SIST EN ISO 15971:2014

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

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.

Contents Page
Foreword .3
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.

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
ISO 15971:2008(E)
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ii © ISO 2008 – All rights reserved

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 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

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.

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 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

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 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

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], Chapter 7; and
References [3] to [5]) are often capable of class 1 performance, but typically have quite a sluggish response to
changes in calorific value because of thermal inertia.
Typical examples of this kind of calorimeter are described in Annex D.
4.3 Indirect methods
4.3.1 General
Instruments that fall into the “indirect” category are those that measure some physicochemical property of the
gas and use a known relationship, established by both practical observation and theoretical analysis, between
calorific value and the property measured, in order to infer the calorific value, either superior or inferior, of the
gas.
4.3.2 Stoichiometric combustion
Instruments of this type depend upon the principle that, for a gas mixture containing only alkane hydrocarbons
and inert constituents, the volume-basis calorific value (either superior or inferior) is a linear function of the
air-to-gas ratio required to achieve stoichiometric combustion.
There are at least two ways to implement this principle in a practical device. In one implementation, the
stoichiometric point is determined by searching for the air-to-gas ratio at which the amount of oxygen in the
products of complete combustion is zero. In an alternative implementation, the stoichiometric point is
determined by searching for the air-to-gas ratio at which maximum flame temperature is achieved.
ISO 15971:2008(E)
One disadvantage of instruments that operate on this principle is the requirement to confirm that the sample
gas contains only alkane hydrocarbons and inert constituents. Any other constituent (for example, alkenes,
hydrogen, carbon monoxide and, most severely, oxygen) can cause the instrument to give a false reading; in
some cases, however, any errors can be accounted for by a correction procedure.
[7]
Instruments based on this principle are readily capable of at least class 2 performance and typically exhibit
rapid response to changes in calorific value.
Some practical details of these devices are given in Annex E.
4.3.3 Catalytic combustion
Instruments of this type are based on the principle that a determination of the amount of heat released during
the complete oxidation (combustion) of a gas at a catalytic surface is a proper representative measure of its
calorific value.
In one implementation, a semi-continuous (i.e. on-off-on) metered flow of fuel gas undergoes oxidation at the
surface of a catalyst-coated conductor; the heat released by this combustion process raises the temperature
of the conductor and so influences its electrical resistance. The electrical resistance can readily be used to
follow the rise in temperature over the “on” period of the gas flow, and the integrated temperature rise for this
period may then be used as an indicator of inferior calorific value.
In another implementation, the oxidation process takes place within a bed of powdered catalytic material. The
flowrate of fuel (at a constant flowrate of air) that is necessary to maintain the reaction chamber at a constant
temperature is then measured and used as an indicator of inferior calorific value.
[8], [9]
Catalytic combustion instruments of the types described in this subclause are at an advanced stage of
development, but are not yet commercially available; their performance capabilities, therefore, cannot yet be
assessed.
4.4 Inferential methods
The dividing line between “indirect” and “inferential” methods is rather indistinct. It can logically be argued that
all determinations of calorific value are, in some sense of the word, inferential. Here, inferential methods are
taken to be those that depend upon an empirical (or possibly semi-empirical) correlation between calorific
value and some other measured property or properties.
Examples of relevant properties that may be used in this way as predictors of calorific value include
[10]
compression factor (which is related to calorific value by means of the SGERG-88 equation ) and speed of
sound. Neither of these properties alone is sufficient to determine calorific value unambiguously (further
information concerning the inert constituents is needed), and no commercial device has yet been produced to
exploit such correlations. Nevertheless, the great precision with which speed of sound can readily be
measured suggests a possible future role for a method based on this principle.
For the present, however, instruments that can best be classed as inferential are much less sophisticated,
both in principle and in construction. In typical instruments of this kind, a supposedly constant proportion of the
heat released by the combustion of a regulated flow of fuel gas is sensed (but not measured) by some
particular device and related empirically to the calorific value.
In one long-established implementation of this principle (Reference [2], Chapter 10 and Reference [6],
Chapter 6), the sensing device is the burner chimney itself, formed from two concentric metal tubes joined
rigidly at the bottom; the two tubes expand differentially by an amount that depends on the heat transferred
[4]
from the flue gases, and this may be used to give an indication of calorific value. In another implementation ,
the sensor is a single metal thermal-expansion tube located in the effluent gas stream; and in a slightly more
modern implementation, it is a similarly located thermopile, the output of which is taken as an indication of
calorific value. In none of these instruments is the water of combustion condensed; consequently all, in
principle, respond to the inferior calorific value.
6 © ISO 2008 – All rights reserved

