ISO 13909-8:2001
(Main)Hard coal and coke — Mechanical sampling — Part 8: Methods of testing for bias
Hard coal and coke — Mechanical sampling — Part 8: Methods of testing for bias
Houille et coke — Échantillonnage mécanique — Partie 8: Méthodes de détection du biais
L'ISO 13909-8:2001 définit des principes et des modes opératoires pour la détection du biais sur des échantillons pour essai de houilles ou de cokes, prélevés conformément aux autres parties de l'ISO 13909. Seule est abordée l'utilisation de méthodes statistiques unidimensionnelles. L'utilisateur est averti que le risque de conclure de manière erronée à l'existence d'un biais, alors qu'aucun biais n'existe pour l'une quelconque des diverses variables mesurées sur le même ensemble d'échantillons, est sensiblement plus important que pour une seule variable. Bien que plusieurs variables puissent être mesurées, la variable unique sur laquelle porteront les résultats de l'essai doit être désignée à l'avance.
Črni premog in koks - Mehansko vzorčenje - 8. del: Metode preskušanja odstopanja
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INTERNATIONAL ISO
STANDARD 13909-8
First edition
2001-12-15
Hard coal and coke — Mechanical
sampling —
Part 8:
Methods of testing for bias
Houille et coke — Échantillonnage mécanique —
Partie 8: Méthodes de détection du biais
Reference number
ISO 13909-8:2001(E)
© ISO 2001
---------------------- Page: 1 ----------------------
ISO 13909-8:2001(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be
edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file,
parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2001
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, elec-
tronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's mem-
ber body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
©
ii ISO 2001 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 13909-8:2001(E)
Contents Page
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Outline of procedure . 4
6 Pre-test inspection . 4
7 Reference methods . 5
8 Choice of variables for the test . 6
9 Choice of fuel for the test . 6
10 Conduct of the test . 7
11 Statistical analysis and interpretation . 9
12 Test report . 20
Annex
A Specimen calculations . 22
Bibliography. 31
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ISO 13909-8:2001(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 3.
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 part of ISO 13909 may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 13909-8 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels,
Subcommittee SC 4, Sampling.
ISO 13909 cancels and replaces ISO 9411-1:1994, Solid mineral fuels — Mechanical sampling from moving streams
— Part 1: Coal and ISO 9411-2:1993, Solid mineral fuels — Mechanical sampling from moving streams — Part 2:
Coke, of which it constitutes a technical revision. It also supersedes the methods of mechanical sampling of coal and
coke given in ISO 1988:1975, Hard coal — Sampling and ISO 2309:1980, Coke — Sampling.
ISO 13909 consists of the following parts, under the general title Hard coal and coke — Mechanical sampling:
— Part 1: General introduction
— Part 2: Coal — Sampling from moving streams
— Part 3: Coal — Sampling from stationary lots
— Part 4: Coal — Preparation of test samples
— Part 5: Coke — Sampling from moving streams
— Part 6: Coke — Preparation of test samples
— Part 7: Methods for determining the precision of sampling, sample preparation and testing
— Part 8: Methods of testing for bias
Annex A of this part of ISO 13909 is for information only.
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ISO 13909-8:2001(E)
Introduction
It is not possible to lay down a standard method for field work by which a sampling procedure can be tested for bias
because details of the procedure will inevitably be affected by local conditions. However, certain principles can be
specified which should be adhered to whenever possible and these are discussed in this part of ISO 13909.
Testing for bias can be a tedious and expensive process, especially if testing of the primary increment sampler is
included. All bias tests therefore include a thorough pre-test inspection, with appropriate action taken regarding any
system deficiencies likely to cause bias.
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INTERNATIONAL STANDARD ISO 13909-8:2001(E)
Hard coal and coke — Mechanical sampling —
Part 8:
Methods of testing for bias
1 Scope
This part of ISO 13909 sets out principles and procedures for testing the bias of test samples of hard coals or cokes,
taken in accordance with other parts of ISO 13909. The use of univariate statistical methods only is addressed.
The user is cautioned that the chance of falsely concluding that there is a bias, when no bias exists in any one of
several variables measured on the same set of samples, is substantially greater than for a single variable. While
several variables may be measured, the single variable on which the outcome of the test will be governed shall be
designated in advance.
NOTE In the text the term 'fuel' is used where both coal and coke would be applicable in the context and either 'coal' or 'coke'
where only one is applicable.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this part of ISO 13909. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 13909 are encouraged to investigate the
possibility of applying the most recent editions of the normative documents indicated below. For undated references,
the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO 13909-1:2001, Hard coal and coke — Mechanical sampling — Part 1: General introduction.
ISO 13909-2:2001, Hard coal and coke — Mechanical sampling — Part 2: Coal — Sampling from moving streams.
ISO 13909-4:2001, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples.
ISO 13909-5:2001, Hard coal and coke — Mechanical sampling — Part 5: Coke — Sampling from moving streams.
ISO 13909-6:2001, Hard coal and coke — Mechanical sampling — Part 6: Coke — Preparation of test samples.
ISO 13909-7:2001, Hard coal and coke — Mechanical sampling — Part 7: Methods for determining the precision of
sampling, sample preparation and testing.
3 Terms and definitions
For the purposes of this part of ISO 13909, the terms and definitions given in ISO 13909-1 apply.
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ISO 13909-8:2001(E)
4Principles
4.1 General
Testing for bias is not always done for a single purpose. The objectives of testing for bias may be for assessing
conformity with contractual requirements, such as guarantees or purchase and acceptance specifications, or for
diagnostic purposes or both, and may or may not involve multiple test parameters. This part of ISO 13909 uses
univariate statistics for testing the performance of the system with respect to a single variable.
It is not possible for any scheme of sampling, or sample preparation and analysis, to be free of errors of
measurement. For this reason, no statistical test can establish that there is no bias, but only that there is not likely to
be a bias of more than a certain magnitude.
The testing of a sampling system for bias is based on taking a series of pairs of samples of essentially the same fuel;
one member of each pair being sampled by the system or component under test, the other member being obtained
by a reference method. For each pair, the difference between the analytical results is determined. The series of
differences between the analytical results thus obtained are subjected to statistical analysis.
The procedure requires the sensitivity of the statistical test of significance to be such that the minimum bias that can
be detected is less than or equal to the maximum tolerable bias,BB. Therefore, shall be established before the test
begins.
NOTE In the absence of other information, a value ofB = 0,20 % to 0,30 % for ash or moisture may be appropriate, subject to
commercial considerations.
The sensitivity of the statistical test used is dependent on the number of pairs compared and the variability of the
differences between them.
The statistical analysis to which results will be subjected assumes three conditions:
— a normal distribution of the variable;
— independence of the errors of measurement;
— statistical homogeneity of the data.
The closeness with which these ideal conditions are achieved, in practice, governs the validity of the statistical
analysis. The execution of the test, including sample reduction, division and laboratory analysis, shall be organized
so as to ensure that deviations from these ideals do not invalidate the statistical analysis.
The statistical test used to make the final judgement is thet-test. A hypothesis is made that the observed mean of
the differences between the two methods is drawn from a population whose mean isB. If the test shows that the
observed difference is significantly less thanB, then the sampler or component is declared free of bias.
In basing decisions on the outcomes of statistical tests, there is always the risk of making either one or the other of
two types of error. If the hypothesis is rejected when it is true, e.g. a bias is not declared even though a bias really
does exist, then an error of the first kind (Type I) has been made. On the other hand, if the hypothesis is accepted
when it is false, e.g. a bias is declared even though a bias really does not exist, then an error of the second kind
(Type II) has been made.
