ISO 18283:2006

Hard coal and coke - Manual sampling

ISO 18283:2006 defines the basic terms used in manual sampling of hard coal and coke and describes the general principles of sampling. ISO 18283:2006 specifies procedures and requirements for establishing a manual sampling scheme, methods of manual sampling, sampling equipment, handling and storage of samples, sample preparation and a sampling report. ISO 18283:2006 applies to manual sampling from fuels in movement. Guidelines for manual sampling from fuels in stationary situations are given, but this method of sampling does not provide a representative test sample and the sampling report shall state this. ISO 18283:2006 does not include sampling of brown coals and lignites, sampling from coal seams or mechanical sampling of coal and coke.

Houille et coke — Échantillonnage manuel

General Information

• Status: Withdrawn
• Publication Date: 05-Sep-2006
• ICS: 73.040 - Coals
• ICS: 75.160.10 - Solid fuels
• Technical Committee: ISO/TC 27/SC 4 - Sampling
• Drafting Committee: ISO/TC 27/SC 4 - Sampling
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 21-Mar-2022
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 18283:2022 - Coal and coke - Manual sampling
• Effective Date: 23-Apr-2020
• Revises: ISO 1988:1975 - Hard coal - Sampling
• Effective Date: 15-Apr-2008
• Revises: ISO 2309:1980 - Coke - Sampling
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18283
First edition
2006-09-15
Hard coal and coke — Manual sampling
Houille et coke — Échantillonnage manuel

Reference number
©
ISO 2006
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ii © ISO 2006 – All rights reserved

Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 Establishing a sampling scheme . 5
4.1 General. 5
4.2 Sampling methods. 6
4.3 Design of the sampling scheme. 6
4.4 Precision of sampling. 9
4.5 Checking the overall precision for the lot by calculation and selection of sampling
scheme. 23
4.6 Determination of acquired precision by replicate sampling . 23
4.7 Size analysis. 24
5 Methods of sampling. 25
5.1 General. 25
5.2 Sampling by time interval . 25
5.3 Sampling by mass interval . 25
5.4 Stratified random sampling . 26
5.5 Extracting the increment. 26
5.6 Fuel in motion . 27
5.7 Moisture/common sample . 29
5.8 Different fuels. 30
5.9 Random selection of increments . 30
6 Sampling equipment. 31
6.1 General. 31
6.2 Examples . 32
7 Handling and storage of samples . 39
7.1 Sample size . 39
7.2 Time. 39
7.3 Divided sample. 39
7.4 Containers . 39
7.5 Moisture loss/breakage or degradation. 40
7.6 Identification/labelling. 41
8 Sample preparation . 41
8.1 General. 41
8.2 Constitution of a sample. 41
8.3 Division . 42
8.4 Reduction . 52
8.5 Mixing. 53
8.6 Air-drying. 53
8.7 Coal — Preparation of test samples . 54
8.8 Coke — Preparation of test samples . 61
9 Packing and marking of samples and sampling report. 64
Annex A (informative) Example of calculation of precision, mass of increments, number of sub-
lots and number of increments per sub-lot. 66
Annex B (informative) Methods of sampling large fuels and fuels from stationary lots. 69
Bibliography . 71

iv © ISO 2006 – All rights reserved

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 18283 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Subcommittee SC 4,
Sampling.
This first edition of ISO 18283 cancels and replaces ISO 1988:1975 and ISO 2309:1980, which have been
technically revised.
Introduction
Mechanical sampling from moving streams is the preferred method for sampling fuels. However, often
mechanical facilities are not available. Moreover, for sized coal or coke, mechanical sampling can be a
problem because of (size) degradation by the sampling system.
The fundamental requirements of sampling are that all particles of the fuel in the lot are accessible to the
sampling instrument and that each individual particle has an equal probability of being selected and included
in the sample.
When sampling manually, conditions are often far from ideal. The methods described in this International
Standard are intended to obtain the most representative sample that can be achieved. Manual sampling
should only be applied if no possibility for mechanical sampling exists.
The purpose of taking and preparing a sample of fuel is to provide a test sample that, when analysed,
provides test results representative of the lot sampled.
The first stage of sampling, known as primary sampling, is the taking from positions distributed over the entire
lot of an adequate number of fuel portions known as primary increments. The primary increments are then
combined into a sample, either “as taken” or after having been divided, in order to reduce the mass of the
sample to a manageable size. From this sample, the required number and types of test samples are prepared
by a series of processes jointly known as sample preparation.
In devising a sampling procedure, it is also essential to guard against bias in the taking of increments. Bias
can arise from
a) incorrect location/timing of increments,
b) incorrect delimitation and extraction of increments,
c) loss of integrity of increments after extraction.
Methods for measuring bias are described in this International Standard.