ISO 15971:2008(E)
As a consequence of their acknowledged simplicity, instruments of this general type cannot usually be
expected to achieve better than class 3 performance except in the most favourable of circumstances.
Very many other principles of operation have been described over the years, but it is not the intention here to
describe ideas which no longer find, have never found, or are unlikely to find reasonably widespread
application. There are countless “dead-end” patents.
5 Performance assessment and acceptance tests
The flowchart given as Figure 1 provides an overview of the procedures that it is typically necessary to carry
out in order to satisfy performance assessment and acceptance testing requirements. Specific details are
provided in 5.1 and 5.2.
5.1 Performance assessment for instrument selection
5.1.1 General
For any application, it is necessary that an instrument for the measurement of calorific value meet some
criteria of acceptability. Two common forms that these criteria may take for a commercially available
instrument are
a) a set of requirements that shall be met in order for the instrument to receive type-approval; in some cases,
this can be issued by a statutory body responsible for the supervision of custody transfer or customer
charging, and
b) a technical specification for purchase contract purposes.
Annex B gives an example of a typical type-approval specification (Clause B.1) and of a typical technical
specification forming part of the purchase documentation (Clause B.2).
5.1 relates to performance assessment tests that it is typically necessary to carry out on (usually) a single
instrument as an exemplar of its type. In 5.1.2 to 5.1.13 are considered the factors that are those most often
specified in a formal set of requirements, such as those referred to in this subclause. A purposeful test
programme is likely to include the investigation of most, if not all, of these aspects of the instrumental
performance.
The test programme shall include a specification for the calibration and other test gases that are required to
carry out many aspects of the detailed testing and calibration, although some test gases might not require
certification. Furthermore, any type-approval documentation issued as a result of the test programme shall
include a specification for the gases for use on-site in the re-calibration and verification procedures, so as to
ensure attainment and maintenance of the specified accuracy.
Instruments of all types considered in this International Standard generally perform optimally when left in
continuous on-line operation; requirements calling for only intermittent or off-line operation demand special
care and extra pre-testing (see 5.1.3).
ISO 15971:2008(E)
Figure 1 — Instrument evaluation, performance assessment and acceptance testing
Depending on the particular circumstances, performance assessment testing may be carried out by a
regulatory authority, by an independent, accredited testing laboratory and/or by the purchaser. If carried out by
a regulatory authority, the tests may lead to type-approval documentation. There is a clear trade-off between
the length of time spent on a series of performance tests and the amount of detail that they yield about an
instrument. Where a regulatory authority requires a thorough characterization of all aspects of performance,
the test programme can easily extend over a period of one year or more.
A pro-forma checklist of the type given as Table 1 can be a useful means of keeping track of the progress of,
and results from, a lengthy evaluation programme.
8 © ISO 2008 – All rights reserved