In any particular test, the probability of a Type I error can be arbitrarily set as a matter of discretion and the risk kept
as small as desired. For a specific test, the probability of an error of Type II can only be quantified in relation to some
other value than the original hypothesis. In this method the value of zero is used. The risk of a Type II error can be
decreased at a fixed probability of an error of Type I only by increasing the number of observations. However, since
the sample estimate of the population standard deviation must be used in the calculations, the risk of a Type II error
is an estimated value. The final statistical test is not carried out until sufficient pairs of observations have been taken
to limitbothaTypeIerror inrelationtoB, andthe estimatedTypeIIerror in relation to zero,to 5%. Thus,ifthe
observed value of the mean difference (the sample estimate of the population mean) is not significantly less thanB,
it shall also be significantly greater than zero.
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ISO 13909-8:2001(E)
The number of paired samples suitable for a test of the overall system relative to the maximum tolerable bias,B, may
be insufficient for testing a given component. In such circumstances, if the performance of a given component is of
critical importance, a separate test shall be considered. For components other than the primary increment sampler,
such tests can usually be implemented with minimum disruption of normal operations and at a lesser cost than for a
test for overall system bias (see 7.2 and 7.3).
If obtaining the number of pairs required is found to be impracticable, changes will need to be made to reduce the
within-set variance. Investigate what improvements can be made in the closeness of members of pairs and/or what
reduction can be achieved in the preparation and testing errors. If such improvements or reductions are not possible,
give consideration to increasing the number of increments in the samples, taking into account the practical problems
associated with taking increments and the relative costs and errors involved in sampling, sample preparation and
testing.
If the required number of pairs of samples is still excessive, the maximum tolerable bias,B, may be reviewed.
When samples of more than one increment are compared, it is necessary that the reference samples and the
samples from the system under test be constituted on the same basis, i.e. for a time-basis system, the individual
increment masses shall be proportional to the flow rate and, for mass-basis systems, the individual increment mass
shall be uniform (see ISO 13909-2 and ISO 13909-5).
4.2 Selection of sample pairs
4.2.1 Composition of sample pairs
The members of each pair of samples can each comprise one or more increments. Individual increments can be
compared or samples compounded from increments taken by the two methods. The test shall be structured so that
the expected mean of the differences of the result would be zero if no systematic error is present in the system or
component under test.
4.2.2 Paired-increment samples
Paired-increment experimental design is the comparing of individual primary increments after being processed by
the system, with the reference samples collected from the stopped belt.
For a given parameter, the variance of the differences between paired samples will normally be smaller than the
variance of either of the two series of samples, taken by the system or component under test and the reference
method respectively, except for fuels that are very homogeneous. For this reason, if the increments taken by the two
methods are taken in close proximity to each other in the fuel stream (without overlapping), the variance of the
differences between them will be minimized and the sensitivity of the test improved.
4.2.3 Paired-batch samples
It is often not practicable to obtain single increment samples from the system. Increments taken by the system can
be compounded as samples, and compared with samples compounded from increments taken over the same period
using the reference method. It is not necessary that the two samples have the same number of increments or that
they are of similar mass. In the extreme, a single stopped-belt reference increment could be used as the reference
sample.
4.3 Location of sampling points
For a test of the overall system, the reference sample shall be taken from the primary fuel stream using the stopped-
belt reference method (see clause 7). The system sample shall be the final sample.
The primary sampler shall be tested by examining the differences between members of each pair consisting of the
samples taken by the primary sampler and the reference method.
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ISO 13909-8:2001(E)
NOTE 1 For high volume, high capacity, conveyor systems, a test of a primary increment sampler as an individual component will
require the collection and processing of increments of large mass in a short time interval. Before such a test is undertaken,
therefore, it is necessary to consider carefully whether such a test can be justified.
With the exception of the primary increment sampler, when testing individual components and subsystems, the test
compares the differences between the sample feed stream and the discharge stream of that component or
subsystem.
NOTE 2 In some cases, the quality of the feed stream can only be obtained indirectly, for example, by calculation from the results
of the divided sample and the corresponding reject-stream sample, weighted according to the division ratio.
For all crushing equipment, differences between samples taken from the fuel both before and after the crusher are
used.
For subsystems and sample dividers, pairs obtained by one of the following methods shall be tested:
a) by taking samples from the feed stream and from the sample discharge stream;
b) by taking samples from the sample discharge stream and the reject stream;
c) by collecting the entire sample discharge stream and the entire reject stream.
When using either method a) or method b), great care shall be exercised to obtain unbiased samples; in the case of
method a) being used, care shall be taken to minimize disturbance of the feed flow, as such disturbances may
introduce bias or distort normal operating conditions.
5 Outline of procedure
The order of operations is as follows:
a) carry out a pre-test inspection (see clause 6);
b) for the overall system, determine where the stopped-belt reference sample will be collected (see 7.1); for
diagnostic testing of the system components, see 7.2 or 7.3;
c) determine the variable for test (see clause 8);
d) choose the fuel to be used for the test (see clause 9);
B
e) decide on the maximum tolerable bias, (see 4.1);
f) decide on the composition of the sample pairs, i.e. whether to compare sample pairs of one increment or more
than one increment;
g) proceed with collection of samples and carry out the tests according to clauses 10 and 11.
6 Pre-test inspection
The primary sources of information regarding compliance with the sampling standard are the equipment
specifications and drawings.
A thorough examination of the sampling system and a review of its component specification shall be made.
The party performing the test shall, however, verify performance by field measurements and observations. The
operation of the sampling system shall be observed both with fuel flowing and with no fuel.
Pre-test inspections of all operations and equipment, both static and under load, should be carried out by persons
experienced in the sampling of segregated, heterogeneous, lumpy bulk materials. It is recommended that operation
under normal conditions be observed for an entire lot.
Do not execute a test for bias until all conditions known to cause bias are corrected, unless it is necessary to
establish the performance of a system or component as it stands. In the latter case, the pre-test inspection provides
essential documentation of what the conditions were at the time of the test.
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ISO 13909-8:2001(E)
7 Reference methods
7.1 Overall system
To test overall system bias, the use of a reference method which is known to be intrinsically unbiased is required. The
preferred method is the stopped-belt method, i.e. the collection of increments from a complete cross-section of the
fuel on the conveyor belt by stopping the belt at intervals. When properly collected, the stopped-belt increment can
be considered as a reference increment.
NOTE 1 If it is not possible to collect stopped-belt increments, other reference methods may be used but, in these cases, an
apparent absence of a lack of bias relative to the reference method may not be conclusive, and the use of such methods may
compromise the validity and authority of the findings.
NOTE 2 With collection of stopped-belt increments, some disruption of normal operations can occur and therefore the plan of
execution may need to be coordinated with the normal operations and organized to minimize such disruption. It should be
recognized that the conveyor system involved may be used for only a few hours per day for normal operations and cannot be
operated solely for bias tests, unless the fuel can be diverted to another discharge point. This can extend the time necessary for
completion of the field work and require special arrangements for supplying fuel to the system for testing.
Stopped-belt increments shall be taken with a sampling frame (see Figure 1), or equivalent, from a complete cross-
section of the solid mineral fuel on the belt at a fixed position, for a length along the belt which is at least three times
the nominal top size of the fuel.
Figure 1 — Sampling frame
If single paired increment samples are tested, each stopped-belt increment shall be taken immediately after the
system increment has been taken, preferably from a position located before the primary sampler. If the stopped-belt
increment has to be taken from a position after the primary sampler, it is important to ensure that there is no change
in conditions between the position at which the stopped-belt sample is taken and that at which the primary increment
is taken.
The sampling frame (or equivalent) shall be placed on the stationary belt at the predetermined position so that the
separator plates at each end are in contact with the belt across its full width. All particles lying inside the sampling
frame end plates shall be swept into the sampling container.
Particles obstructing the insertion of the end plate on the left-hand side shall be pushed into the increment while
those obstructing the insertion of the end plate on the right-hand side shall be pushed out of the increment or vice-
versa. Whichever practice is used initially, this practice shall be implemented throughout the test.
