Corn sheller and moisture meter

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As the ear of corn reaches maturity, the individual grains swell and then harden to form a closely-packed cylindrical cob. The kernels on the cob are within the husks which endow some degree of protection from damage caused by insects, fungi and the ambient climate. Recovery of the grains requires the removal of the husks, a reduction in the field moisture content to a safe storage and processing level and removal of the grains from the cob (shelling). Shelling is, therefore, one of the essential processes in corn production

On a large scale, mechanized corn shellers are extensively used on previously harvested and dried cobs, and most farmers in the corn belt area of Thailand use a mechanized corn shelling. Alternatively, grain corn can be recovered directly from the field using a suitable corn picker, picker sheller, or combine harvester, but these kinds of machines have not yet been introduced in Thailand. The large size of corn grains renders them rather vulnerable to mechanical damage.

Shelled corn occupies less storage space on the farm, but because of greater resistance to air circulation, the safe moisture content for storage might be lower than that of ear corn.



There is a wide variety of hand-operated corn shelling devices and designs available which seek to improve upon manual shelling. However, shellers are often required in the more remote and less prosperous rural areas where factory-made machines may be unattainable. There is seldom a choice of models available to a potential consumer. In these circumstances it may be appropriate to consider construction of a sheller using local materials in a published design.

Pinson and Walker (1984)1 described four basic styles of manual shellers as shown in Table 1. Each style is named as (1) traditional hand methods, (11) hand-held devices, (111) small rotary shellers, and (IV) large hand shellers. Illustrations of typical models are shown in Fig.2. Each style of sheller caters for different economic conditions, such as for, domestic purposes, ownership by one or two small scale farmers, use by labourers on a larger farm, and use by independent farmers on a daily hire basis. There are wide differences in machine performance and operator acceptability, much of which is due to variations in detailed engineering quality.



(1) Principles and Types of Mechanized Corn Sheller.

Table 1. Maize shellers and shelling techniques evaluated in laboratory trials

"Spring sheller" works on this same principle, feeding the ears endways into an opening bounded by a rotating flute cylinder, a rotating toothed disk, and a spring pressure plate. The cylinder shells the ears as the disk revolves them. The cob passes through undamaged after the kernels are removed.

The "cylinder sheller" has largely replaced the spring sheller because of its greater capacity, its ability to shell corn, and its simplicity, although more power is required because more cob is broken and crushed. The cylinder sheller consists of a cylinder with spiral flutes or paddles which turns inside a cage with longitudinal bars. The cylinder clears the bars by approximately 5 cm, and the bars are spaced apart enough to let kernels fall through but retain cobs. The ears are fed into an opening at one end of the cage. The spiral flutes feed them through the cage and at the same time shell the ears by rolling and crushing action against the cage and each other. In some machines the cage bars are slightly flexible to reduce crushing of cobs and kernels. An adjustable cob gate serves to control the flow of corn through the cylinder, and it is adjusted to hold the corn in the cylinder long enough to be completely shelled.

For the cylinder sheller, many different types and configurations of shelling devices exist, but very few have reached the stage of even limited field use. The three types generally employed in present day combines are cross-flow rasp-bar cylinders, axial-flow rasp-bar cylinders, and spike-tooth cylinders. Spiketooth cylinders were used almost exclusively in both combines and corn shellers prior to about 1930, but most combines now produced have rasp-bar cylinders. Spike-tooth cylinders are now used to a limited extent in OS or European countries, but in Thailand spiketooth cylinders are most abundantly used.

Most cross-flow, rasp-bar cylinders have opengrate concaves with rectangular bars paralled to the cylinder axis. The clearance between the concave bars and the corrugated cylinder bars is adjustable. High speed motion pictures have shown that in cereals the main shelling effect results from the impact or shattering action of the cylinder bars hitting the head at high speed. Although rubbing undoubtedly contributes to the shelling action, the primary function of the concave appears to be holding or bringing the material into the cylinder-bar path for repeated impacts.