vi © ISO 2006 – All rights reserved

INTERNATIONAL STANDARD ISO 18283:2006(E)

Hard coal and coke — Manual sampling
CAUTION — Sampling can involve hazardous materials, operations, equipment and situations.
However, it is beyond the scope of this International Standard to address all of the safety problems
associated with the use of this International Standard. It is, therefore, the responsibility of the parties
concerned to establish appropriate safety and health practices and to determine the applicability of
regulatory limitations prior to use.
1 Scope
ISO 18283 defines the basic terms used in manual sampling of hard coal and coke and describes the general
principles of sampling. It specifies procedures and requirements for establishing a manual sampling scheme,
methods of manual sampling, sampling equipment, handling and storage of samples, sample preparation and
a sampling report.
This International Standard applies to manual sampling from fuels in movement. Guidelines for manual
sampling from fuels in stationary situations are given in Annex B, but this method of sampling does not
provide a representative test sample and the sampling report shall state this.
ISO 18283 does not include sampling of brown coals and lignites, which is described in ISO 5069-1 and
ISO 5069-2, nor sampling from coal seams, for which guidance is given in ISO 14180. Mechanical sampling of
coal and coke is covered in ISO 13909 (all parts).
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 579, Coke — Determination of total moisture
ISO 589:2003, Hard coal — Determination of total moisture
ISO 687, Solid mineral fuels — Coke — Determination of moisture in the general analysis test sample
ISO 3310-1, Test sieves — Technical requirements and testing — Part 1: Test sieves of metal wire cloth
ISO 13909-7, Hard coal and coke — Mechanical sampling — Part 7: Methods for determining the precision of
sampling, sample preparation and testing
ISO 13909-8, Hard coal and coke — Mechanical sampling — Part 8: Methods of testing for bias
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
air-drying
process of bringing the moisture content of the sample near to equilibrium with the atmosphere in the area in
which further reduction and division of the sample are to take place
NOTE Air-drying to equilibrium with the atmosphere applies to coal. Drying of coke is generally to facilitate sample
preparation.
3.2
bias
systematic error that leads to the average value of a series of results being persistently higher or persistently
lower than those that are obtained using a reference sampling method
3.3
common sample
sample collected for more than one intended use
3.4
continuous sampling
taking of a sample from each consecutive sub-lot so that increments are taken at uniform intervals whenever
the fuel is handled at the point of sampling
3.5
divided increment
part obtained from the division of the increment in order to decrease its mass
NOTE Such division can be done with or without prior size reduction.
3.6
fixed-mass division
method of sample division in which the mass retained is predetermined and independent of the mass of the
feed
3.7
fixed-ratio division
method of sample division in which the division ratio is predetermined, i.e. the mass of sample retained is a
fixed proportion of the mass of the feed
3.8
fuel
hard coal or coke
3.9
general analysis test sample
sample prepared to pass a sieve of nominal size of openings 212 µm in accordance with ISO 3310-1, used for
the determination of most chemical and some physical characteristics
3.10
increment
portion of fuel extracted in a single operation of the sampling device
3.11
intermittent sampling
taking of samples from only certain sub-lots of fuel
2 © ISO 2006 – All rights reserved