Take care to minimize the risk of bias being introduced in the course of preparation of the increments and samples
and check all sample division equipment and procedures for bias with respect to relevant test parameters.
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ISO 13909-8:2001(E)
It is recommended that all increments/samples be weighed immediately after collection, as well as before and after
each crushing and dividing operation.
Pay close attention to minimizing unintended mass losses.
Report all observed mass losses.
7.2 Primary increment samplers
Primary increment samplers shall be tested for bias using only the stopped-belt method described in 7.1 as the
reference method.
7.3 Subsystems and components
Subsystems and components will ordinarily be tested under routine continuous-operating conditions by sampling the
feed, discharge and reject streams using a previously validated sampler. At the final stage, the entire stream is
collected (i.e. this stream is not sampled). The quality of the feed to the sample divider shall be calculated from the
results of the divided sample and the corresponding reject-stream sample, weighted according to the division ratio.
If it is not practicable to test under normal operating conditions, a separate test may be arranged in which either of
the following methods of sampling may be used, as appropriate.
a) Sample the feed and product streams simultaneously to obtain paired members. When using this method, great
care should be exercised to obtain unbiased samples.
b) Alternately collect the entire feed stream and the entire product stream associated with successive primary
increments.
8 Choice of variables for the test
Tests for bias can be carried out for ash, moisture or any other variable required, but tests for ash and moisture
generally suffice. Bias in ash on a dry basis is most commonly caused by errors in size distribution. Bias in moisture
may be caused by a wide variety of factors, including but not limited to errors in size distribution, moisture losses
associated with crushers, excessive ventilation within the sampling system, less than the closest possible coupling
between system components, excessive retention time in the system, or any combination of these.
Direct tests on particle-size distribution are often necessary for coke but the following points should be noted:
a) breakage may occur in the process of increment collection or between the sampling points, thus making zero size
bias practically impossible to achieve;
b) bias in other variables, e.g. ash, may have complex and dissimilar relationships to bias in particle-size
distribution, thus making meaningful interpretation of those variables difficult if not impossible.
It is therefore recommended that, for parameters other than size, a direct comparison for that parameter should be
made.
9 Choice of fuel for the test
9.1 Coal
If more than one coal is to be sampled by the system, the coal chosen for the test shall be one that is expected to
show up any bias in the sampling system. For example, bias for ash on primary samplers and sample dividers is
commonly caused by the exclusion of larger sized particles (see clause 8). If a coal is chosen where the ash of such
particles is similar to that of the coal as a whole, then no bias will be detected even though those particles are being
excluded. If subsequently the sampler is used to sample a coal where the large particles have an ash which differs
from the mean, then the results will be biased. In this case, therefore, examine the coals to be sampled to find the
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ISO 13909-8:2001(E)
relationship between ash and the size fraction and the maximum tolerable bias (see clause 4) compared with the
maximum bias possible, e.g. if the top 10 % of the size distribution were excluded.
It is recommended that the complete bias test be carried out on a coal from a single source. When this is not
possible, the data shall be examined statistically to ensure that their combination for the statistical bias tests is valid
(see 11.5 and 11.6).
9.2 Coke
The same requirements as those specified in 9.1 for coal with respect to its ash shall apply when choosing a coke for
test with respect to its moisture content.
10 Conduct of the test
10.1 Establishing detailed test procedures
10.1.1 General
The test may encompass the primary sampler alone, individual components or subsystems, or the entire system.
The design of the test will differ depending on the objectives. Users of these methods are cautioned to clearly define
the objectives of the test as the first step in planning.
A bias test for the whole system is carried out by comparing the reference samples taken from the main flow with the
samples collected at the final stage of the on-line system. The paired batch experimental design (see 4.2.3) is the
preferred practice because it minimizes the disruption of normal operations. However, the paired-increment
experimental design (see 4.2.2) is an acceptable method, provided precautions are taken to avoid introduction of
bias caused by the test itself.
Bias may be hardware induced, system logic induced, or a combination of both. It follows that even with the paired
increment experimental design, routine operating conditions are best simulated by operating sampling systems
under the control of system logic at routine operating condition settings. Stop the conveyor for collection of reference
increments by tripping it out with the safety line. Under such test conditions, the sampling system must not also be
stopped by system interlocks, and a bias-test mode of operation must be provided that prevents this, as well as to run
system timers continuously.
Conveyor belt systems for handling fuel are often not designed for repeated starting and stopping under load. The
paired increment experimental design does not necessarily preclude collection of stopped-belt increments, provided
arrangements are made
...
SLOVENSKI STANDARD
SIST ISO 13909-8:2002
01-junij-2002
1DGRPHãþD
SIST ISO 9411-1:1998
SIST ISO 9411-2:1998
ýUQLSUHPRJLQNRNV0HKDQVNRY]RUþHQMHGHO 0HWRGHSUHVNXãDQMD
RGVWRSDQMD
Hard coal and coke -- Mechanical sampling -- Part 8: Methods of testing for bias
Houille et coke -- Échantillonnage mécanique -- Partie 8: Méthodes de détection du biais
Ta slovenski standard je istoveten z: ISO 13909-8:2001
ICS:
73.040 Premogi Coals
SIST ISO 13909-8:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 13909-8:2002
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SIST ISO 13909-8:2002
INTERNATIONAL ISO
STANDARD 13909-8
First edition
2001-12-15
Hard coal and coke — Mechanical
sampling —
Part 8:
Methods of testing for bias
Houille et coke — Échantillonnage mécanique —
Partie 8: Méthodes de détection du biais
Reference number
ISO 13909-8:2001(E)
© ISO 2001
---------------------- Page: 3 ----------------------
SIST ISO 13909-8:2002
ISO 13909-8:2001(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be
edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file,
parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2001
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, elec-
tronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's mem-
ber body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
©
ii ISO 2001 – All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 13909-8:2002
ISO 13909-8:2001(E)
Contents Page
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Outline of procedure . 4
6 Pre-test inspection . 4
7 Reference methods . 5
8 Choice of variables for the test . 6
9 Choice of fuel for the test . 6
10 Conduct of the test . 7
11 Statistical analysis and interpretation . 9
12 Test report . 20
Annex
A Specimen calculations . 22
Bibliography. 31
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SIST ISO 13909-8:2002
ISO 13909-8:2001(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 3.
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 part of ISO 13909 may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 13909-8 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels,
Subcommittee SC 4, Sampling.
ISO 13909 cancels and replaces ISO 9411-1:1994, Solid mineral fuels — Mechanical sampling from moving streams
— Part 1: Coal and ISO 9411-2:1993, Solid mineral fuels — Mechanical sampling from moving streams — Part 2:
Coke, of which it constitutes a technical revision. It also supersedes the methods of mechanical sampling of coal and
coke given in ISO 1988:1975, Hard coal — Sampling and ISO 2309:1980, Coke — Sampling.
ISO 13909 consists of the following parts, under the general title Hard coal and coke — Mechanical sampling:
— Part 1: General introduction
— Part 2: Coal — Sampling from moving streams
— Part 3: Coal — Sampling from stationary lots
— Part 4: Coal — Preparation of test samples
— Part 5: Coke — Sampling from moving streams
— Part 6: Coke — Preparation of test samples
— Part 7: Methods for determining the precision of sampling, sample preparation and testing
— Part 8: Methods of testing for bias
Annex A of this part of ISO 13909 is for information only.
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Introduction
It is not possible to lay down a standard method for field work by which a sampling procedure can be tested for bias
because details of the procedure will inevitably be affected by local conditions. However, certain principles can be
specified which should be adhered to whenever possible and these are discussed in this part of ISO 13909.
Testing for bias can be a tedious and expensive process, especially if testing of the primary increment sampler is
included. All bias tests therefore include a thorough pre-test inspection, with appropriate action taken regarding any
system deficiencies likely to cause bias.