Axial-flow (helical-flow), rasp-bar cylinders are similar to conventional cross-flow cylinders except for the number and arrangement of rasp-bars on the rotor. Each of the 2 rotors has 2 pairs of rasp bars 180 apart, instead of 8 to 10 uniformly spaced bars. Another arrangement, on single-rotor machines, has three equally spaced, helical rasp-bars (that move the material along the rotor) and a staggered arrangement of short, axial sections between the helical bars.

The open-grate concaves on axial-flow cylinders are of the same general type as those used with crossflow rasp-bar cylinders and are adjustable in a similar manner to change cylinder-concave clearances. A major functional difference is that the material passes through the shelling zone between the axial-flow cylinder and concave several times as it moves rearward in a helical path, rather than making a single pass as with a cross-flow cylinder.

The arrangement of a spike-tooth cylinder and concave is such that the cylinder teeth pass midway between staggered teeth on the concave, thus producing a combining action in addition to the highspeed impacts upon the heads. The concave assembly is adjusted laterally to give equal clearances on both sides of the cylinder teeth.

The teeth in the spike-tooth concave are mounted on perforated, removable sections, usually with two rows of teeth per section. The total number of rows of teeth needed in the concave (usually 2,4 or 6) depends on the shelling conditions. Perforated blank sections, or grid-type grates are added to fill the concave space.

A spike-tooth cylinder has a more positive feeding action than a rasp-bar cylinder, with added advantages that it does not plug as easily, and requires less power. Rasp-bar cylinders are readily adaptable to a wide variety of crop conditions, are easy to adjust and maintain, and are relatively simple and durable. A raspbar cylinder with an open-grate concave has greater seed separating capacity than a spike-tooth cylinder.

Rubber Roller Shellers.

In the conventional cylinder shellers, the corn kernels are subjected to mechanical damage while passing through the shelling section, which consist of the steel cylinder and steel concave. Pickard4 and USDA engineers5 studied the use of a relatively soft material, like rubber, instead of steel for the rasp-bar and for the cylinder. This is similar to the groundnut rubber roll sheller developed by Khon Kaen University.



The shelling efficiency will be evaluated by the amount of corn left unshelled on the cob (called cylinder loss) and damage on the kernels.

Shelling effectiveness is related to (a) the peripheral speed of the cylinder, (b) the cylinder-concave clearance, (c) the number of times the material passes the concave (e g., axial-flow cylinder), (d) the number of rows of concave teeth used with a spike-tooth cylinder, (e) the condition of the crop in terms of moisture content, maturity, etc., and (f) the rate at which material is fed into the machine.

Cylinder Loss

Such factors as cob moisture content, kernel cylinder-concave clearance Infuence the performance for cross-flow cylinder shellers, and these factors plus the restriction at the cob gate influence the performance of the axial-flow sheller. (18.75 mm) for rasp-bar cylinders.

Johnson et al indicated the relationship of cylinder loss as influenced by kernel moisture content. In this case a range of loss was proposed which represented the values occurring from both types of shelling cylinders, various reasonable adjustments, and several operating conditions (Fig. 8 The range of cylinder loss which comes about because of cylinder adjustments).

Goss et al8, report similar data on cylinder loss versus moisture content. These data are presented in Fig.9 (see Fig. 9 Cylinder loss and its relation to kernel moisture content. Self-propelled unit-cylinder-concave clearances were 314 to 1-inch at front, 5/8 to 314-inch rear; cylinder speed was 2760 to 2905 fpm; feed-rate not including corn, was 190 to 370 lb per minute. Combine with snapping roll-clearances were 1 114-inch at front and 9/16-inch at rear; cylinder speed was 2750 to 2865 fpm; feed-rate, not including corn, was 37 to 85 lb per minute.) show that a difference in loss level exists. Important is that cylinder loss tends to increase with kernel moisture but with careful adjustment this loss can be maintained below 1%.

Some workers reported that kernel moisture does not correlate as well to cylinder loss as does cob moisture. Burrough and Harbage concluded that when the cob moisture was high, considerable cob breakage occurred because the cob was too "spongy" for good shelling.15 Their results, presented in Fig.10 (see Fig. 10 Effect of cob moisture content on the loss of unshelled kernels by an axial-flow shelling unit), were from a picker-sheller travelling at 4.0 km/hr.

