Check of the calibration of moisture meters

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Part 1: Moisture meters for cereals

INTRODUCTION

The calibration of moisture meters may, for stable samples and ideal measuring conditions, prove entirely satisfactory. On the other hand, the results obtained with the same moisture meter can be affected by many variables of cultivation, ripeness, humidity, temperature, harvesting, transport and level of impurities, particularly for cereals received with high moisture content.

SCOPE AND FIELD OF APPLICATION

This part of ISO 7700 specifies a method of checking the calibration of moisture meters in service for measuring the moisture content of cereal grains, by checking some values or a range covering all the values for which the moisture meter is used.

It is applicable to oats, durum wheat, wheat, maize, barley, rice, rye and sorghum.,

REFERENCES

ISO 712, Cereals and cereal productsDetermination of moisture content (Routine method).

ISO 5223, Test sieves for cereals.

ISO 6540, Maize-Determination of moisture content (on milled grains and on whole grains).

PRINCIPLE

Preparation of several test samples, or a range of test samples, with different moisture contents, under specified conditions, determination of their moisture contents by a reference method and measurement with the moisture meter to be checked.

REAGENT

Use only distilled water or water of equivalent purity.

Sodium hypochlorite (bleach) solution of approximately 5, 7 % (m/m) active chlorine (18 chlorometri degrees).

APPARATUS

Usual laboratory apparatus, and in particular,

Bottles, with airtight seals, of capacity approximately 21, cleaned with a bactericide and fungicide, such as the bleach (4.1), rinsed three times with distilled water and dried.

Apparatus required for the routine reference method of determining moisture content (see ISO 712 of ISO 6540).

Sieves, for cleaning the grain, complying with the requirements of ISO 5223, and in particular sieves with long rounded apertures of width 1,80; 200; and 2,24 mm and with round holes of diameter 4,50 mm, or a mechanical separator.

PROCEDURE

Selection and cleaning of samples

Select a variety or varieties or better still a mixture of varieties of a cereal from those which are the most prevalent in the region where the moisture meter is used. In the case of maize, the choice depends on the type of grain (dent, flint, dent-flint rather than on the variety.

Clean the samples by removing undersize material, including shrivelled grains, by manual sieving using appropriate sieves (see 5.3) and removing larger impurities by hand or using a mechanical separator (5.3).

As an indication, use the following sieves:

• sieve with long rounded apertures of width 1,80 mm for rye and durum wheat;
• sieve with long rounded apertures of width 2,00 mm for wheat;
• sieve with long rounded apertures of width 2,24 mm for barley;
• sieve with long round holes of diameter 4,50 mm for maize.

Preparation of test samples

1. Procedure when checking several values

1.1 According to the number of values to be checked, prepare a greater or lesser number of test samples, each having a mass of approximately 1 kg and various moisture contents between 10 and 25 % (m/m) for cereals other than maize, and between 12 and 45 % (m/m) for maize.

Preferably choose moisture contents close to those which are most frequently observed iri the region where the moisture meter is used.

1.2 To prepare the test samples, in the case of cereals other than maize, use by preference the samples selected in which in their natural state have the moisture contents necessary for the test, or if necessary samples specially conditioned by the procedure specified in 3.

In the case of maize, it is essential to select samples which, in their natural state, have the moisture contents necessary for the test, as any drying or wetting is prohibited for instruments measuring electrical properties.

Place the samples in the bottles and seal them.

2. Procedure when checking a range

2.1 Prepare a range of 10 test samples each having a mass of approximately 1 kg and various moisture contents, at intervals as regular as possible, between 10 and 25 % (m/m) for cereals other than maize, and between 12 and 45 % (m/m) for maize.

2.2 To prepare this range of test samples, proceed as specified in 1.2.

3. Conditioning of samples

This subclause does not apply to maize; maize shall not be conditioned (see 1.2).

3.1 If conditioning is necessary, take a sample having a moisture content at the time of harvesting equal to or less than the lowest moisture content selected for the calibration range, or failing that, bring the required quantity of sample selected to this minimum moisture content by drying very gradually at a temperature not exceeding 30°C, using ventilation if necessary.

3.2 For each desired moisture content, place in a bottle a quantity of the sample such that the bottle is between a half and two-thirds full.

3.3 Calculate the quantity of distilled water necessary to bring each sample to the selected moisture content, using the formula

where

H₁ is the moisture content, expressed as a percentage by mass, of the sample;

H₂ is the moisture content, expressed as a percentage by mass, selected for the calibration.

Using a burette or graduated pipette, add in rapid drops to esch sample, in one or two portions, as the case may be (see 3.4), the volume of distilled water thus calculated, while shaking the bottle. Seal the bottle.