3.12
lot
defined quantity of fuel for which the quality is to be determined
NOTE A lot can be divided into sub-lots.
3.13
manual sampling
extraction of increments by human effort
3.14
mass-basis sampling
taking of increments whereby the position of each increment to be extracted from the stream of fuel is
measured by a mass interval of stream flow and the increment mass is fixed
3.15
mechanical sampling
extraction of increments by mechanical means
3.16
moisture sample
sample taken specifically for the purpose of determining total moisture
NOTE For coke, this sample can also be used for general analysis.
3.17
nominal top size
aperture size of the smallest sieve in the range included in the R 20 Series (see ISO 565, square hole) on
which not more than 5 % of the sample is retained
3.18
physical sample
sample taken specifically for the determination of physical characteristics, e.g. physical strength indices or
size distribution
3.19
precision
closeness of agreement between independent test results obtained under stipulated conditions
NOTE 1 This is often defined using an index of precision, such as 2 standard deviations.
NOTE 2 A determination might be made with great precision and the standard deviation of a number of determinations
on the same sub-lot might, therefore, be low; but such results are accurate only if they are free from bias.
3.20
primary increment
increment extracted at the first stage of sampling, prior to any sample division and/or sample reduction
3.21
random sampling
extracting of increments at random mass or time intervals
3.22
replicate sampling
extracting, at intervals, of increments that are combined in rotation into different containers to give two or more
samples of approximately equal mass
3.23
sample
quantity of fuel, representative of a larger mass for which the quality is to be determined
3.24
sample division
process in sample preparation whereby the sample is divided into representative, separate portions
3.25
sample preparation
process of bringing samples to the condition required for analysis or testing
NOTE Sample preparation covers mixing, particle size reduction, sample division and sometimes air-drying of the
sample and may be performed in several stages
3.26
sample reduction
process in sample preparation whereby the particle size of the sample is reduced by crushing or grinding
3.27
size analysis sample
sample taken specifically for particle size analysis
3.28
standard deviation
square root of the variance
3.29
stratified random sampling
extracting of an increment at random within the mass interval or time interval determined for mass-basis
sampling or time-basis sampling respectively
3.30
sub-lot
part of a lot for which a test result is required
3.31
systematic sampling
extracting of increments at uniform mass or time intervals according to a predetermined plan
3.32
test sample
sample which is prepared to meet the requirements of a specific test
3.33
time-basis sampling
extracting of increments whereby the position of each increment to be collected from the stream of fuel is
measured by a time interval and the increment mass is proportional to the flow rate at the time the increment
is taken
3.34
unit mass
quantity of material, which is defined by the sampling process (usually the primary increment)
3.35
variance
measure of dispersion, which is the sum of the squared deviations of observations from their average divided
by one less than the number of observations
4 © ISO 2006 – All rights reserved

4 Establishing a sampling scheme
4.1 General
4.1.1 Sampling
The preferred method for manual sampling of fuels is during handling: e.g. (un)loading of ships, barges,
wagons, trucks or during formation of or reclaiming from stockpiles. For safety and practical reasons, manual
sampling of moving streams is sometimes not possible.
NOTE Manual sampling in stationary situations (see Annex B) refers to static lots, where no formation of or
reclaiming from piles/heaps takes place.
Increments should be collected by trained samplers. Instructions should be as complete and as simple as
possible; in particular, the position of sampling and the times at which increments are taken should be
specified and not left to the personal judgement of the sampler. These instructions, which should preferably be
set out in writing, should be prepared by the sampling supervisor from the information given in this
International Standard.
4.1.2 Sampling scheme
The general procedure for establishing a sampling scheme is as follows.
a) Define the quality parameters to be determined and the types of samples required.
b) Define the lot.
c) Select or assume the required overall precision for the lot (see 4.4.2).
d) Determine or assume the variability of the fuel (see 4.4.5 and, if relevant, 4.4.6) and the variance of
preparation and testing (see 4.4.7).
e) Ascertain the nominal top size of the fuel for the purpose of determining the mass of increment and
sample (see 4.4.9 and 4.4.10).
The nominal top size should initially be ascertained by consulting the consignment details or by visual
estimation and should be verified by preliminary test work.
f) Select the sampling device (see Clause 6).
g) Establish the number of sub-lots and the number of increments per sub-lot required to attain the desired
precision (see 4.5).
h) Determine the method of combining the increments into samples and the method of sample preparation
(see Clause 8).
i) Define the sampling interval in terms of time or mass (see Clause 5).
j) Determine where to take the increments (see Clause 5).
4.1.3 Parameters
In order to ensure that the result obtained is to the required precision, the following parameters are
considered:
a) variability of the fuel;
b) number of samples to be taken from the lot;
c) number of increments comprising each sample;
d) mass of sample relative to nominal top size.
4.1.4 Sampling methods
In this International Standard, the following sampling methods are considered:
a) continuous sampling;
b) intermittent sampling.
4.2 Sampling methods
4.2.1 Continuous sampling
In continuous sampling, every sub-lot is sampled and the number of increments collected from each sub-lot
shall be determined in accordance with 4.4.8.2. There are as many sample results for the lot as there are sub-
lots. Each sub-lot should be approximately the same size; however, for practical reasons sometimes sub-lots
of different sizes are used. The mean result for the lot should be of the required precision, but if it is desired to
check that the required precision has been attained, it is possible to do this by using the procedures of
replicate sampling (see 4.6).
4.2.2 Intermittent sampling
If fuel of the same source and quality is sampled frequently, it can be satisfactory to collect increments from
some of the sub-lots but not from others. This is called intermittent sampling. The same number of increments
shall be taken from every sub-lot that is sampled (see 4.4.8.3). The sub-lots to be sampled shall be chosen at
random, unless it can be demonstrated that no bias, for example as a result of time-dependent variance, is
introduced by choosing sub-lots systematically. Such demonstration shall be repeated from time to time and at
random intervals. There are as many sample results per lot as there are sub-lots sampled, but because some
sub-lots are not sampled, it is not possible to say whether the average of these results has the required
precision for the lot unless information about the variation between sub-lots is available (see 4.4.4 and 4.4.6).
If the variation between sub-lots is too large, it can be necessary to introduce continuous sampling to achieve
the desired precision. Use of intermittent sampling shall be agreed between contracting parties and shall be
recorded in the sampling report.
4.3 Design of the sampling scheme
4.3.1 General
The basic first step in the design of a sampling scheme is a review of the requirements for operations in order
to draw up instructions for the sampling operator(s). The instructions should cover all sampling problems likely
to be encountered.
It is important that the sampling operator receive instructions that are simple, easily understood and capable
of only one interpretation. These instructions, which should be set out in writing, should be prepared by the
sampling supervisor after inspecting the sampling site and referring to the information given in this
International Standard. The following items in the following list and described in 4.3.2 to 4.3.6 should be
considered by the supervisor when compiling instructions:
a) fuel to be sampled and considerations for sampling;
b) bias of results;
c) precision of results;
6 © ISO 2006 – All rights reserved

