ISO 13909-8:2016

DRAFT INTERNATIONAL STANDARD
ISO/DIS 13909-8
ISO/TC 27/SC 4 Secretariat: SABS
Voting begins on: Voting terminates on:
2014-07-08 2014-10-08
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
ICS: 75.160.10;73.040
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ISO/DIS 13909-8:2014(E)
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ISO/DIS 13909-8:2014(E)
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ISO/DIS 13909-8
Contents Page
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Principles.1
4.1 General .1
5 Pre-test inspection .2
6 Reference methods .2
7 Test design.3
7.1 Choice of test parameters .3
7.2 Number of paired samples .3
7.3 Selection of sample pairs .3
7.4 Choice of fuel for test.4
7.5 Coke.4
8 Conduct of the test.4
8.1 General .4
8.2 Collection and preparation of test samples.5
9 Outline of test procedure.5
9.1 Special precautions for moisture-test samples .5
9.2 Documentation .6
10 Statistical analysis and interpretation.6
10.1 Outline of statistical procedure .6
10.2 Calculations .6
11 Test report.11
Annex A(informative)Example calculations .13
Bibliography.23
ISO/DIS 13909-8
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. All bias tests therefore include a thorough pre-test
inspection, with appropriate action taken regarding any system deficiencies likely to cause bias.
DRAFT INTERNATIONAL STANDARD ISO/DIS 13909-8
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.
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 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 13909-1, Hard coal and coke — Mechanical sampling — Part 1: General introduction
ISO 13909-2, Hard coal and coke — Mechanical sampling — Part 2: Coal — Sampling from moving streams
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 13909-5, Hard coal and coke — Mechanical sampling — Part 5: Coke — Sampling from moving streams
ISO 13909-6, Hard coal and coke — Mechanical sampling — Part 6: Coke — Preparation of test samples
ISO 13909-7, Hard coal and coke — Mechanical sampling — Part 7: Methods for determining the precision of
sampling, sample preparation and testing
ISO 21398:2007, Hard coal and coke — Guide to the inspection of mechanical sampling systems.
3 Terms and definitions
For the purposes of this part of ISO 13909, the terms and definitions given in ISO 13909-1 apply.
4 Principles
4.1 General
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 sensitivity of the statistical test is dependent on the number of pairs compared, the variability of the
differences between members of the pairs and the number of parameters used for the test.
The statistical analysis to which results will be subjected assumes three conditions:
ISO/DIS 13909-8
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 sample 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.
It is recommended that all increments/samples be weighed immediately after collection. Close attention shall
be given to minimizing unintended mass losses.
All observed mass losses shall be reported.
7 Test design
7.1 Choice of test parameters
The standard test for general purposes shall include moisture and ash (ash on a dry basis). Use of these two
parameters will generally suffice. Other variables can be included if desired. However, given a fixed number
of sample pairs, the test often becomes less sensitive to detection of a bias as additional variables are
included.
Bias in ash content on a dry basis is most commonly caused by errors in size distribution. Bias in moisture
content 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. It is recommended that tests for size
distribution be conducted as a separate test and that size distribution parameters not be included in the
standard test for analytical parameters.
7.2 Number of paired samples
The minimum number of paired samples for the test shall be 30. However, if use of a smaller number of
paired samples results in a detection of bias, no additional samples need be collected. After completing the
statistical calculations the resulting confidence region can be evaluated. If no bias is detected yet the
confidence region covers levels of bias that are of commercial concern, more paired samples can be collected
of fuel originating from this same source in order to reduce the size of the confidence region.
7.3 Selection of sample pairs
7.3.1 Composition of sample pairs
The members of each pair of samples can each be comprised of portions of one or more increments.
Individual increments can be paired or samples of compounded increments can be paired. 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 under test.
7.3.2 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.
ISO/DIS 13909-8
NOTE 1 A final system sample consisting only of fuel collected from a single primary increment may be of insufficient
mass to provide an analytical result following the requirements specified in ISO 13909-4 and ISO 13909-6 for preparation
and analysis by the methods used routinely during regular operations.
NOTE 2 In causing the mechanical system to separately collect and process individual primary increments for a test for
bias, the system sample may lose more moisture than while the system is operating as designed.
7.3.3 Paired-batch samples
It is often not practical to obtain single increment samples from the system. Increments taken by the system
can be compounded as samples and paired with samples compounded from increments taken over the same
period using the reference method. It is not necessary that the two sample members, reference and system,
have the same number of increments or be of similar mass. Single stopped-belt reference increments are
often used as the reference sample and paired with compounded system sample increments. In collecting
paired-batch samples, the timing at which stopped-belt increments are collected from each moving batch shall
be determined using a stratified random sampling scheme.
7.4 Choice of coal for test
If more than one coal is to be sampled by the system, the coal chosen for the bias 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. If a coal is chosen where the
ash content of such particles is similar to that of the coal as a whole, yet the particles are too large for
collection, 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 content which differs from the mean,
the sampling system could be biased by not accurately representing those particles.
It is recommended that the complete bias test be carried out on a coal from a single source.
7.5 Coke
The same requirements as those specified in 7.4 for coal with respect to its ash content shall apply when
choosing a coke for the bias test with respect to its moisture content.
8 Conduct of the test
8.1 General
The whole system shall be tested by comparing stopped-belt refer
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