BS ISO 13909-7:2016 pdf download – Hard coal and coke — Mechanical sampling Part 7: Methods for determining the precision of sampling, sample preparation and testing
For the purposes of this document, the terms and definitions given in ISO 13909-1 apply.
4 General When designing a sampling scheme in order to meet a required precision of results, formulae are necessary that link certain fuel and sampling characteristics to that precision. The main factors to be considered are the variability of primary increments, preparation and testing errors, the number of increments and samples taken to represent the lot, and the mass of the samples. These formulae are derived in Clause 5. Methods for estimating the parameters used in those formulae are given in Clause 6. Once a sampling system has been designed and installed, the precision which is being achieved on a routine basis should be checked. An estimate of the precision can be obtained from the primary increment variance, V I , the numbers of increments, n, and sub-lots, m, (see Clause 5) and the preparation and testing variance, V PT . The preparation component of V PT is made up of on-line sample processing and off-line sample preparation.
Sampling variance is a function of product variability, so the same number of increments, sub-lots, and preparation and testing errors will yield different precision with fuels that exhibit different product variability. Depending on the extent to which serial correlation exists and which method of estimating primary increment variance is used, such an estimate could represent a considerable overestimate of the numerical value of the precision (i.e. indicate that it is worse than is really the case). In addition, in order for the results to be meaningful, large numbers of increments (in duplicate) would need to be prepared and analysed for the estimation of V I and V PT .
Quality variations obtained in the form of primary increment variances on existing systems are not absolute and therefore designers should exercise caution when using such results in a different situation. The estimated value of the primary increment variance, V I , should be derived experimentally for each fuel and at each sampling location. Whenever a sampling scheme is used for determining increment variance, the operating conditions should be as similar as possible to the conditions known, or anticipated, to prevail during the sampling for which the increment variance is needed, whether it be carried out by the same or by a different sampling system. An estimate of the precision actually achieved can be obtained by taking the sample in a number of parts and comparing the results obtained from these parts. There are several methods of doing this, depending on a) the purpose of the test, and b) the practical limitations imposed by the available sampling procedures and equipment. Where a sampling system is in existence, the purpose of the test is to check that the scheme is in fact achieving the desired precision (see Clause 7). If it is not, it may need to be modified and rechecked until it meets the precision required. In order to do this, a special check scheme should be devised which may be different from the regular scheme but which measures the precision of the regular scheme. For regular sampling schemes, the most rigorous approach is that of duplicate sampling of sub-lots. In many existing mechanical sampling systems, however, the capacity of individual components and the interval between increments in the regular scheme is insufficient to allow the taking of extra increments. In such cases, duplicate samples can be constituted from the normal number of increments and the result adjusted for the smaller number of increments in each sample (see 7.3).
The need may arise to sample a particular lot and to know the precision of the result obtained (see Clause 8). Once again, a special check scheme needs to be devised, but in this case, it is the precision achieved by that scheme on that lot which is required. For the measurement of the precision achieved for a particular lot, replicate sampling is the best method. Methods for detailed checking of preparation and testing errors are given in Clause 9. The results may also be used to provide data for the formulae used in Clause 5.
5 Formulae relating to factors affecting precision
5.1 General Precision is a measure of the closeness of agreement between the results obtained by repeating a measurement procedure several times under specified conditions and is a characteristic of the method used. The smaller the random errors of a method, the more precise the method is. 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 part of ISO 13909.