Morrison, in making comparative tests between a combine and a conventional sheller, presented information on the effect of several crop and machine factors on the cylinder loss.

Fig. 11 Cylinder loss, combine with rasp-bar cylinder compared to a conventional stationary sheller.

Effect of Adjustment and Design

Packard conducted tests of crop and machine variables on the influence of cylinder loss. His data are presented in Fig.12 and 13 (see Fig. 12 Comparison of results for various bars with respect to shelling and crackage and Fig. 13 Effect of cylinder-concave clearance, kernel moisture, cylinder speed, and ear orientation upon shelling efficiency and kernel crackage). From this work he concluded:

Kernel Damage by Cylinder Actions.

Breakage is of concern because unless the fines are screened out they interfere with drying and increase the possibility of grain spoiling in storage.

Damage from a rasp-bar cylinder increases with peripheral speed, especially at speed above 15.2 m/s. 11.12 Laboratory studies have indicated that damage is considerably less when the ears are fed into the cylinder with their axes parallel to the cylinder axis, rather than longitudinally or In random orientation.13 Kernel moisture content has a significant effect on damage from either a combine cylinder or a cage-type sheller. Results from various studies 13,44 indicate that crackage increases as the moisture content is changed in either direction from a minimum crackage zone of about 20 to 22% (Fig. 14 Effect of kernel moisture content upon grain crackage from two types of shelling units, as determined with a 4.8-mm (12164-in.) screen.).

Shelling actions can be obtained by machine adjustment that will remove all kernels. However, any adjustment must also be considered in terms of the kernel crackage which results. The more severe the shelling action, the greater will be the crackage.

Crackage or damaged kernel ratio is determined by (a) visually separated, (b) screening by 16/64 (6.25 mm) and 12164 (4.80 mm) round hole sieves, or (c) a colorimetric technique.

The visual separation method is reliable for determining broken kernel ratio, however, it takes a lot of time, and also for the determination of injured kernel or slightly cracked kernels, it greatly depends on the person's skill.

The sieving method was developed for grading corn into a certain grade depending on the ratio of crackage and impurities. This method is good for determining marketing value of a certain lot of corn, but for evaluation of corn sheller especially in the ease of fungus infection concerns, this method might not be suitable.

A calorimetric technique was developed by Chowdhury 6,18 to provide a fast and reliable system of grain damage evaluation. The technique is based on determining the amount of surface area of the corn kernel that is damaged. The starch molecule of damaged tissues and cells are reacted with a fast green FCF dye, and subsequently, the amount of dye absorbed by the exposed starch molecules is established by extracting the dye with a dye-recovery solution. The concentration of the dye in the recovery solution is determined by measuring the percentage transmission of light through the solution by using a spectrophotometer.

The influence of moisture content on crackage are shown in Fig.13 and 15 (see Fig. 15 Effect of kernel moisture on the percentage of kernels cracked by the sheller.). 10,16 In all cases, crackage significantly increased as kernel moisture increased.

The data presented thus far have related crackage to kernel moisture content mainly at moisture contents above 20%. Barkstorm 17 and Gross 8 reported on tests where crackage was observed for moisture contents below 20% (Fig. 16 Crackage as influenced by kernel moisture and cylinder speed.). The trend in these data is one of increased crackage with decreasing kernel moisture content. There is no inconsistency between these data and those presented before. Crackage appears to reach a minimum in the 18-19% moisture range. At kernel moistures above 20%, the soft kernels are easily crushed from an impact loading. At the low moistures the kernel becomes hard and brittle. In this condition it again becomes easily fractured under the impact loading of the cylinder.

Fig. 17 Kernel crackage as related to moisture content using the 22-inch rasp-bar cylinder

Besides influencing the kernel crackage, the shelling mechanism also affects the cob breakage. The size of the cob discharged from the cylinder affects the resulting separating characteristics of corn from the cob. Table 2 presents the size distribution of cob parts resulting from a combine cylinder operated at 670 rpm and a 5/8-inch (15.63 mm) cylinder-concave clearance (rear).