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INTERNATIONAL STANDARD ISO 13909-8:2001(E)
Hard coal and coke — Mechanical sampling —
Part 8:
Methods of testing for bias
1 Scope
This part of ISO 13909 sets out principles and procedures for testing the bias of test samples of hard coals or cokes,
taken in accordance with other parts of ISO 13909. The use of univariate statistical methods only is addressed.
The user is cautioned that the chance of falsely concluding that there is a bias, when no bias exists in any one of
several variables measured on the same set of samples, is substantially greater than for a single variable. While
several variables may be measured, the single variable on which the outcome of the test will be governed shall be
designated in advance.
NOTE In the text the term 'fuel' is used where both coal and coke would be applicable in the context and either 'coal' or 'coke'
where only one is applicable.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this part of ISO 13909. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 13909 are encouraged to investigate the
possibility of applying the most recent editions of the normative documents indicated below. For undated references,
the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO 13909-1:2001, Hard coal and coke — Mechanical sampling — Part 1: General introduction.
ISO 13909-2:2001, Hard coal and coke — Mechanical sampling — Part 2: Coal — Sampling from moving streams.
ISO 13909-4:2001, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples.
ISO 13909-5:2001, Hard coal and coke — Mechanical sampling — Part 5: Coke — Sampling from moving streams.
ISO 13909-6:2001, Hard coal and coke — Mechanical sampling — Part 6: Coke — Preparation of test samples.
ISO 13909-7:2001, Hard coal and coke — Mechanical sampling — Part 7: Methods for determining the precision of
sampling, sample preparation and testing.
3 Terms and definitions
For the purposes of this part of ISO 13909, the terms and definitions given in ISO 13909-1 apply.
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4Principles
4.1 General
Testing for bias is not always done for a single purpose. The objectives of testing for bias may be for assessing
conformity with contractual requirements, such as guarantees or purchase and acceptance specifications, or for
diagnostic purposes or both, and may or may not involve multiple test parameters. This part of ISO 13909 uses
univariate statistics for testing the performance of the system with respect to a single variable.
It is not possible for any scheme of sampling, or sample preparation and analysis, to be free of errors of
measurement. For this reason, no statistical test can establish that there is no bias, but only that there is not likely to
be a bias of more than a certain magnitude.
The testing of a sampling system for bias is based on taking a series of pairs of samples of essentially the same fuel;
one member of each pair being sampled by the system or component under test, the other member being obtained
by a reference method. For each pair, the difference between the analytical results is determined. The series of
differences between the analytical results thus obtained are subjected to statistical analysis.
The procedure requires the sensitivity of the statistical test of significance to be such that the minimum bias that can
be detected is less than or equal to the maximum tolerable bias,BB. Therefore, shall be established before the test
begins.
NOTE In the absence of other information, a value ofB = 0,20 % to 0,30 % for ash or moisture may be appropriate, subject to
commercial considerations.
The sensitivity of the statistical test used is dependent on the number of pairs compared and the variability of the
differences between them.
The statistical analysis to which results will be subjected assumes three conditions:
— a normal distribution of the variable;
— independence of the errors of measurement;
— statistical homogeneity of the data.
The closeness with which these ideal conditions are achieved, in practice, governs the validity of the statistical
analysis. The execution of the test, including sample reduction, division and laboratory analysis, shall be organized
so as to ensure that deviations from these ideals do not invalidate the statistical analysis.
The statistical test used to make the final judgement is thet-test. A hypothesis is made that the observed mean of
the differences between the two methods is drawn from a population whose mean isB. If the test shows that the
observed difference is significantly less thanB, then the sampler or component is declared free of bias.
In basing decisions on the outcomes of statistical tests, there is always the risk of making either one or the other of
two types of error. If the hypothesis is rejected when it is true, e.g. a bias is not declared even though a bias really
does exist, then an error of the first kind (Type I) has been made. On the other hand, if the hypothesis is accepted
when it is false, e.g. a bias is declared even though a bias really does not exist, then an error of the second kind
(Type II) has been made.
In any particular test, the probability of a Type I error can be arbitrarily set as a matter of discretion and the risk kept
as small as desired. For a specific test, the probability of an error of Type II can only be quantified in relation to some
other value than the original hypothesis. In this method the value of zero is used. The risk of a Type II error can be
decreased at a fixed probability of an error of Type I only by increasing the number of observations. However, since
the sample estimate of the population standard deviation must be used in the calculations, the risk of a Type II error
is an estimated value. The final statistical test is not carried out until sufficient pairs of observations have been taken
to limitbothaTypeIerror inrelationtoB, andthe estimatedTypeIIerror in relation to zero,to 5%. Thus,ifthe
observed value of the mean difference (the sample estimate of the population mean) is not significantly less thanB,
it shall also be significantly greater than zero.
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The number of paired samples suitable for a test of the overall system relative to the maximum tolerable bias,B, may
be insufficient for testing a given component. In such circumstances, if the performance of a given component is of
critical importance, a separate test shall be considered. For components other than the primary increment sampler,
such tests can usually be implemented with minimum disruption of normal operations and at a lesser cost than for a
test for overall system bias (see 7.2 and 7.3).
If obtaining the number of pairs required is found to be impracticable, changes will need to be made to reduce the
within-set variance. Investigate what improvements can be made in the closeness of members of pairs and/or what
reduction can be achieved in the preparation and testing errors. If such improvements or reductions are not possible,
give consideration to increasing the number of increments in the samples, taking into account the practical problems
associated with taking increments and the relative costs and errors involved in sampling, sample preparation and
testing.
If the required number of pairs of samples is still excessive, the maximum tolerable bias,B, may be reviewed.
When samples of more than one increment are compared, it is necessary that the reference samples and the
samples from the system under test be constituted on the same basis, i.e. for a time-basis system, the individual
increment masses shall be proportional to the flow rate and, for mass-basis systems, the individual increment mass
shall be uniform (see ISO 13909-2 and ISO 13909-5).
4.2 Selection of sample pairs
4.2.1 Composition of sample pairs
The members of each pair of samples can each comprise one or more increments. Individual increments can be
compared or samples compounded from increments taken by the two methods. The test shall be structured so that
the expected mean of the differences of the result would be zero if no systematic error is present in the system or
component under test.
4.2.2 Paired-increment samples
Paired-increment experimental design is the comparing of individual primary increments after being processed by
the system, with the reference samples collected from the stopped belt.
For a given parameter, the variance of the differences between paired samples will normally be smaller than the
variance of either of the two series of samples, taken by the system or component under test and the reference
method respectively, except for fuels that are very homogeneous. For this reason, if the increments taken by the two
methods are taken in close proximity to each other in the fuel stream (without overlapping), the variance of the
differences between them will be minimized and the sensitivity of the test improved.
4.2.3 Paired-batch samples
It is often not practicable to obtain single increment samples from the system. Increments taken by the system can
be compounded as samples, and compared with samples compounded from increments taken over the same period
using the reference method. It is not necessary that the two samples have the same number of increments or that
they are of similar mass. In the extreme, a single stopped-belt reference increment could be used as the reference
sample.
4.3 Location of sampling points
For a test of the overall system, the reference sample shall be taken from the primary fuel stream using the stopped-
belt reference method (see clause 7). The system sample shall be the final sample.
The primary sampler shall be tested by examining the differences between members of each pair consisting of the
samples taken by the primary sampler and the reference method.
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NOTE 1 For high volume, high capacity, conveyor systems, a test of a primary increment sampler as an individual component will
require the collection and processing of increments of large mass in a short time interval. Before such a test is undertaken,
therefore, it is necessary to consider carefully whether such a test can be justified.
With the exception of the primary increment sampler, when testing individual components and subsystems, the test
compares the differences between the sample feed stream and the discharge stream of that component or
subsystem.
NOTE 2 In some cases, the quality of the feed stream can only be obtained indirectly, for example, by calculation from the results
of the divided sample and the corresponding reject-stream sample, weighted according to the division ratio.
