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Products to be stored may have characteristics that do not permit immediate warehousing. Excessive moisture content or the presence of insects and impurities can compromise long-term quality conservation of products.
To ensure good technical and marketing management, it is important periodically to check the quantity and quality of the stored products. Before the actual storage operations, products should undergo appropriate checking in order to verify weight, the percentage of impurities and moisture content.
During storage, the state of conservation of the products must be monitored. To this end, in addition to moisture content and the degree of insect infestation, the temperature of the grain will be periodically measured.
Weighing first takes place when the products arrive at the storage centres. Depending on delivery conditions (products delivered in bags or in bulk, transport system, etc.), weighing can be done with simple mechanical scales or platform scales.
In relatively small storage centres, when the products are delivered in bags, weighing is done with simple mechanical scales.
These scales can hold 200 kg, and so several bags of grain can be weighed at the same time.
A storage centre must be equipped with enough scales to permit a year-round normal flow of products to the warehouse.
In some cases, in order to shorten reception time, if the bags entering the warehouse are of a standard gross weight, partial weighing of batches can be done. The total weight is then obtained by multiplying the number of bags by the average weight of the bags weighed.
Platform scales permit the weighing of batches by weighing the loaded or unloaded vehicles as they enter and leave the storage centres.
Because of their high cost and great load-bearing capacity, these weighing systems are advantageous only in big storage centres where products are delivered in bags or in bulk by vehicles.
The use of platform scales requires minor construction. In particular, a pit must be dug and developed to hold the platform-scale mechanisms. This pit is generally 1.60 m deep; some special models of platform scale can be installed in pits only 90 cm deep.
Some manufacturers have also provided an open-sky installation without a pit. In this case the platform scale is raised above the ground and so ramps must be built so the vehicles can have access to the platform.
Finally, it must be remembered that the carrying capacity indicated by the manufacturers refers to the maximum weight the platform scale can hold. This weight includes both the weight of the transport vehicle and the weight of its load.
When products arrive at the storage centres and during warehousing, it is important to check their quality and state of conservation.
It is practically impossible to analyse all the batches. Therefore, a representative sample of the total product must be taken, from which the appropriate analyses can be made.
To obtain a representative sample, several primary samples must be taken: once they have been gathered and mixed together in a clean receptacle, they constitute the global sample on which the necessary tests will be made.
If the global sample is too big, it must be divided up to obtain a smaller, but still representative, sample.
Methods of taking samples differ depending on whether products are delivered or stored in bags or in bulk.
Sampling of grains delivered in bags
For a given batch of grains, the number of bags from which samples must be taken depends on the total number of bags, as shown in the following table:
|COMPOSITION OF BATCH||BAGS SAMPLED|
|1 to 10 bags||All bags|
|10 to 100 bags||10 bags chosen at random|
|More than 100 bags||The square root of the total number of bags|
After the bags are selected, there are two ways of making up the global sample: by probing or by emptying the bags.
Sampling by probing
The primary samples are taken directly by introducing hollow probes into the selected bags (bag probes, probing rods, etc.).
The samples should be of about 50 g per 100-kg bag and, in any case, sufficient for composition of a global sample of at least 500 g; after they are taken, the samples must be carefully mixed together.
Sampling by emptying the bags
The contents of each bag are stirred and spread in a layer 10 cm thick on a clean surface; a primary sample of about 1 kg is taken from each layer of grain; the various primary samples are then carefully mixed together to obtain the global
Sampling of grains delivered in bulk
A sampling of grains delivered in bulk can be obtained by taking primary samples either when the product is stationary (e.g. on a truck or a trailer), or when it is in motion (e.g. while a silo is being filled).
Sampling of a stationary product
A sampling of a stationary product is obtained by taking several samples inside the grain mass and mixing the resultant primary samples.
To the extent possible, samples should be taken throughout the entire thickness of the layer of grain.
Deep samples, obtained with special probes, are preferable to those taken on the surface with suction flasks or other small receptacles.
The number of samples and the points at which they are taken depend on the total quantity of grain, as shown in the diagrams below.
Number of samples: A 5 samples; B 8 samples; C 11 samples.
For particularly heterogeneous batches, it is recommended that samples be taken of every five tonnes of product, with a minimum of 10 samples.
Sampling of a moving product
More reliable and exact than the technique of sampling of a stationary product, this technique consists of taking one or several samples at once from the mass of moving grain.
The samples can normally be taken with simple tools (suction flasks, shovels, etc.) or by means of automatic samplers located in the ducts where the grain is circulating.
If the global sample obtained is too big, methods must be used to obtain smaller, but still representative, samples.
One of the simplest of these methods, although not a very precise one, is called the "cone" method.
Sample seduction: cone method
Smaller samples can be obtained by dividing the global sample that has been stirred and piled on a clean surface into two or four equal parts.
Sample reduction can also be obtained by using special devices like the conical sampledivider, also called "Boerner divider", and the riffle divider, more often used for seeds.
Determining the percentage of impurities
Impurities are not only bad for the storage of products; they are an element in the very quality of these products.
It is therefore important to determine the percentage of impurities in a batch of grain before storing it or trading it.
Generally considered to be impurities are:
The percentage of impurities is determined on samples of relatively low weight (a few hundred grams).
In practice, healthy grain is separated from impurities by visual sorting and sifting. Then the impurities are classified and weighed on a scale.