In 1988, a Study on the relation between types of corn sheller, operational conditions and aflatoxin contamination was done by the Post-Harvest Section Maize Quality and Improvement Centre, DOA, Thailand.

This study was intended to clarify the relation between type of mechanized corn sheller, kernel moisture content, occurrence of mechanical damage to kernels and aflatoxin contamination, and then to contribute to corn sheller improvement.

Methods and Procedures.

Four types of corn sheller; namely, the rasp-bar cylinder sheller with special rubber attached on the spiral bars (ALVAN BRANCH Model AB/MS/8000S, England), the spike-tooth cylinder sheller (NIPPON SHARYO Model "NCR-1200': Japan), the plate-tooth cylinder sheller (a variation of the spike-tooth cylinder sheller: "LOTUS 77' Thailand), and the two row, spring sheller (CHIKUMA, Japan), were tested. All the cylinder shellers tested were of the axial-flow type.

Variable factors were the periphral speed of the cylinder or shelling disc and the kernel moisture content. Factors other than these two remained constant. The peripheral speed of the cylinder was set at three or four levels according to the results of performance tests and the standard cylinder speed of each machine prescribed by the manufacturers. Intervals of the speed was varied based on rpm, and each interval was 50 rpm. Proposed moisture contents were 32, 27, 23 and 18% (wb). It was intended that preparation of these was to be accomplished in the field from which the sample came.

Samples were harvested divided into four times at Koktum district in Lopburi Province, than the middle of August to early September. After harvest, ear corn was put in gunny sacks and kept one night underneath the farmer's house. On the next morning of the harvest day, the corn was delivered from Lopburi to the MQIRC, Bangkhen, Bangkok. The variety of corn was "Suwan 1".

To estimate suitable rpm levels for this trial and to fix feeding rate (Kg ear maize/min), performance test for each corn sheller was conducted prior to the shelling experiments. In this test, measurements of machine elements were also done.

Occurence of kernel damage and contamination of foreign material were investigated using samples extracted from corn kernels produced from approximately 400 kg of ear corn for the cylinder shellers, and 40 kg of ear corn for the spring sheller. In 1988, these procedures were done by the visual separation method.

Shelling efficiency was evaluated using samples extracted from the second and third outlet. All the kernels remained on the cobs were shelled manually, and both fractions were weighed to calculate the cylinder loss.

For four consecutive weeks after shelling, shelled corn samples each around 80 kg for the cylinder shellers and 20 kg for the spring sheller were stored under ambient air conditions, and during the storage period, samples for quantitative analysis of aflatoxin content were extracted on the 0, 1, 3, 7, 14, 21 and 28th day. Kernel moisture contents of the samples were determined by a calibrated single kernel moisture meter.

Results and Discussions

In the case of rasp-bar cylinder sheller, usually the rasp-bars are on the cylinder but in this sheller the raspbars are attached inside the concave drum.

In the spike-tooth sheller the cylinder teeth are columnar shape while traditional spike-tooth cylinder has rectangular teeth. This sheller does not have a blower, only a sieve. All the fine foreign materials and crackage were mixed with the whole kernel fractions.

The plate-tooth sheller is the most common type in Thailand, and is considered as a variation of the spiketooth cylinder sheller. The other type is the rectanglar spike-tooth closed cylinder sheller.

The spring sheller showed the lowest brakage in all moisture contents tested, but capacity is extreamly low because of its shelling mechanism. Also shelling efficiency of this sheller was extremely low to high moisture corn. For example, it was not able to shell about 41% of kernels by weight in the case of 32% m.c. while cylinder loss of the cylinder sheller was 0.5 to 5.0% at the same m.c.. (Table 3. Mechanical Damage on Kernels and Shelling Efficiency and continued)

Moisture content of kernels was deeply correlated with the broken kernel ratio. (Fig.33) For the plate-tooth cylinder and the spike-tooth cylinder sheller, an inflection point of the broken kernel ratio appeared at around 23% moisture content. (Table 3) As with Similar results before, the rubber attached on the cylinder was not effective in decreasing the damage kernel ratio.