For all crushing equipment, differences between samples taken from the fuel both before and after the crusher are
used.
For subsystems and sample dividers, pairs obtained by one of the following methods shall be tested:
a) by taking samples from the feed stream and from the sample discharge stream;
b) by taking samples from the sample discharge stream and the reject stream;
c) by collecting the entire sample discharge stream and the entire reject stream.
When using either method a) or method b), great care shall be exercised to obtain unbiased samples; in the case of
method a) being used, care shall be taken to minimize disturbance of the feed flow, as such disturbances may
introduce bias or distort normal operating conditions.
5 Outline of procedure
The order of operations is as follows:
a) carry out a pre-test inspection (see clause 6);
b) for the overall system, determine where the stopped-belt reference sample will be collected (see 7.1); for
diagnostic testing of the system components, see 7.2 or 7.3;
c) determine the variable for test (see clause 8);
d) choose the fuel to be used for the test (see clause 9);
B
e) decide on the maximum tolerable bias, (see 4.1);
f) decide on the composition of the sample pairs, i.e. whether to compare sample pairs of one increment or more
than one increment;
g) proceed with collection of samples and carry out the tests according to clauses 10 and 11.
6 Pre-test inspection
The primary sources of information regarding compliance with the sampling standard are the equipment
specifications and drawings.
A thorough examination of the sampling system and a review of its component specification shall be made.
The party performing the test shall, however, verify performance by field measurements and observations. The
operation of the sampling system shall be observed both with fuel flowing and with no fuel.
Pre-test inspections of all operations and equipment, both static and under load, should be carried out by persons
experienced in the sampling of segregated, heterogeneous, lumpy bulk materials. It is recommended that operation
under normal conditions be observed for an entire lot.
Do not execute a test for bias until all conditions known to cause bias are corrected, unless it is necessary to
establish the performance of a system or component as it stands. In the latter case, the pre-test inspection provides
essential documentation of what the conditions were at the time of the test.
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7 Reference methods
7.1 Overall system
To test overall system bias, the use of a reference method which is known to be intrinsically unbiased is required. The
preferred method is the stopped-belt method, i.e. the collection of increments from a complete cross-section of the
fuel on the conveyor belt by stopping the belt at intervals. When properly collected, the stopped-belt increment can
be considered as a reference increment.
NOTE 1 If it is not possible to collect stopped-belt increments, other reference methods may be used but, in these cases, an
apparent absence of a lack of bias relative to the reference method may not be conclusive, and the use of such methods may
compromise the validity and authority of the findings.
NOTE 2 With collection of stopped-belt increments, some disruption of normal operations can occur and therefore the plan of
execution may need to be coordinated with the normal operations and organized to minimize such disruption. It should be
recognized that the conveyor system involved may be used for only a few hours per day for normal operations and cannot be
operated solely for bias tests, unless the fuel can be diverted to another discharge point. This can extend the time necessary for
completion of the field work and require special arrangements for supplying fuel to the system for testing.
Stopped-belt increments shall be taken with a sampling frame (see Figure 1), or equivalent, from a complete cross-
section of the solid mineral fuel on the belt at a fixed position, for a length along the belt which is at least three times
the nominal top size of the fuel.
Figure 1 — Sampling frame
If single paired increment samples are tested, each stopped-belt increment shall be taken immediately after the
system increment has been taken, preferably from a position located before the primary sampler. If the stopped-belt
increment has to be taken from a position after the primary sampler, it is important to ensure that there is no change
in conditions between the position at which the stopped-belt sample is taken and that at which the primary increment
is taken.
The sampling frame (or equivalent) shall be placed on the stationary belt at the predetermined position so that the
separator plates at each end are in contact with the belt across its full width. All particles lying inside the sampling
frame end plates shall be swept into the sampling container.
Particles obstructing the insertion of the end plate on the left-hand side shall be pushed into the increment while
those obstructing the insertion of the end plate on the right-hand side shall be pushed out of the increment or vice-
versa. Whichever practice is used initially, this practice shall be implemented throughout the test.
Take care to minimize the risk of bias being introduced in the course of preparation of the increments and samples
and check all sample division equipment and procedures for bias with respect to relevant test parameters.
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It is recommended that all increments/samples be weighed immediately after collection, as well as before and after
each crushing and dividing operation.
Pay close attention to minimizing unintended mass losses.
Report all observed mass losses.
7.2 Primary increment samplers
Primary increment samplers shall be tested for bias using only the stopped-belt method described in 7.1 as the
reference method.
7.3 Subsystems and components
Subsystems and components will ordinarily be tested under routine continuous-operating conditions by sampling the
feed, discharge and reject streams using a previously validated sampler. At the final stage, the entire stream is
collected (i.e. this stream is not sampled). The quality of the feed to the sample divider shall be calculated from the
results of the divided sample and the corresponding reject-stream sample, weighted according to the division ratio.
If it is not practicable to test under normal operating conditions, a separate test may be arranged in which either of
the following methods of sampling may be used, as appropriate.
a) Sample the feed and product streams simultaneously to obtain paired members. When using this method, great
care should be exercised to obtain unbiased samples.
b) Alternately collect the entire feed stream and the entire product stream associated with successive primary
increments.
8 Choice of variables for the test
Tests for bias can be carried out for ash, moisture or any other variable required, but tests for ash and moisture
generally suffice. Bias in ash on a dry basis is most commonly caused by errors in size distribution. Bias in moisture
may be caused by a wide variety of factors, including but not limited to errors in size distribution, moisture losses
associated with crushers, excessive ventilation within the sampling system, less than the closest possible coupling
between system components, excessive retention time in the system, or any combination of these.
Direct tests on particle-size distribution are often necessary for coke but the following points should be noted:
a) breakage may occur in the process of increment collection or between the sampling points, thus making zero size
bias practically impossible to achieve;
b) bias in other variables, e.g. ash, may have complex and dissimilar relationships to bias in particle-size
distribution, thus making meaningful interpretation of those variables difficult if not impossible.
It is therefore recommended that, for parameters other than size, a direct comparison for that parameter should be
made.
9 Choice of fuel for the test
9.1 Coal
If more than one coal is to be sampled by the system, the coal chosen for the test shall be one that is expected to
show up any bias in the sampling system. For example, bias for ash on primary samplers and sample dividers is
commonly caused by the exclusion of larger sized particles (see clause 8). If a coal is chosen where the ash of such
particles is similar to that of the coal as a whole, then no bias will be detected even though those particles are being
excluded. If subsequently the sampler is used to sample a coal where the large particles have an ash which differs
from the mean, then the results will be biased. In this case, therefore, examine the coals to be sampled to find the
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relationship between ash and the size fraction and the maximum tolerable bias (see clause 4) compared with the
maximum bias possible, e.g. if the top 10 % of the size distribution were excluded.
It is recommended that the complete bias test be carried out on a coal from a single source. When this is not
possible, the data shall be examined statistically to ensure that their combination for the statistical bias tests is valid
(see 11.5 and 11.6).
9.2 Coke
The same requirements as those specified in 9.1 for coal with respect to its ash shall apply when choosing a coke for
test with respect to its moisture content.
10 Conduct of the test
10.1 Establishing detailed test procedures
10.1.1 General
The test may encompass the primary sampler alone, individual components or subsystems, or the entire system.
The design of the test will differ depending on the objectives. Users of these methods are cautioned to clearly define
the objectives of the test as the first step in planning.
A bias test for the whole system is carried out by comparing the reference samples taken from the main flow with the
samples collected at the final stage of the on-line system. The paired batch experimental design (see 4.2.3) is the
preferred practice becaus
...
