BS ISO 13909-2:2016 pdf download – Hard coal and coke — Mechanical sampling Part 2: Coal — Sampling from moving streams
f) Establish the number of sub-lots and the number of increments per sub-lot required to attain the desired precision (see 4.3.4).
g) Decide whether to use time-basis or mass-basis sampling (see Clause 5) and define the sampling intervals in minutes for time-basis sampling or in tonnes for mass-basis sampling.
h) Ascertain the nominal top size of coal for the purpose of determining the minimum mass of sample (see 4.4 and Table 1).
NOTE The nominal top size may initially be ascertained by consulting the consignment details, or by visual estimation, and may be verified, if necessary, by preliminary test work.
i) Determine the minimum average increment mass (see 4.5).
4.2 Design of the sampling scheme
4.2.1 Material to be sampled The first stage in the design of the scheme is to identify the coal to be sampled. Samples may be required for technical evaluation, process control, quality control and for commercial reasons by both the producer and the customer. It is essential to ascertain exactly at what stage in the coal-handling process the sample is required and, as far as practicable, to design the scheme accordingly. In some instances, however, it may prove impracticable to obtain samples at the preferred points and, in such cases, a more practicable alternative is required.
4.2.2 Division of lots A lot may be sampled as a whole or as a series of sub-lots, e.g. coal dispatched or delivered over a period of time, a ship load, a train load, a wagon load or coal produced in a certain period, e.g. a shift. It may be necessary to divide a lot into a number of sub-lots in order to improve the precision of the results. For lots sampled over long periods, it may be expedient to divide the lot into a series of sub-lots, obtaining a sample for each.
4.2.3 Basis of sampling Sampling may be carried out on either a time-basis or a mass-basis. In time-basis sampling, the sampling interval is defined in minutes and seconds and the increment mass is proportional to the flow rate at the time of taking the increment. In mass-basis sampling, the sampling interval is defined in tonnes and the mass of increments constituting the sample is uniform. Of these two alternatives, time-basis sampling is easier to implement and verify, because only a fixed speed cutter and a timing device are required. On the other hand, for mass-basis sampling, a conveyor belt weightometer is required as well as a device that is controlled sufficiently to adjust the primary cutter speed increment by increment to achieve uniform increment mass.
4.2.4 Precision of sampling After the desired sampling precision has been selected, the number of sub-lots and the minimum number of increments per sub-lot collected shall be determined as described in 4.3.4, and the average mass of the primary increments shall be determined as described in 4.5. For single lots, the quality variation shall be assumed as the worst case (see 4.3.2). The precision of sampling achieved may be measured using the procedure of replicate sampling (see ISO 13909-7). At the start of regular sampling of unknown coals, the worst-case quality variation shall be assumed, in accordance with 4.3.2. When sampling is in operation, a check may be carried out to confirm that the desired precision has been achieved, using the procedures described in ISO 13909-7.
If any subsequent change in precision is required, the number of sub-lots and of increments shall be changed as determined in 4.3.4 and the precision attained shall be rechecked. The precision shall also be checked if there is any reason to suppose that the variability of the coal being sampled has increased. The number of increments determined in 4.3.4 applies to the precision of the result when the sampling errors are large relative to the testing errors, e.g. for moisture content.