Aflatoxin contents decreased as the initial moisture contents of samples decreased except in the case of 23% m.c.. The relation between the damaged kernel ratio and aflatoxin contamination was not clear for each moisture content. Evaluation method of crackage we took was the visual separation method. Determination amount of this method for slightly injured kernels was fluctuated largely depending on the operaters' skill. However, exposed starch molecule from the slightly injured parts is also attacked by A. flavus, and have possibilities to produce aflatoxin. Then in 1989 alternative evaluation method such as the fast dye method is necessary to take.

To introduce an improved machinery into users, improvement based on to the established and popular model might be a fast way. In 1988, the spike-tooth cylinder sheller performed a low breakage level among the cylinder shellers. Then the following years the studies on improvement of corn sheller is focused on the spike-tooth cylinder sheller and its variation in Thailand.

From the results in 1988 and also the results by Azuma (1988, unpublished and not shown), it is suggested that the length of the shelling part of the spike-tooth and plate-tooth cylinder sheller is too long, so duration of impact loading becomes unnecessarily too long. In 1989, the effects of the range of the teeth placed on the cylinder, the shape of the teeth, and arrangement of the teeth on crackage were examined.




Moisture content is one of the most important factors in quality control of grain especially in controlling fungus infestation. Products with a high moisture content will not keep for extended periods in storage, so it is important, therefore, that equipment be available for accurate determination of the moisture content. As aflatoxin contamination in corn is of major concern in Thailand, the necessity for suitable methods to determine moisture content with less time and higher accuracy assumes greater importance. The same equipment might also be used for determining the optimal time of harvest.

From our survey results, most of the farmers in Thailand do not own any devices to determine grain moisture content. In most cases, the farmers rely on visual and sensory grading techniques to estimate the moisture content. For grain, this might entail biting, rattling, feeling, estimating from the planting date or duration, and observation.

The moisture content is an index of the probable keeping quality of the product and can be expressed on either the wet or dry basis. Moisture meters for grain are generally designed to give percentage moisture content on a wet basis.

The method of determining the moisture content of products may be divided into two broad classifications: (1) direct and (2) indirect.

The direct methods are usually accepted as standards for calibration for and comparison with the indirect methods.

Regardless of the method used for determining the moisture content, there are possibilities of errors in making the determination. The major problem is that of securing a sample which is representative of the entire lot of material. To reduce the possibilities of error, several samples should be obtained from different locations in the bin or field. Usually a large sample is obtained which is taken to the laboratory for determining the moisture content. If the sample which is taken to the laboratory is not properly divided, errors may again occur. The sample can be divided equally into two parts with a suitable sample divider. These can be further subdivided for a suitable size of working samples.

The standard deviation of the moisture content of individual kernels in a sample taken from a bin of grain of uniform moisture content critically depends on the measuring device

Direct Methods

Several national and international organizations have developed standard air-oven methods for testing moisture content. These are organizations that deal primarily in cereals and cereal products.

1) International Association for Cereal Chemistry (ICC).

ICC Standard No. 110/1 specifies drying the ground seed of wheat, rice, barley, millet, rye and oats for two hours at 130-133 C. Ground maize seed is dried fore hours at 130-133 C.

2) International Organization for Standardization (ISO).

ISO adopted its moisture testing procedures from the ICC. The crops, and drying times in ISO Standard 712-1985 (E) are the same as in ICC Standard No.110/1.

3) International Seed Testing Association (ISTA).

ISTA procedures for cereals and maize described in Chapter 9 of the rules (Seed Science and Technology, vol.13, p.338-341, 493-495) conform to those of ISO. The procedures in detail are described in Appendix II.

4) European Economic Commission (EEC).

The EEC has also adopted ICC Standard No.110/1 as its procedure.

5) American Association of Cereal Chemists (AACC).

AACC Method 44-15A specifies that soybeans, rice, peas, lentils and grain are to be ground and dried one hour at 130 + 1 C. Whole seed of maize and beans are dried 72 hours at 103 + 1 C. Whole seed of flax are dried four hours at 130 + 1 C.

6) Association of Official Analytical Chemists (AOAC).