3.4 In order to ensure that the water is evenly distributed, shake the bottles by inverting them by hand, under the following conditions:

- If the difference between the desired moisture content and the moisture content of the sample is less than 10 % (m/m) (absolute), add the quantity of the water calculated above in one portion, and shake energetically over 4 days as described in table 1, taking great care to turn the bottle upright again after shaking.

- If the difference between the desired moisture content and the moisture content of the sample is more than 10 % (m/m) (absolute), add the quantity of the water calculated above in two equal portions at an interval of 24 h and shake energetically at regular intervals over 5 days as described in table 2, taking great care to turn the bottle upright again after shaking.

In all cases, the bottles shall be kept at a temperature of approximately 5°C, for example in a refrigerator.

NOTE- Instead of shaking by hand, it is possible to use an apparatus allowing either very slow, continuous shaking to be carried out over $ days at 5°C, or vigorous shaking in accordance with the timetable given in Tables 1 and 2 for manual shaking.

Table 1

Table 2

Checking the moisture meter

1. Procedure when checking several values

1.1 If the test samples have not been conditioned, ensure that they have been kept under the same temperature conditions as the moisture meter before the test; if not keep them under these conditions to allow them to reach thermal equilibrium with the moisture meter. If the test samples have been conditioned, remove the bottles from the refrigerator at feast 16 h (usually overnight) before the test, to allow them to reach thermal equilibrium with the moisture meter. In all cases, note the temperature at which thermal equilibrium is reached.

Reject the test samples if they emit an odour of fermentation or are mouldy (in the case of a range, it is necessary to start again).

1.2 On each previously mixed test sample, carry out the following operations:

a) Determine the moisture content by the routine reference method specified in ISO 712 or, in the case of maize, in ISO 6540.

Take as the result the arithmetic mean of two determinations. It is imperative to comply with this condition.

b) Using the moisture meter, carry out four successive measurements using four test portions taken from the test sample.

In the case of moisture meters designed to take readings on whole grains, which in general require large test portions, transfer each test portion back into the bottle containing the test sample after each measurement, and mix by shaking the bottle before taking a new test portion.

In the case of moisture meters for taking readings on ground grains, which, in general, require small test portions (less than 509), carry out the grinding and measurement strictly according to the manufacturer's instructions. Discard all test portions after use.

c) After four measurements have been taken, again determine the moisture content by the routine reference method, proceeding as described in a).

2. Procedure when checking a range

On each test sample, carry out the same operations specified above and repeat the operations specified in 1.2 at an interval of 24 h using the same test samples for cereals other than maize, and different test samples for maize.

EXPRESSION OF RESULTS

Procedure when checking several values

1. For each test sample, the following values are available:

- two results obtained by the routine reference method, x.

The difference between these two results shall not exceed 0,15 9 of moisture per 100 9 of sample for products not requiring preconditioning (defined in 6.2.3), and 0,20 9 of moisture per 100 g of sample for products requiring preconditioning. Otherwise, repeat the test;

- four measurements carried out with the moisture meter, y.

2. For each test sample, calculate the difference between the result of each measurement carried out with the moisture meter, y, and the mean of the two results obtained by the routine reference method, x, i.e. y - x.

The values y - x shall be less than the maximum permitted errors such as those specified in the Annex.

Procedure when checking a range

1. Deal separately with the two series of measurements carried out at an interval of 24 h and compare them to ensure that there has been no development of the grain (maize excluded) and/or variation in the response of the moisture meter over this 24 h period.

2. For each test sample and for each series of measurements, the following values are available:

- two results obtained by the routine reference method, x.

The difference between these two results shall not exceed 0,15 9 of moisture per 100 9 of sample for products not requiring preconditioning and 0,20 9 of moisture per 100 9 of sample for products requiring precondi-requiring preconditioning. Otherwise, repeat the test.

- four measurements carried out with the moisture meter, y.

3. For each test sample and for each series of measurements, calculate the difference between the mean of the four measurements carried out with the moisture meter, y, and the mean of the two results determined by the routine reference method, x, i.e.y-x.

For each series of measurements, the values of y x should be less than the maximum permitted errors, such as those specified in the annex. If a value of y - x is greater than the maximum permitted error, repeat the measurements on the corresponding test sample.

TEST REPORT

The test report shall show the method used, the test temperature, the type of moisture meter used and its precision class, and the results obtained. In addition, it shall mention any operating details not specified in this International Standard, or regarded as optional, as well as any incidents likely to have influenced the results.

The test report shall include all the information necessary for the complete identification of the sample.