The relationship, expressed in percentage, between the weight of the impurities and the weight of the sample gives the percentage of impurities in the batch of grain analysed.
Measuring moisture content
Determining the moisture content of grain is an operation of particular importance at all stages of the post-harvest system.
Before the harvest, for example, it permits an estimation of the degree of maturity of the grain. After threshing, it is used for deciding on methods and durations of drying. Before processing of products, it shows whether the grain is in condition for treatment.
At the time of marketing, it reveals the quantity of dry matter that is being bought, and thus permits establishment of the fair sales price (water does not have the same market value as grain).
Before and during storage, it helps to determine warehousing conditions and to evaluate the state of conservation of grain.
In the field, farmers are used to approximating the moisture content of grain by empirical evaluations.
These methods, based on individual experience, do not give a true objective measurement, but estimate the degree of moisture by subjective sensory perception (touch, sight and smell) of some of the grain's characteristics.
Thus some farmers are in the habit of nibbling grains, or scoring them with a thumbnail, or crushing them between their fingers, to evaluate their hardness and consistency and thereby to estimate their moisture content.
Others rely on the good or bad smell that comes from a handful of grain; still others base their evaluations on the dull or sharp rattle produced by shaking a few grains in a metal box.
Some assess the fluidity of the grain by trying to push an open hand into a fairly big grain mass contained in a bag or in a thick layer of bulk grain.
Of all the empirical methods, perhaps the least approximate is the salt test. This consists of mixing a sample of grain in a glass receptacle with some dry ordinary kitchen salt.
After the receptacle has been shaken several times, its walls are examined to see whether the salt has stuck to them.
If the salt sticks to the walls, it means that the moisture content of the sample of grain is higher than about 15 percent.
Although these empirical methods are employed mainly by farmers, they ought to be progressively replaced by the use of instruments that permit a true measurement of the moisture content of grain.
The use of empirical methods in storage centres or during commercial transactions should be emphatically opposed.
Today, the methods that make use of suitable measuring instruments can be divided into two categories: direct and indirect measurement.
The sample to be measured is weighed on precision scales, then dried in a drying-room, and weighed again.
The difference in weight before and after drying indicates the amount of water contained the sample.
The technological sophistication of the instruments and the complexity of the measurements necessitate the presence of qualified staff.
Some of the measuring instruments that operate on the principle of drying the sample are:
As precise as they are complex, these instruments are best adapted to the needs of specialized laboratories that are responsible for the calibration of other indirect measurement devices (e.g. hygrometers) or for the determination of the moisture content in big storage corporations or in the context of commercial transactions.
These methods permit determination of moisture content by measuring the electrical characteristics of grain moisture.
Some of the measuring instruments using this principle are:
Hygrometers are electric devices which allow the moisture content of a sample of grain to be directly and immediately read on a dial.
Hygrometers measuring the electric resistance of grain are portable instruments that are relatively imprecise but extremely practical and inexpensive.
The more expensive and complex hygrometers that measure the dielectric constant of grain are generally used in big storage centres and for commercial transactions.
Temperature control is indispensable for monitoring the conservation of stored products.
An abnormal rise in grain temperature can signal the onset of degradation.
Therefore, regular checks must be made in order to avoid substantial losses of product.
Furthermore, since the grain masses are not homogeneous, samples must be taken from different points in each stored mass.
We shall first consider the characteristics required for effective measuring devices, and then the various types of devices that exist.
Given the heterogeneity of the grains in the stored mass, measuring devices need not be of high precision, and deviations of as much as one degree centigrade are negligible.
On the other hand, such devices must be very sensitive, in order to discern as fast as possible the slightest abnormal change in temperature.
They should be easy to read and require a minimum of handling.
Finally, in order to resist the shocks incurred during repeated handling, these devices must be solid in structure and dependably sturdy.
The various types have different principles of operation and methods of use.
These devices are indispensable for bulk-storage installations but can also be useful for monitoring products stored in bags.
Aside from the devices of varying degrees of sophistication described below, digital thermometers that are easy to read and cost relatively little can be found.
These function on the principle of the expansion of a liquid (mercury or alcohol) under the influence of temperature.
They are placed inside the metal probes that can be inserted into the bags or grain mass.
This is a relatively inexpensive system that can be used for grain stored in bags or in bulk; its disadvantage, however, is the imprecision of the reading taken, owing to the removal of the thermometer from the measurement or reading point.
The operating principle of these thermometers is based on measurement of the electrical current running through a filament made of platinum, copper, steel or nickel, with a resistance that varies according to the temperature.
Placed in a long sheath hung from the top of the storage bin, this thermometer is inserted into the grain mass.
The advantage of such thermometers is that a single reading can be taken of the overall temperature of the stored products, whereas with liquid thermometers measurements must be repeated at several points in the grain mass.
Probes with thermistors or thermocouples
These are probes equipped with sensors or sensitive points (thermistors or thermocouples), which behave like electric thermometers to be read at a distance.
Introduced into the grain after the bins have been filled, these probes are linked electrically by telephone cable to individual portable reading cases or centralized control cabinets.
Positions of probes in a silo: 1 Probes; 2 Cable; 3 Connection box; 4 Control panel.
Because the temperature in the grain mass varies, the sensors must be adequately distributed throughout the stored mass.
In general, they should be placed:
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