AOAC Method 14.063 prescribes a drying period of one hour at 130 + 3 C for ground wheat, rye, oats, maize, buck wheat, rice and barley.

7) Official Grain Standard of USDA.

The Official Grain Standard specify the same methods and crops as AACC.

8) American Society of Agricultural Engineers (ASAE).

ASAE Standard S352 lists oven temperatures and heating times for whole seeds of 31 kinds of agricultural crops. Most of these methods were developed by Hart, Feinstein and Golumbic and published in USDA Marketing Research Report 304 in 1959. The procedures in detail are described Appendix I.



Direct methods that are widely accepted are called basic methods. Basic methods do not require calibration against some other methods and are themselves used in calibrations. Several basic methods have been used for cereals, oil seeds and edible legumes, but there is no general agreement among countries as to which method is best. Several basic methods that have been used in developing air-oven methods for grain are described here.

1) Air-Oven Methods

Air-oven methods are widely used and have been officially adopted by numerous governmental agencies and international organizations. They are relatively simple and inexpensive to conduct and give reproducible results between laboratories. Most technique development has been with cereals, oil seeds and edible legumes, and these methods are often assumed to be adequate for other seeds.

In performing the air-oven method, a weighed quantity of seed is heated at a certain temperature for a specific period of time. The loss in weight during heating is considered to be the moisture content of the sample. However, no single air-oven procedure can be used with the same accuracy on al kinds of seed. Results vary depending on the time and temperature used. Drying to constant weight is not satisfactory because different constant weights are obtained at different temperatures. In many cases, however, temperature and time of heating appear to have been established empirically, based on attainment of constant weight.

Water exists in seeds as "free" water, which is loosely held by capillary forces, and as "bound" water, which is more tightly held. It is difficult to remove all the water without also removing other volatile materials, or causing chemical decomposition that may produce more water, or increasing dry weight through oxidation. The apparent moisture content at any drying temperature is thus a balance of all these factors. Although oven methods are considered basic, they are in fact rather empirical, with results depending on the testing conditions. Proper testing conditions for greatest accuracy must be determined by calibration with another primary method that does not depend on drying temperature and time.

2) Vacuum-Oven Methods

In vacuum-oven methods, seeds are dried a temperatures lower than 100 C in a partial vacuum to reduce the amount of other volatiles released. Temperature still plays a role in determining the apparent moisture content at constant weight, even under vacuum. Non-aqueous volatiles may still be lost because they are released at lower temperatures under vacuum. Since all oven methods are empirical, accurate air-oven methods may not necessary be established by comparison with drying to constant weight in a vacuum-oven.

Air-oven methods developed by the ICC for cereals correspond to the results obtained with the vacuumP2O5 method. In this method, phosphorus pentoxide is placed in the drying tubes to absorb the moisture released from the seeds. ICC Standard No.109/1 specifies drying ground seed to constant weight at 50C, a procedure requiring approximately 150 hours or more.

ISO has adopted the basic method of ICC. The crops, temperature and times in ISO Standard 7111985 (E) are the same as in ICC Standard No.109/1.

The air-oven methods of the AACC were developed to correspond with the results obtained after drying to constant weight (more than five hours) at 98 C -100C in a vacuum-oven (AACC Method 44-40).

The AOAC methods appear to be identical to those of the AACC. The vacuum-oven method is described in AOAC Method 14.003.

3) Distillation Methods

In distillation methods, seeds are placed in a distilling liquid and distilled until no more water is given off. The water is collected and measured by volume. The results obtained depend on the boiling temperature of the liquid used. Toluene is most commonly used and gives satisfactory results in most cases.

Table 4. Caracteristics of Dessicants

Dessicants Amount of Water Vapour in the Dry Air (g/m) Drying Temperature for Reclamation
Anndrous calcium chloride 0.2 300
Anhydrous calcium sulfate 0.004 230 ~ 250
Phosphrous pent- oxide 0.00002 unable
Molecule sieve 0.0001 200 ~ 400
Activated alumina 00018 180
Silica gel 0.006 150

4) Karl-Fischer Methods

The Karl-Fischer procedure is frequently used as a basic reference method for other procedures. It is useful in measuring moisture content of solids, liquids and gases, and is appropriate for use on seeds. Both "free" and "bound" water are measured. A major advantage is that results are not dependent on temperature or duration of drying. In this procedure, moisture is extracted from a ground sample of seed with methanol and kept with Karl-Fischer reagent to determine the amount. The method is superior to oven methods because the reagents are specific for determination of water, and prolonged heating at high temperature is avoided.