NORME ISO
INTERNATIONALE 13909-8
Première édition
2001-12-15
Houille et coke — Échantillonnage
mécanique —
Partie 8:
Méthodes de détection du biais
Hard coal and coke — Mechanical sampling —
Part 8: Methods of testing for bias
Numéro de référence
ISO 13909-8:2001(F)
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ISO 13909-8:2001(F)
Sommaire Page
Avant-propos. iv
Introduction . v
1 Domaine d'application. 1
2 Références normatives . 1
3 Termes et définitions. 2
4 Principes. 2
5 Brève description du mode opératoire. 4
6 Inspection avant l'essai. 5
7 Méthodes de référence. 5
8 Choix des variables pour l'essai . 7
9 Choix du combustible pour l'essai . 7
10 Déroulement de l'essai. 8
11 Analyse statistique et interprétation. 10
12 Rapport d'essai . 23
Annexe A (informative) Calculs relatifs aux échantillons . 24
Bibliographie . 34
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ISO 13909-8:2001(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiée
aux comités techniques de l'ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du
comité technique créé à cet effet. Les organisations internationales, gouvernementales et non
gouvernementales, en liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec
la Commission électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI,
Partie 3.
Les projets de Normes internationales adoptés par les comités techniques sont soumis aux comités membres
pour vote. Leur publication comme Normes internationales requiert l'approbation de 75 % au moins des
comités membres votants.
L'attention est appelée sur le fait que certains des éléments de la présente partie de l'ISO 13909 peuvent faire
l'objet de droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour
responsable de ne pas avoir identifié de tels droits de propriété et averti de leur existence.
L'ISO 13909-8 a été élaborée par le comité technique ISO/TC 27, Combustibles minéraux solides,
sous-comité SC 4, Échantillonnage.
L'ISO 13909 annule et remplace l'ISO 9411-1:1994, Combustibles minéraux solides — Échantillonnage
mécanique sur minéraux en mouvement — Partie 1: Charbon et l'ISO 9411-2:1993, Combustibles minéraux
solides — Échantillonnage mécanique sur minéraux en mouvement — Partie 2: Coke, dont elle constitue une
révision technique. Elle remplace également les méthodes d'échantillonnage mécanique du charbon et du
coke données dans l'ISO 1988:1975, Charbons et lignites durs — Échantillonnage et l'ISO 2309:1980,
Coke — Échantillonnage.
L'ISO 13909 comprend les parties suivantes, présentées sous le titre général Houille et coke —
Échantillonnage mécanique:
Partie 1: Introduction générale
Partie 2: Charbon — Échantillonnage en continu
Partie 3: Charbon — Échantillonnage sur lots statiques
Partie 4: Charbon — Préparation des échantillons pour essai
Partie 5: Coke — Échantillonnage en continu
Partie 6: Coke — Préparation des échantillons pour essai
Partie 7: Méthodes pour la détermination de la fidélité de l'échantillonnage, de la préparation de
l'échantillon et de l'essai
Partie 8: Méthodes de détection du biais
L'Annexe A de la présente partie de l'ISO 13909 est donnée uniquement à titre d'information.
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Introduction
Il n'est pas possible d'établir une méthode normalisée pour le travail sur le terrain dans le cadre de laquelle il
serait possible de détecter le biais d'un mode opératoire d'échantillonnage. En effet, les détails du mode
opératoire seront inévitablement touchés par les conditions locales. Cependant, il est possible de spécifier
certains principes qu'il convient d'observer autant que possible. Ils sont examinés dans le cadre de la
présente partie de l'ISO 13909.
La détection du biais est un processus qui peut s'avérer fastidieux et coûteux, en particulier s'il comprend
l'essai de l'échantillonneur de prélèvements élémentaires primaires. C'est pourquoi tous les essais de
détection du biais incluent une inspection approfondie avant l'essai, des mesures appropriées étant prises
pour toute lacune du système risquant de provoquer un biais.
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Houille et coke — Échantillonnage mécanique —
Partie 7:
Méthodes de détection du biais
1 Domaine d'application
Dans la présente partie de l'ISO 13909 sont définis des principes et des modes opératoires pour la détection
du biais sur des échantillons pour essai de houilles ou de cokes, prélevés conformément aux autres parties
de l'ISO 13909. Seule est abordée l'utilisation de méthodes statistiques unidimensionnelles.
L'utilisateur est averti que le risque de conclure de manière erronée à l'existence d'un biais, alors qu'aucun
biais n'existe pour l'une quelconque des diverses variables mesurées sur le même ensemble d'échantillons,
est sensiblement plus important que pour une seule variable. Bien que plusieurs variables puissent être
mesurées, la variable unique sur laquelle porteront les résultats de l'essai doit être désignée à l'avance.
NOTE Dans le texte, le terme «combustible» est employé lorsque le charbon et le coke sont tous deux concernés
dans le contexte, et «charbon» ou «coke» sera utilisé si seul ce terme convient.
2 Références normatives
Les documents normatifs suivants contiennent des dispositions qui, par suite de la référence qui y est faite,
constituent des dispositions valables pour la présente partie de l'ISO 13909. Pour les références datées, les
amendements ultérieurs ou les révisions de ces publications ne s'appliquent pas. Toutefois, les parties
prenantes aux accords fondés sur la présente partie de l'ISO 13909 sont invitées à rechercher la possibilité
d'appliquer les éditions les plus récentes des documents normatifs indiqués ci-après. Pour les références non
datées, la dernière édition du document normatif en référence s'applique. Les membres de l'ISO et de la CEI
possèdent le registre des Normes internationales en vigueur.
ISO 13909-1:2001, Houille et coke — Échantillonnage mécanique — Partie 1: Introduction générale
ISO 13909-2:2001, Houille et coke — Échantillonnage mécanique — Partie 2: Charbon — Échantillonnage en
continu
ISO 13909-4:2001, Houille et coke — Échantillonnage mécanique — Partie 4: Charbon — Préparation des
échantillons pour essai
ISO 13909-5:2001, Houille et coke — Échantillonnage mécanique — Partie 5: Coke — Échantillonnage en
continu
ISO 13909-6:2001, Houille et coke — Échantillonnage mécanique — Partie 6: Coke — Préparation des
échantillons pour essai
ISO 13909-7:2001, Houille et coke — Échantillonnage mécanique — Partie 7: Méthodes pour la détermination
de la fidélité de l'échantillonnage, de la préparation de l'échantillon et de l'essai
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ISO 13909-8:2001(F)
3 Termes et définitions
Pour les besoins de la présente partie de l'ISO 13909, les termes et définitions donnés dans l'ISO 13909-1
s'appliquent.
4 Principes
4.1 Généralités
La détection du biais n'a pas toujours qu'un seul objectif. La détection du biais peut servir à évaluer la
conformité par rapport à des exigences contractuelles, telles que des garanties ou des spécifications d'achat
et d'acceptation; elle peut également avoir un objectif de diagnostic ou être utilisée pour servir ces deux
objectifs à la fois. Elle peut ou non impliquer plusieurs paramètres d'essai. La présente partie de l'ISO 13909
utilise des statistiques unidimensionnelles pour tester les performances du système concernant une seule
variable.
Il n'est pas possible qu'un programme d'échantillonnage, la préparation des échantillons et l'analyse ne
comportent aucune erreur de mesure. Par conséquent, un essai statistique quel qu'il soit ne peut déterminer
l'absence totale de biais, mais peut uniquement préciser qu'un biais supérieur à un certain ordre de grandeur
est improbable.
La détection du biais sur un système d'échantillonnage repose sur la collecte d'une série de paires
d'échantillons composés essentiellement du même combustible; une unité de chaque paire est échantillonnée
par le système ou l'élément soumis à l'essai, tandis que l'autre unité est obtenue par une méthode de
référence. Pour chaque paire, la différence entre les résultats analytiques est déterminée. La série de
différences entre les résultats analytiques ainsi obtenue fait l'objet d'une analyse statistique.
Le mode opératoire exige que la sensibilité du test statistique d'hypothèse soit telle que le biais minimum
pouvant être détecté soit inférieur ou égal au biais maximum tolérable, B. Dès lors, B doit être déterminé
avant le début du test.