ISO 15971:2008(E)
5.1.2 Continuity of operation
It is likely that an explicit requirement be for the instrument to be in-service and to operate correctly (and, for
most applications, continuously) for a specified period of time. If the particular application requires intermittent
operation only, then the trueness tests (see 5.1.3) should address this complicating factor as a priority before
proceeding with other tests.
In the case of continuous operation, the instrument should be tested simply by letting it run without interruption
or undue interference (such as unscheduled adjustments to settings) for a continuous period that exceeds the
specified minimum required operating period by a specified percentage. Depending on the application, this
may be anywhere from a few days to several months. It can, however, be possible to carry out other tests
during this period, for example trueness and repeatability tests, without prejudice to the continuity (reliability)
test.
Repeating the continuity (reliability) test, after routine maintenance in accordance with the manufacturer's
instructions, at least once, is likely to be a worthwhile option. If the instrument cannot operate without
breakdown or obvious malfunction for the specified period, then it fails this test.
The results from a completed test may be analysed in order to assess the period for which the specified
instrument performance has been achieved. If this is less than the minimum for which correct operation is
required, then the instrument has again failed. In this case, it can still be possible to use the instrument in
applications where a shorter period of continuous satisfactory operation is acceptable.
If there is no explicit requirement for a minimum operating period, then it is possible to allow the frequency of
maintenance operations to be determined by operational experience, i.e. the maintenance operations are
performance-driven rather than requirement-driven.
Table 1 — Example of checklist for type-approval and acceptance testing
Type approval testing Factory acceptance test On-site acceptance test
Property or test
required result pass/fail specified result pass/fail specified result pass/fail
1 Continuity of operation
continuous operation within
maximum specified error of
0,25 MJ/m
test period 6 months 7 months pass 1 month 1 month pass 3 months 3 months pass
2 Trueness of calorific value
number of test gases 7  5  application gases
range, MJ/m 31 linearity, MJ/m < 0,10 0,07 pass < 0,10 0,05 pass not tested
3 Repeatability
multiple readings type b)
number of test readings 40  20  20
spread, MJ/m < 0,10 0,08 pass < 0,10 0,05 pass < 0,10 0,06 pass
4 Response to calorific value
step-change
step, MJ/m 38 to 42  40 to 41  40 to 41
95 % response time < 4 min 2 min pass < 3 min 1 min pass < 3 min 1 min pass
35 s 53 s 42 s
ISO 15971:2008(E)
Table 1 — Example of checklist for type-approval and acceptance testing (continued)
Type approval testing Factory acceptance test On-site acceptance test
Property or test
required result pass/fail specified result pass/failspecified result pass/fail
5 Temperature dependence
maximum calorific-value
error for temperature
change
5.1 Steady state
range, °C 15 to 30  15 to 25  natural variations (3 months)
error, MJ/m < 0,10 0,13 fail < 0,10 0,08 pass < 0,05 0,04 pass
5.2 Dynamic
range, °C 15 to 30  natural variations natural variations (3 months)
rate, K/h 5
error, MJ/m < 0,20 0,14 pass not tested < 0,05 0,04 pass
6 Atmospheric pressure
dependence
range, hPa 970 to  natural variations natural variations (3 months)
3 < 0,10 0,11 fail < 0,05 0,05 pass < 0,05 0,03 pass
error MJ/m
7 Other environmental factors
7.1 Relative humidity interval
range, % 30 to 80  natural variations natural variations
error, MJ/m < 0,10 0,05 pass < 0,10 0,05 pass < 0,10 0,03 pass
7.2 Electrical supply variations
voltage, AC 200 to 240  200 to 240 local site variations
error, MJ/m < 0,03 0,00 pass < 0,03 0,00 pass not tested
frequency, Hz 40 to 65  local site variations local site variations
error, MJ/m < 0,03 0,00 pass not tested not tested
brown-out 5 cycles at  not applicable not applicable
100 VAC
error MJ/m < 0,25 0,12 pass not tested not tested
7.3 Electromagnetic interference
and compatibility (EMC)
conformance with ISO/IEC
requirements
standard tests  pass not applicable not applicable
8 Installation factors
8.1 Gas supply pressure
range, kPa (mbar) 0,5 to 4,0 1,0 to 4,0 1,0 to 4,0
(5 to 40) (10 to 40) (10 to 40)
error, MJ/m < 0,02 0,01 pass < 0,02 0,01 pass < 0,02 0,00 pass
8.2 Air supply pressure
range, kPa (mbar) 0,1 to 1,0 0,3 to 0,5
(1 to 10) (3 to 5)
error, MJ/m < 0,02 0,04 fail < 0,02 0,00 pass < 0,02 0,00 pass

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