The air-oven methods of the ASAE for whole seeds of 31 kinds of crops were taken from USDA Marketing Research Report 304 and are all calibrated against the Karl-Fischer methods.

5) Other Basic Methods

Near-infrared spectrometry, gas chromatography (GC), neutron technology moisture analysers nuclear magnetic resonance (NMR) and other basic methods for determining moisture content have been developed. Their use on grain has been limited and they still do not have played an important role in moisture determination.



Indirect methods involve the measurement of a property of the material including mechanical, electrical, or thermal property, which is related to the moisture content. One of the direct means is required to calibrate the indirect method. The moisture content is usually expressed on a wet basis for the indirect methods.

1) Electrical Resistance Methods

The electrical resistance or conductivity of a material depends on its moisture content. This principle is used as a basis for a number of moisture meters. These moisture meters must be calibrated for each grain against a standard method. In as much as temperature affects the electrical resistance of a material, corrections for variations in temperature must be made for tests conducted at temperatures other than those at which the calibration is reported. The electrical resistance units are rather simple in design and require only a few seconds for making a moisture determination. The resistance of grain is measured between two steel rolls or plates which serve as electrodes. A different spacing of rolls or plates can be made. One of the problems with the meter is the difficulty of maintaining calibration because wear of the bearings, rolls or springs changes the spacing between the electrodes and gives a lower moisture content. By proper maintenance, however, improper moisture content value due to spacing can be minimised. The relationship between moisture content and resistance is indicated in the following equation;

log p = — am + c or log (log 1/p) = bm

where a, b and c are constants; m is moisture content; p is resistance.

The pressure exerted on the sample with the electrical resistance method affects the resistance of the product. A straight line must result when moisture content determinations are plotted against oven determinations, so calibration consists only of determining the compression data which will give a straight line on a 45 slope.

The electrical resistance of grain decreases when the pressure is increased.

Above 17% moisture content, there is a parabolic relationship between the moisture content and the logarithm of the electrical resistance. Most meters do not give readings below 7% moisture, because there is very little change in the electrical conductivity. Grain that has been recently dried with heated air gives lower readings than the actual moisture content. This occurs because the tendency of these meters is to measure the resistance on the surface. If moisture has been added to the grain, the readings are higher than the actual moisture content of the product.

The characteristics of the resistance type moisture meter are summarised:

2) Dielectric Methods (Capacitance)

The dielectric properties of products depend on the moisture content. The capacity of a condenser is affected by the dielectric properties of the material placed between the condenser plates. Wet materials have a high dielectric constant, and dry materials have a low dielectric constant. Water has a dielectric constant of 80 at 20C Most grains have a value less than 5, and air in a vacuum has a value of 1.

The characteristics of the capacitance type moisture meter are summarised:

Dielectric constant (capacitance) gauges consist of a pair of electrodes that set up a radio frequency field. As product the space containing the electrodes, dielectric properties of the process material change the radio frequency field. These changes are measured, and the moisture content is inferred, because the dielectric properties of water are different from most dry substances.

The dielectric constant principle was the first reasonable approach to moisture measurement in the grain industry. Such manufacturers as Motomco and DlCKEY-john were pioneers in grain industry moisture measurement.

Cost of this technology is moderate, but the main disadvantage is its sensitivity to composition and particle size changes. Fat content variations, protein content variations, or slight changes in product formulation have a very noticeable effect on moisture accuracy. In grain industries, where these gauges are frequently used, almost every hybrid variety of the grain needs its own calibration curve. The measurement is sensitive to temperature and bulk density changes. Freshly wetted products or products with surface moisture are not suitable for measurement with this technology. The relationship between dielectric constant and moisture content is not linear.

Details of Some Available Proprietary Moisture Meters

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