d) lot size and number of sub-lots;
e) method of sampling;
f) requirements for test samples;
g) number of increments;
h) mass of sample.
4.3.2 Fuel to be sampled and considerations for sampling
The first stage in the design of the scheme is to identify the fuel to be sampled. Samples can be required for
technical evaluation, process control, quality control and for commercial reasons by both the producer and/or
seller and the customer. It is essential to ascertain exactly at what stage in the fuel-handling process the
sample is required and, as far as practicable, to design the scheme accordingly. In some instances, however,
it can prove impracticable to obtain samples at the point preferred and, in such cases, a more practicable
alternative is required, provided a representative sample can be taken.
The following identifications are indispensable for the design of a manual sampling scheme:
a) fuel properties, e.g. fines, lumpy and, more specifically, the nominal top size; furthermore, whether dry,
wet or free flowing;
b) location and the handling system;
c) transport means/carriers;
d) where to sample in the handling process, taking into account contract terms and the practicability for
sampling;
e) human safety risks.
4.3.3 Division of lots
The lot may be sampled as a whole, resulting in one sample, or divided into a number of sub-lots resulting in a
sample from each. A lot may be sampled as a whole or as a series of sub-lots each to a maximum of 10 000 t,
e.g. fuel despatched or delivered over a period of time, a ship load, a train load, a wagon load, or fuel
produced during a certain period, e.g. a shift.
Such division into a number of sub-lots can be necessary to
a) achieve the required precision (calculated by the procedure in 4.5),
b) maintain the integrity of the sample, e.g. avoiding bias that can result from the loss of moisture due to
standing or of calorific value due to oxidation,
c) create convenience when sampling lots over a long period, e.g. on a shift basis,
d) keep sample masses manageable, taking into account the maximum lifting capacity,
e) distinguish different components of a mixture of fuels, e.g. different coal types within one lot.
4.3.4 Bias of results
It is of particular importance in sampling to ensure as far as possible that the parameter to be measured is not
altered by the sampling and sample preparation process or by subsequent storage prior to testing. This can
require, in some circumstances, a limit on the mass of the primary increment, the divided sample and the test
sample to maintain integrity (see 4.4.9 and 4.4.10).
It can be necessary, when collecting samples for moisture determination from lots over an extended period, to
limit the standing time of samples by dividing the lot into a number of sub-lots. For establishing the loss of
integrity of the sample, a bias test can be carried out to compare a series of reference samples immediately
after extraction with samples after standing for the normal time to establish moisture or calorific value loss
(see ISO 13909-8).
Bias testing for manual sampling can be performed according to the same principles as for mechanical
sampling using a reference method to judge a manual sampling practice (ISO 13909-8).
4.3.5 Precision of results
After the overall precision of the lot has been decided, the number of sub-lots and the number of increments
per sub-lot collected shall then be determined as described in 4.4.8 and the reference mass of the primary
increments shall be determined as described in 4.4.9.
For single lots, the quality variation shall be assumed as the worst case (see 4.4.5). The precision of sampling
achieved may be measured using the procedure of replicate sampling (see 4.6).
At the start of regular sampling of unknown fuels, the worst-case quality variation shall be assumed in
accordance with 4.4.3 and 4.4.5.
If any subsequent change in precision is required, the number of sub-lots and of increments shall be changed
as determined in 4.5 and the precision attained rechecked. The precision shall also be checked if there is any
reason to suppose that the variability of the fuel being sampled has increased. The number of increments
determined in 4.5 applies to the precision of the result when the sampling errors are large relative to the
sample preparation and testing errors, e.g. moisture. However, in some tests, the testing errors are
themselves large. In this case, it can be necessary to prepare two or more test portions from the sample and
use the mean of the determinations to give a better precision.
4.3.6 Requirements for test samples
In the sampling scheme and in the scheme of preparation of samples, attention shall be paid to requirements
on the samples for testing.
A number of tests are carried out on crushed or pulverized samples of prepared top sizes as mentioned in the
relevant testing standards, e.g. ash on a − 0,212 mm sample. However, a number of tests require samples
either in the original state or prepared to a particle size somewhere between original state and 0,212 mm.
Examples of physical tests on samples in their original state are size-distribution tests, float and sink tests,
coking tests, etc.
Examples of tests on partly crushed and prepared samples are total moisture, hardgrove index and dilatation.
In view of the above, consideration of the sampling and preparation schemes should foresee either whether all
required samples can be taken and prepared from a common sample or whether it is necessary to take a
number of separate samples. In all cases, the masses of the common sample and the required test samples
should be maintained in accordance with the minimum masses as prescribed in this International Standard
and in the standard specifying the test method. In case of differences between standards, the greater mass
should be maintained.
In case the mass of the sample as calculated in accordance with this International Standard is insufficient for
the masses of the required test samples, the number of increments should be increased to provide the greater
mass.
8 © ISO 2006 – All rights reserved

