4.1.1. Introduction
4.1.2. Definitions
4.1.3. Principles of representative sampling
4.1.4. Method for sampling from bags
4.1.5. Method for sampling from static bulk
4.1.1.1. Correct sampling to obtain a representative sample from a lot of grain is an essential part of the grading system. If the sampling method does not take into account the possible non-uniform distribution of quality factors such as foreign matter, insect and defective grains, the true grade of a lot cannot be determined. Certain principles of representative sampling must be observed.
4.1.1.2. It is not the intention of this standard to lay down fixed rules to be followed in every case because local conditions and contract requirements may specify particular inspection procedures such as the individual examination of primary samples. However, it is important that biased sampling is avoided at all times.
4.1.2.1 Consignment: the quantity of grain dispatched or received at one time that is covered and defined in one contract.
4.1.2.2. Lot: a portion of the consignment assumed to be uniform which will allow the quality to be assessed. The consignment should be divided into primary units of equal size or status, any or all of which may be sampled. For bagged grain, each bag may be regarded as a lot. For bulk grain, the lot may be expressed in terms of weight if the grain is being moved, or volume, when it is static e.g. a truck or bin.
4.1.2.3. Primary sample: the quantity taken from a single position in the lot. A number of primary samples will usually be taken from the lot.
4.1.2.4. Bulk sample: the amount of grain formed by combining and mixing the primary samples from a particular lot.
4.1.2.5. Laboratory sample: the quantity of grain obtained by dividing the bulk sample in such a way as to be a representative sample of the lot.
4.1.3.1. The consignment shall be divided into lots which should have an equal opportunity of being sampled. This is possible only during the construction or dismantling of a stack, the loading or off-loading of a truck, or when bulk grain is being moved.
4.1.3.2. The method should select, without bias, a representative number of lots from the consignment.
4.1.3.3. Primary samples should be taken from lots in such a manner, and of such a size, as to be representative of the consignment. The method and equipment for extracting the primary sample must be efficient and without bias, ensuring that the sample size is large enough to include the less frequent components of the grain e.g. pest-damaged grain.
4.1.4.1. If the grain is present in bags then each bag shall be regarded as a lot. The number of bags to be sampled is specified in Table 4.1.
Table 4.1 Number of bags to be sampled from consignments of different sizes.
|
Number of bags in consignment |
Number of bags to be sampled |
|
Up to 10 |
Each bag |
|
10 to 100 |
10 drawn at random |
|
More than 100 |
The square root (approximately) of the total number of bags in the consignment, drawn at random according to a suitable scheme |
4.1.4.2. Sampling of individual bags shall be carried out when bag stacks are being constructed and when they are being dismantled for onward distribution. If long term storage occurs, then the stack should be periodically inspected (ideally every 2 weeks) by drawing samples from the most accessible bags. It will not be possible to sample from bags located in the deep interior of the stack but bags on the top of the stack should be removed to sample bags below the surface.
4.1.4.3. Equipment for obtaining primary samples from bagged grain:
· Simple bag sampling spears: A sampling spear with a 25 mm diameter should be used for obtaining samples of maize. To obtain a good cross-sectional sample the spear should be 40 to 45 cm in length. The tapered type of sampling spear is not recommended since it takes unequal portions of grain from along the line of penetration, which could lead to distorted assessments of grain quality. More even sampling is achieved with the cylindrical type of sampling spear.· Double-tube sampling spears: these spears comprise two metal tubes, one fitting closely inside the other and each with several common slots, turning the inner tube through 180° opens or closes the intake apertures, and so collects grain from a transverse section of the bag. Spears 45 cm long and 12 mm wide are suitable for bag sampling. They are superior to the simple bag sampling spear, but are still instruments of haphazard rather than representative sampling.
4.1.4.4. The main disadvantage of obtaining samples with the above instruments is that it does not conform to the basic principles of representative sampling. If foreign matter or defective grain happens to be very unevenly distributed in the bag, the haphazard nature of spear sampling could lead to a distorted quality assessment.
4.1.4.5. Whole bag sampling: the only way in which a truly representative primary sample can be obtained from a bag is to empty and divide the whole contents using a suitable method and equipment.
4.1.4.5.1. The produce flow sampler: The produce flow sampler can extract a truly representative sample from a hag of grain. The bag of grain should be tipped into the hopper at the top, which has a capacity of at least 100 kg. The outlet of the hopper is closed by a bung (stopper) until the operator is ready to commence sampling. When the bung is removed the grain falls through on to a cone, which is positioned to ensure that the flow of grain is distributed equally on all sides. Most of the grain is directed into a bag attached to the underside of the collecting tunnel, but some of the grain is trapped by tour vents arranged equidistantly around the base of the cone and directed via a separate spout into a sample container. The size of the sample depends on the dimensions of the vents, which can be altered by interchangeable parts. The produce flow sampler was designed for sampling bags as they are off-loaded from a truck.
4.1.5.1. Bulk grain can be sampled when it is static, i.e. when it is contained in a truck, barge or storage bin. Each truck, barge etc. which forms the contract shall be considered as a single unit for the purposes of sampling.
4.1.5.2. If sampling takes place from laden wagons, containers or lorries the primary samples shall be taken throughout the whole depth of the grain layer at the following points:
Figure 4.1 Sampling points on a static wagon, truck or hold of ship