NOTE En l'absence d'autres informations, une valeur de B = 0,20 % à 0,30 % pour les cendres ou l'humidité peut
être appropriée, sous réserve de considérations d'ordre commercial.
La sensibilité du test statistique utilisé dépend du nombre de paires comparées et de la variabilité des
différences entre elles.
L'analyse statistique dont feront l'objet les résultats présuppose trois conditions:
une distribution normale de la variable;
l'indépendance des erreurs de mesure;
une homogénéité statistique des données.
Le degré de proximité avec lequel ces conditions idéales sont atteintes détermine en pratique la validité de
l'analyse statistique. L'exécution de l'essai, y compris la réduction de l'échantillon, la division et l'analyse en
laboratoire, doit être organisée de façon à garantir que les écarts par rapport à ces conditions idéales
n'invalident pas l'analyse statistique.
Le test statistique utilisé pour le jugement final est le test t. Selon une hypothèse, la moyenne observée des
différences entre les deux méthodes provient d'une population dont la moyenne est B. Si le test indique que la
différence observée est sensiblement inférieure à B, alors l'échantillonneur ou l'élément est considéré comme
n'ayant aucun biais.
En basant les décisions sur les résultats des tests statistiques, il y a toujours un risque de provoquer l'un ou
l'autre des deux types d'erreurs suivants. Si l'hypothèse est rejetée quand elle est vraie, par exemple un biais
n'est pas déclaré même s'il y en a réellement un, une erreur du premier type (Type I) a alors été commise.
D'autre part, si l'hypothèse est acceptée lorsqu'elle est fausse, par exemple, un biais est déclaré même s'il n'y
en a pas, c'est alors une erreur du deuxième type (Type II) qui a été commise.
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Dans n'importe quel essai particulier, la probabilité d'une erreur de Type I peut être définie arbitrairement
selon appréciation et le risque maintenu aussi faible que cela est souhaité. Pour un essai spécifique, la
probabilité d'une erreur de Type II ne peut être quantifiée que par rapport à certaines valeurs autres que
l'hypothèse d'origine. Dans cette méthode, la valeur zéro est utilisée. Le risque d'une erreur de Type II ne
peut être réduit à une probabilité déterminée d'une erreur de Type I qu'en augmentant le nombre
d'observations. Cependant, étant donné que l'estimation de l'écart-type de la population doit être reprise dans
les calculs, le risque d'une erreur de Type II est une valeur estimative. Le test statistique final n'est pas
effectué tant qu'un nombre suffisant de paires d'observations n'a pas été récolté pour limiter à 5 % à la fois
l'erreur de Type I par rapport à B et l'erreur estimée de Type II par rapport à zéro. Si la valeur observée de la
différence moyenne (l'estimation de la moyenne de la population de l'échantillon) n'est pas sensiblement
inférieure à B, elle sera donc également sensiblement supérieure à zéro.
Le nombre d'échantillons appariés convenant pour un essai de l'ensemble du système par rapport au biais
maximum tolérable, B, peut ne pas suffire pour l'essai d'un élément spécifique. Dans de telles circonstances,
si la performance d'un élément donné est particulièrement importante, un essai distinct doit être envisagé.
Pour tout ce qui ne concerne pas l'échantillonneur de prélèvements élémentaires primaires, de tels essais
peuvent généralement être mis en œuvre avec une interruption minimale des opérations normales et à un
coût inférieur à celui de l'essai pour la détection du biais sur l'ensemble du système (voir 7.2 et 7.3).
S'il s'avère irréalisable de recueillir le nombre requis de paires, des modifications devront être apportées pour
réduire la variance dans l'ensemble. Chercher ce qu'il est possible d'améliorer en termes de proximité des
unités des paires et/ou de réduire dans le cadre des erreurs de préparation et d'essai. Si de telles
améliorations ou diminutions ne sont pas possibles, envisager d'augmenter le nombre de prélèvements
élémentaires dans les échantillons, en tenant compte des problèmes pratiques associés à la collecte des
prélèvements et des coûts et erreurs relatifs associés à l'échantillonnage, à la préparation des échantillons et
aux essais.
Si le nombre requis de paires d'échantillons est encore excessif, le biais maximum tolérable, B, peut être revu.
Lorsque des échantillons de plus d'un prélèvement élémentaire sont comparés, il est nécessaire que les
échantillons de référence et les échantillons du système soumis à l'essai soient constitués sur la même base.
Ainsi, pour un système basé sur le temps, les masses des prélèvements élémentaires individuels doivent être
proportionnelles au débit et, pour les systèmes basés sur la masse, la masse des prélèvements élémentaires
individuels doit être uniforme (voir l'ISO 13909-2 et l'ISO 13909-5).
4.2 Sélections des paires d'échantillons
4.2.1 Composition de paires d'échantillons
Les unités de chaque paire d'échantillons peuvent chacune comporter un ou plusieurs prélèvements
élémentaires. Les prélèvements individuels ou les échantillons composés de prélèvements provenant des
deux méthodes peuvent être comparés. L'essai doit être structuré de sorte que la moyenne attendue des
différences du résultat soit égale à zéro si aucune erreur systématique n'est présente dans le système ou
l'élément soumis à l'essai.
4.2.2 Échantillons à prélèvements élémentaires appariés
Le schéma expérimental pour les prélèvements élémentaires appariés consiste à comparer des prélèvements
élémentaires primaires individuels une fois traités par le système, les échantillons de référence étant recueillis
sur la bande transporteuse à l'arrêt.
Pour un paramètre donné, la variance des différences entre les échantillons appariés sera normalement
inférieure à la variance d'une des deux séries d'échantillons collectées respectivement par le système ou
l'élément soumis à l'essai et par la méthode de référence, à l'exception des combustibles très homogènes.
C'est pourquoi, si les prélèvements provenant des deux méthodes sont collectés à proximité immédiate les
uns des autres dans l'écoulement de combustible (sans chevauchement), la variance des différences entre
ceux-ci sera réduite au minimum et la sensibilité de l'essai améliorée.
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4.2.3 Échantillons de lots appariés
Il est souvent irréalisable de récolter des échantillons avec un simple prélèvement à partir du système. Les
prélèvements élémentaires collectés par le système peuvent être composés pour former des échantillons et
comparés aux échantillons composés faits de prélèvements élémentaires recueillis pendant la même période
de temps à l‘aide de la méthode de référence. Il n'est pas nécessaire que les deux échantillons aient le même
nombre de prélèvements ou qu'ils aient une masse similaire. Au dernier degré, un simple prélèvement de
référence sur bande transporteuse à l'arrêt peut être utilisé comme échantillon de référence.
4.3 Emplacement des points d'échantillonnage
Pour un essai sur l'ensemble du système, l'échantillon de référence doit être pris sur l'écoulement de
combustible primaire à l'aide de la méthode de référence sur bande transporteuse à l'arrêt (voir l'Article 7).
L'échantillon du système doit être l'échantillon final.
L'échantillonneur primaire doit être mis à l'essai en étudiant les différences entre les unités de chaque paire
constituée des échantillons prélevés par l'échantillonneur primaire et par la méthode de référence.
NOTE 1 Pour les systèmes transporteurs ayant un volume et une capacité élevés, l'essai d'un échantillonneur de
prélèvements primaires en tant qu'élément individuel nécessitera la collecte et le traitement de prélèvements d'une masse
importante pendant un court intervalle de temps. Avant qu'un essai de ce type ne soit entrepris, il est donc nécessaire
d'étudier soigneusement si cet essai est justifié.
À l'exception de l'échantillonneur de prélèvements élémentaires primaires, lorsque l'essai est effectué sur des
éléments individuels et des sous-systèmes, il compare les différences entre le courant d'alimentation de
l'échantillon et le courant de décharge de cet élément ou du sous-système.