4.4 Precision of sampling
4.4.1 General
In all methods of sampling, sampling preparation and analysis, errors are incurred and the experimental
results obtained from such methods for any given parameter deviate from the true value of that parameter. As
the true value cannot be known exactly, it is not possible to assess the accuracy of the experimental results,
i.e. the closeness with which they agree with the true value. However, it is possible to make an estimate of the
precision of the experimental results, i.e. the closeness with which the results of a series of experiments made
on the same fuel agree among themselves.
It is possible to design a sampling scheme that, in principle, can achieve an arbitrary level of precision, such
level to be determined.
The required overall precision on a lot should be agreed between the parties concerned. In the absence of
such agreement, a value of 10 % of the ash content may be assumed.
4.4.2 Precision and total variance
Precision is the closeness of agreement between the results obtained by applying the experimental procedure
several times under prescribed conditions, and is a characteristic of the sampling scheme used and the
variability of the fuel being sampled. The smaller the random errors of the scheme, the more precise is the
scheme. A commonly accepted index of precision is two times the sample estimate of the population standard
deviation, and this index of precision is used throughout this International Standard.
If a large number of replicate samples are taken from a sub-lot of fuel, prepared and analysed separately, the
precision, P, of a single observation is given by Equation (1):
Ps==22V (1)
SPT
where
s is the sample estimate of the population standard deviation;
V is the total variance of the results for replicate samples.
SPT
The total variance in Equation (1) is a function of the primary increment variance, the number of increments,
and the errors associated with sample preparation and testing.
For a single sample, this relationship is expressed by Equation (2):
V
I
VV=+ (2)
SPT PT
n
where
V is the primary increment variance;
I
V is the preparation and testing variance;
PT
n is the number of primary increments in the sample.
4.4.3 Continuous sampling
Where the result of Equation (2) is the arithmetic mean of a number of sample values, resulting from dividing
the lot into a series of sub-lots and taking a sample from each, V is given by Equation (3):
SPT
VV
IPT
V =+ (3)
SPT
Nn N
where
n is the number of primary increments comprising each sample;
N is the number of sample results used to obtain the mean.
Since a sample is equivalent to one member of a set of replicate samples, combining Equations (1) and (3) for
continuous sampling results in Equations (4) and (5):
P VV
SL IPT
P== 2 + (4)
L
Nn N
N
SL SL
SL
PP=⋅N (5)
SL L SL
where
P is the overall precision of sampling, sample preparation and testing for the lot at 95 % confidence
L
level, expressed as % absolute;
P is the overall precision for the sub-lot at 95% confidence level, expressed as % absolute;
SL
V is the primary increment variance;
I
n is the number of increments per sub-lot;
N is the number of sub-lots in the lot;
SL
V is the preparation and testing variance.
PT
If the quality of a fuel of a type not previously sampled is required, then in order to devise a sampling scheme,
assumptions have to be made about the variability (see 4.4.5).
4.4.4 Intermittent sampling
Whilst the value used for the primary increment variance is assumed to be consistent for all the sub-lots in a
lot, there can be variations between the means of sub-lots. Providing all sub-lots are sampled and tested, this
is not a source of additional variance. However, if only one or some sub-lots are sampled and tested (i.e.
intermittent sampling), then a term to correct for sub-lot variance should be included in Equation (3) and the
equations derived from it, as given in Equation (6):
⎛⎞
VV NV
IPT SLS SL
V=+ + 1− (6)
⎜⎟
SPT
Nn N N N
SLS SLS SL SLS
⎝⎠
where
N is the number of sub-lots in the lot;
SL
N is the number of sub-lots sampled;
SLS
V is the sub-lot variance.
SL
10 © ISO 2006 – All rights reserved

