatographs (which are not the subject of this

International Standard), they are the only types of instrument that can sensibly be used for routine

measurements of calorific value on natural gas passing through transmission and distribution systems.

The principle of operation may be either direct, indirect or inferential, within the meaning of these terms in

accordance with ISO 14532. This International Standard is concerned mostly with the performance of these

kinds of instruments. Some instruments have the additional facility of measuring relative density; in these

cases, this capability is equivalent to making available the determination both of the calorific value on the

mass basis and of the Wobbe index.

Depending upon the particular application, instruments can be required to record either the superior or the

inferior calorific value. Although each particular type of instrument responds, in principle, to one or the other of

these, most types can be set up so as to record, with little loss of accuracy for typical natural gases, the

alternate value. To achieve this, the main requirement is that the instrument be set up using calibration gases

that are correspondingly certified (see also 5.1.10.2).
4 © ISO 2008 – All rights reserved
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ISO 15971:2008(E)
4.2 Direct combustion calorimetry

Only those instruments that are true combustion calorimeters, in the sense that the energy released as heat

by the combustion of gas is determined by means of thermometric measurements, fall into the

“direct-measurement” category. All current commercial implementations determine the volume-basis calorific

value.

In this type of instrument, the gas sample is metered volumetrically on a continuous basis, often through the

use of a water-sealed “wet meter” (Reference [2], Chapter 4, and ISO 6145-1), before passing to a burner.

The main measurement is of the quasi-stationary (equilibrium) rise in temperature of a continuously flowing

(metered) heat-exchange medium with which the hot products of combustion do not mix.

The heat-exchange medium is usually air; water-flow calorimeters do exist in a wide variety of forms but all of

these are now obsolete. The temperature rise is usually measured using resistance thermometry. Calibration

is usually achieved by the use of gaseous reference materials (working standards) certified for calorific value.

Calorific values are usually measured by this method at ambient temperature and pressure. It is necessary,

however, to refer the values recorded to specified reference conditions of temperature and pressure of both

metering and of combustion. For this reason, prior information concerning the stability of the output with

respect to variations of ambient temperature can be important (see 5.1.6).

It is also important to define the reference condition of water content for the gas, in particular if the instrument

controls the water content of the gas (either by saturation or by drying) prior to or during the measurement

process. At standard reference conditions, the difference between the superior calorific value of a dry gas and

a saturated gas is approximately 1,7 %.

Instruments of this type are usually set up so as to record the superior calorific value. One of the main

advantages of true combustion calorimeters is that there is no restriction on the composition of the sample gas

for which they are expected to give the correct result.

Calorimeters based on this generic methodology (Reference [2], Chapter 10; Reference [6],

...

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