NOTE 2 Dans certains cas, la qualité du courant d'alimentation ne peut être obtenue qu'indirectement, en calculant par
exemple les résultats de l'échantillon divisé et de l'échantillon de rejet correspondant, pesé conformément au rapport de
division.
Pour tout le matériel de broyage, les différences entre les échantillons pris dans le combustible à la fois avant
et après le broyage sont utilisées.
Pour les sous-systèmes et les diviseurs d'échantillons, les paires obtenues par une des méthodes suivantes
doivent être soumises à l'essai:
a) en prélevant des échantillons à partir du courant d'alimentation et du courant de décharge des
échantillons;
b) en prélevant des échantillons à partir du courant de décharge et du courant de rejet;
c) en recueillant tout le courant de décharge et tout le courant de rejet.
Lorsque les méthodes a) ou b) sont utilisées, d'importantes précautions doivent être prises pour obtenir des
échantillons non biaisés; dans le cas de la méthode a), il faudra veiller à minimiser la perturbation du débit
d'alimentation, étant donné que cela pourrait entraîner un biais ou fausser les conditions normales de
fonctionnement.
5 Brève description du mode opératoire
L'ordre des opérations est le suivant:
a) effectuer une inspection avant l'essai (voir l'Article 6);
b) déterminer, pour l'ensemble du système, où l'échantillon de référence sur bande transporteuse à l'arrêt
sera collecté (voir 7.1); en ce qui concerne l'essai pour diagnostic des éléments du système, voir 7.2 ou
7.3;
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c) déterminer la variable pour l'essai (voir l'Article 8);
d) sélectionner le combustible à utiliser pour l'essai (voir l'Article 9);
e) choisir le biais maximum tolérable, B (voir 4.1);
f) décider de la composition des paires d'échantillons, c'est-à-dire comparer des paires d'échantillons d'un
seul prélèvement ou de plusieurs prélèvements;
g) procéder à la collecte d'échantillons et effectuer les essais conformément aux Articles 10 et 11.
6 Inspection avant l'essai
Les sources d'informations de premier ordre concernant le respect de la norme d'échantillonnage sont les
spécifications du matériel et les dessins.
Un examen approfondi du système d'échantillonnage et une analyse de la spécification de son élément
doivent être effectués.
La partie réalisant l'essai doit toutefois en vérifier la performance au moyen d'observations et de mesures sur
place. Le fonctionnement du système d'échantillonnage doit être suivi à la fois avec et sans débit de
combustible.
Il convient que les contrôles avant essai de toutes les opérations et du matériel, à la fois statique et chargé,
soient effectués par des personnes ayant de l'expérience dans l'échantillonnage de matériaux en vrac,
granuleux, hétérogènes et ségrégés. Il est recommandé que le fonctionnement dans des conditions normales
soit observé pour un lot complet.
Ne pas effectuer d'essai de détection du biais tant que toutes les conditions pouvant provoquer un biais n'ont
pas été corrigées, à moins qu'il ne soit nécessaire de définir la performance d'un système ou d'un élément
dans son état actuel. Dans ce dernier cas, l'inspection avant essai fournit des informations essentielles quant
aux conditions au moment de l'essai.
7 Méthodes de référence
7.1 Sur l'ensemble du système
Pour détecter le biais sur l'ensemble du système, le recours à une méthode de référence connue pour être
intrinsèquement non biaisée est requis. La méthode privilégiée est celle de la bande transporteuse à l'arrêt,
c'est-à-dire la collecte de prélèvements élémentaires depuis l'intégralité d'une coupe transversale du
combustible sur la bande transporteuse, en arrêtant cette dernière par intervalles. Lorsqu'il a été correctement
collecté, le prélèvement sur bande transporteuse à l'arrêt peut être considéré comme un prélèvement de
référence.
NOTE 1 S'il n'est pas possible de collecter des prélèvements élémentaires sur bande transporteuse à l'arrêt, d'autres
méthodes de référence peuvent être utilisées. Toutefois, dans ce cas, l'absence de biais par rapport à la méthode de
référence peut ne pas être concluante. L'utilisation de méthodes de ce genre peut alors compromettre la validité et
l'autorité des résultats.
NOTE 2 Lors de la collecte de prélèvements élémentaires sur bande transporteuse à l'arrêt, une perturbation du
fonctionnement normal peut se produire. Il est dès lors possible que le plan d'exécution ait besoin d'être coordonné par
rapport aux opérations habituelles et organisé de manière à minimiser toute perturbation de ce type. Il convient d'admettre
que le transporteur en question ne peut être utilisé que quelques heures par jour pour un fonctionnement normal. Il ne
peut pas fonctionner uniquement pour des essais de détection du biais, à moins que le combustible puisse être dévié vers
un autre point de décharge. Cela prolongera la durée nécessaire pour l'achèvement du travail sur le terrain et nécessitera
des arrangements particuliers pour alimenter le système en combustible pour l'essai.
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Des prélèvements pris sur bande à l'arrêt doivent être pris à l'aide d'un cadre de prélèvement (voir Figure 1)
ou son équivalent à partir d'une section complète du combustible solide sur le convoyeur à une position fixe,
qui sera d'une longueur au moins égale à trois fois la dimension du calibre nominal maximum du combustible.
Figure 1 — Châssis d'échantillonnage
Si des échantillons uniques de prélèvements élémentaires appariés sont soumis à l'essai, chaque
prélèvement sur bande transporteuse à l'arrêt doit être recueilli immédiatement après que le prélèvement du
système a été collecté, de préférence à partir d'un emplacement situé avant l'échantillonneur primaire. Si le
prélèvement sur bande transporteuse à l'arrêt doit être collecté après l'échantillonneur primaire, il est
important de veiller à ce qu'aucune modification ne soit apportée aux conditions entre la position à laquelle
l'échantillon sur bande transporteuse à l'arrêt est prélevé et celle où le prélèvement élémentaire primaire est
collecté.
Le châssis d'échantillonnage (ou équivalent) doit être placé sur la bande transporteuse immobile à un
emplacement prédéterminé de façon à ce que les plaques du séparateur à chaque extrémité soient en
contact avec la bande transporteuse sur toute sa largeur. Toutes les particules se situant entre les extrémités
du châssis d'échantillonnage doivent être évacuées vers le récipient d'échantillonnage.
Les particules obstruant l'insertion de la plaque d'extrémité du côté gauche doivent être poussées dans le
prélèvement, tandis que celles empêchant l'insertion de la plaque d'extrémité du côté droit doivent être
poussées en dehors du prélèvement ou vice versa. Quelle que soit la pratique utilisée au départ, elle doit être
mise en œuvre tout au long de l'essai.
Veiller à minimiser le risque de biais survenant au cours de la préparation des prélèvements élémentaires et
des échantillons et vérifier le matériel de division des échantillons ainsi que les modes opératoires pour la
détection du biais par rapport aux paramètres d'essai adéquats.
Il est recommandé que tous les prélèvements élémentaires/échantillons soient pesés dès leur collecte, ainsi
qu'avant et après chaque opération de concassage ou de division.
Faire particulièrement attention à minimiser les pertes de masses involontaires.
Enregistrer toutes les pertes de masses observées.
7.2 Échantillonneurs de prélèvements élémentaires primaires
Les échantillonneurs de prélèvements élémentaires primaires ne doivent faire l'objet d'essais de détection du
biais qu'à l'aide de la méthode sur bande transporteuse à l'arrêt décrite en 7.1 comme étant la méthode de
référence.
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7.3 Sous-systèmes et éléments
Les sous-systèmes et éléments seront généralement testés dans le cadre de conditions de fonctionnement
régulières et continues en échantillonnant les courants d'alimentation, de décharge et de rejet à l'aide d'un
échantillonneur préalablement validé. À la dernière étape, l'écoulement est collecté dans son intégralité (
...
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