Miller Magazine Issue: 112 April 2019

104 ARTICLE APRIL 2019 but is usually neither practical nor economical when a lar- ge quantity is involved. The choice is either not to examine the consignment at all or to take samples to obtain some information, acknowledging that anything less than a total examination is bound to affect the accuracy of the results. Principles of representative sampling The results of sample analyses can be expressed in preci- se terms. However, precise analytical results may be of little practical value, and may be misleading if the samples are obtained without taking into account the non-random or aggregated distributions of foreign matter, damaged grains, insects, etc. Certain principles of representative sampling must be observed: • The consignment should be divided into primary units of equal size or status, which may be sampled. For bagged grain, each bag may be regarded as a primary unit. For bulk grain, the primary unit may be expressed in terms of weight, if the grain is being moved, or volume, when it is static - as in a truck or bin. • All primary units should have an equal opportunity of being sampled. This is possible only during the construction or dismantling of a stack, the loading or offloading of a truck, or when bulk grain is being moved. • The method should select, without bias, a representative number of primary units from the consignment. Many countries adopt the recommendations of ISO 950 “Cereals - Sampling (as grain)”. Double-tube sampling spears These spears comprise two metal tubes, one fitting closely inside the other and each with several common slots. Spears may vary in length from 45cm to 3.5m, and in width from 12mm to 50mm. turning the inner tube through 180° opens or closes the intake apertures, and so collects grain from a transverse section of the container. Double-tube sampling spears are designed primarily for obtaining samples from vertical lines of penetration in bulk grain, although small versions may be used for sampling bag- ged grain. They are superior in many ways to the simple bag sampling spear, but are still instruments of haphazard rather than representative sampling. Equipment for obtaining primary samples from bulk grain Bulk grain is sampled either when it is static, i.e. when it is contained in a truck, barge or storage bin, or when it is on the move, i.e. when it is being discharged through a spout or on a conveyor belt. A wide range of sampling equipment has been developed to meet the special requirements of these various situations, some for small-scale operations and other items for situations where grain is handled in very large quantities. Double-tube sampling spears Spears 1.8m long and 3.5cm outer diameter are common- ly used, but longer 3.7m double-tube spears are available for sampling grain in exceptionally deep trucks and barges. The sampling spear should always be inserted into bulk grain at a slight angle from the vertical, with the slots facing upward. The slots must be opened only when the spear has reached the sampling position, and must be closed before it is removed. Manually-operated deep bin probes The simplest probe of this type consists of a hollow spear head, which serves as a sample cup, with a spring-loaded cap attached to a metal or wooden rod about 1 meter long. Extension rods are attached to increase the depth of penet- ration. When the sampling point has been reached a slight upward pull on the rod lifts the cap of the spear head, allowing grain to fill the cup. The probe is then withdrawn completely and the sample removed. A single probe yields up to 300g of sample material. The deep bin fin-probe consists of a double-tube samp- ler with a set of extension rods. When the sampling po- sition is reached a twist of the extension rod opens the sample intakes. This action is facilitated by the fin whi- ch prevents the outer tube from turning. A reverse twist closes the sample intakes before the probe is withdrawn from the grain. Up to 600g of sample representing a 1.5m long vertical ‘cut’ may be obtained. A considerable amount of physical effort is required to push any of these probes into grain. None can be expec- ted to penetrate more than approximately 5 meters. Pneumatic grain samplers Pneumatic grain samplers overcome the main disad- vantages of manual operation by using powered-sucti- on to penetrate the static bulk of grain, and by taking a continuous sample. They are quicker to operate than manual samplers, and can be used easily to obtain samp- les from the sides and floors of bulk grain containers. Working sample size In practice, it is necessary to compromise between what