⎛⎞
N
SLS
The term 1− V compensates for the fact that, as the proportion of sub-lots sampled and tested
⎜⎟
SL
N
⎝⎠SL
increases, the influence of sub-lot variance decreases, until it disappears when N = N .
SLS SL
The equivalent to Equation (4) can be derived by combining Equations (1) and (6), as given in Equation (7):
⎡⎤
VV ⎛⎞NV
IPT SLS SL
P=+21+− (7)
⎢⎥
⎜⎟
L
Nn N N N
⎢⎥
SLS SLS⎝⎠SL SLS
⎣⎦
4.4.5 Primary increment variance
The primary increment variance, V , depends upon the type and nominal top size of fuel, the degree of pre-
I
treatment and mixing, the absolute value of the parameter to be determined and the mass of increment taken.
For some fuels, the increment variance for ash is higher than that for moisture and, hence, for the same
precision, the number of increments required for the general analysis sample is adequate for the moisture
sample and the common sample.
The value of the primary increment variance, V , required for the precision using Equation (4) can be obtained
I
by either
a) assuming a value determined for a similar fuel from a similar fuel handling and sampling operation, or
b) determining it directly on the fuel to be sampled by taking at least 50 increments spread over an entire lot
or over several lots of the same type of fuel and analysing each increment separately on the required
parameters, preferably ash (dry basis) and total moisture.
For calculating the variance, Equation (8) can be used:
⎡⎤
x
()
1 ∑ i
⎢⎥
Vx=− −V (8)
IP∑ i T
⎢⎥
nn−1
⎢⎥
⎣⎦
where
V is the primary increment variance;
I
n is the number of increments taken;
x is the value of the analysed parameter;
i
V is the preparation and testing variance.
PT
If neither of these values is available, a value of V = 20 for ash content can be assumed initially and checked
I
after sampling has been carried out.
4.4.6 Sub-lot variance
In some cases (e.g. see 4.4.4), the sub-lot variance, V , can be calculated, because, just like the primary
SL
increment variance, this value gives an indication of the homogeneity of the fuel. For calculation of V
SL
Equation (9) can be used:
⎡⎤
x
()
∑ SL
1⎢⎥
Vx=− −V (9)
SL ∑ SL PT
⎢⎥
NN−1
⎢⎥
⎣⎦
where
V is the sub-lot variance;
SL
N is the number of sub-lots in the lot;
x is the value of the analysed parameter from the sub-lot;
SL
V is the preparation and testing variance.
PT
If the variance of different lots/sub-lots or different cargoes of the same fuel varies considerably, the primary
increment variance found for any lot or cargo cannot be used for calculation of the number of increments for
the next lot or cargo.
4.4.7 Preparation and testing variance
The value of the preparation and testing variance, V , required for the calculation of the precision using
PT
Equation (4) or (7) can be obtained by either
a) assuming a value determined for a similar fuel using a similar sample preparation scheme, or
b) determining it directly on the fuel to be sampled by constituting at least 20 sub-samples spread over the
entire lot or over several lots of the same type of fuel. Each sub-sample is divided into two parts and
prepared so that split portions of each sub-sample are taken at the first division stage. Each portion shall
be prepared and tested for the parameters of interest, preferably ash (dry basis) and total moisture. The
same analysing methods are applied as are used in routine operations. The difference between the two
results shall be calculated for each pair and the preparation and testing variance, V , can be calculated
PT
as follows:
d
i
∑
V = (10)
PT
2n
p
where
V is the preparation and testing variance;
PT
d is the difference between individual pair members;
i
n is the number of pairs.
p
Alternately, split one or more sub-lot samples into a minimum of 20 test samples. Prepare and analyse each
test sample for the parameters of interest, preferably ash (dry basis) and total moisture. The preparation and
testing variance shall be calculated as given in Equation (11):
⎡⎤
x
⎢⎥()i
∑
Vx=− (11)
⎢⎥
PT ∑ i
nN−1
⎢⎥TS
⎢⎥
⎣⎦
where
V is the preparation and testing variance;
PT
N is the number of test samples;
TS
x is the value of the analysed parameter.
i
12 © ISO 2006 – All rights reserved

If neither of these values is available, a value of V = 0,2 for ash content can be assumed initially and
PT
checked if necessary after preparation and testing has been carried out.
If high overall precision, P , is required, then lower V values of 0,1 or 0,05 for ash content are required to
L PT
obtain the required overall precision using a practical number of primary increments and sub-lots (see 4.4.8).
4.4.8 Number of sub-lots and number of increments per sub-lot
4.4.8.1 General
The number of increments taken from a lot in order to achieve a particular precision is a function of the
variability of the quality of the coal in the lot irrespective of the mass of the lot. The lot may be sampled as a
whole, resulting in one sample, or divided into a number of sub-lots resulting in a sample from each. Such
division can be necessary in order to achieve the required precision and the necessary number of sub-lots
shall be calculated using the procedure given in 4.4.8.2 or 4.4.8.3 as appropriate.
Another important reason for dividing the lot is to maintain the integrity of the sample, i.e. to avoid bias after
taking the increment, particularly in order to minimize loss of moisture due to standing. The requirement to do
this is dependent on factors such as the time taken to collect the samples, ambient temperature and humidity
conditions, the ease of keeping the sample in sealed containers during collection and the particle size of the
coal. It is recommended that, if moisture loss is suspected, a bias test is carried out to compare the quality of
a reference sample immediately after extraction with that of the sample after standing for the normal time. If
bias is found, the sample standing time should be reduced by collecting samples more frequently, i.e.
increasing the number of sub-lots.
There can be other practical reasons for dividing the lot:
a) for convenience when sampling over a long period,
b) to keep sample masses manageable.
Establish the number of sub-lots and number of increments required per sub-lot in accordance with 4.4.8.2 or
4.4.8.3 as appropriate.
NOTE The equations given in 4.4.8.2 and 4.4.8.3 generally give an overestimation of the required number of
increments. This is because they are based on the assumption that the quality of coal has no serial correlation; however,
serial correlation is always present to some degree. In addition, because a certain amount of preparation and testing is
required when measuring the increment variance or the sub-lot variance, the preparation and testing errors are included
more than once.
The designer of a sampling scheme should make provisions for the worst case anticipated and then tend to
use higher values for V and V than may actually occur when the system is in operation. On implementing a
I SL
new sampling scheme, a check on the actual precision being achieved should be carried out using the
methods described in ISO 13909-7. This can be necessary to achieve the required precision, in which case
the number of sub-lots is calculated using the procedures given in 4.4.8.2 and 4.4.8.3.
4.4.8.2 Continuous sampling
Determine the number of sub-lots required for practical reasons (see 4.4.8.1) and then estimate the number of
increments for a desired precision from Equation (12), obtained by transposing Equation (4):
4V
I
n= (12)
NP⋅− 4V
LPT
A value of infinity or a negati
...