MAXIMUM PERMITTED ERRORS

The maximum permitted errors for moisture meters In service, In accordance with draft OIML International Recommendation No. 59 concerning moisture meters for cereals and oilseeds, are as follows.

A.1 Class I moisture meters (see also the figure)

- For cereals other than maize, rice and sorghum: 0,7 (absolute) for a moisture content, x, less than 10 % (m/m); 0,4 (absolute) plus 3 % (relative) for a moisture content, x, greater than 10 % (m/m).

- For maize, rice and sorghum: 0,8 (absolute) for a moisture content, x, less than 10 % (m/m); 04, (absolute) plus 4 % (relative) for a moisture content, x, greater than 10 % (m/m).

A.2 Class II moisture meters

• For cereals other than maize, rice and sorghum: 0,8 (absolute) for a moisture content, x, less than 10 % (m/m); 0,4 (absolute) plus 4 % (relative) for a moisture content, x, greater than 10 % (m/m).
• For maize, rice and sorghum: 0,9 (absolute) for a moisture content, x, less than 10 % (m/m); 0,4 (absolute) plus 5 % (relative) for a moisture content, x, greater than 10 % (m/m).

Details of Some Available Proprictary Moisture Meters

Table

Table (continued)

Table (continued)

Table 1 - Oven Temperature and Heating Period for Moisture Content Determinations

REFERENCES

1. PINSON, G.S. and D.J. WALKER, A Comparison of Maize Shelling Methods for Small Scale Farmers., Tropical Science, 25:3-18 (1985)
2. ARNOLD, R.E. Experiments with rasp bar threshing drums. I: Some factors affecting performance. J. Agricultural Engineering Research, 9.99-131 (1964)
3. DODDS, M. Power requirement of a self-propelled combine. Canadian Agricultural Engineering, 10:7476 (1968)
4. PICHARD, G.E., Laboratory Studies of Corn Combining, Agricultural Engineering Dec. (1955)
5. USDA, Hugs and Squeezes., Agricultural Research, 16 (Nov), 17 (1967)
6. CHOWDHURY, M.H. and W.F. BUCHELE, Effects of the Operating Parameters of the Rubber Roll Sheller., Transactions of the ASAE, 18:482-486 (1975)
7. JOHNSON,W.H., B.J.LAMP,JR., lE.HENRY,G.E. HALL, Corn Harvesting Performance at Various Dates, Transactions of the AEAE, 6:268-272 (1963)
8. GOSS,J,R., ROY BAINER, R.G.CURLEY, D.G. SMELTZER, field Tests of Combines in Corn, Agricultural Engineering, 36:794 (1955)
9. MORRISON,CS., Attachments for Combining Corn, Agircultural Enginnering, 36:797 (1955)
10. PICKARD,G.E., Laboratory Studies in Corn Combining, Agricultural Engineering, 36:792 (1955)
11. LAMP, B.J., Jr., Corn Harvesting 1959-60, Department Report, Department of Agricultural Engineering, Ohio Agricultural Expeiment Station (1960)
12. MORRISON,E.G. Harvesting corn with combine. Agricultural Engineering, 10:231-232 (1929)
13. MAHMOUD,A.R.,& W.F.BUCHELE. Corn ear orientation effects on mechanical damage and forces on concave., transactions of the ASAE, 18(3):444-447, 452 (1975)
14. BYG,D.M.,& G.E.HALL. Corn losses and kernel damage in field shelling of corn. Transactions of the ASAE, 11(2):164-166 (1968)
15. JOHNSON, W.H.,& B.J.LAMP. Principles Equipment and Systems for Com Harvesting. Agricultural Consulting Associates, Inc., Ohio (1966)
16. BURROUGH, D.E. and R.P.HARBAGE, Performance of a Corn Picker-Sheller, Agricultural Engineering, 34:22 (1953)
17. BARKSTROM,R., Attachments for Combining Corn, Agricultural Engineering, 36:800 (1955)
18. CHOWDHURY, M.H. and W.H. BUCHELE, The Nature of Corn Kernel Damage Inflicted in the Shelling Crescent of Grain Combines., ASAE paper No.76-1557, (1976)
19. GRAIN DIVISION, CONSUM,ER AND MARKETION SERVICE IN USDA, "Oven Methods for Determining Moisture Content of Grain and Related Commodities." (1971)
20. T.BAN et al., Japanese J. of Agricultural Engineering, 34: No.4 (1973)

ASAE Standard: ASAE S352

Developed by the ASAE Physical Properties of Agricultural Products Committee; approved by Electric Power and Processing Division Standards Committee; adopted February 1972; reconfirmed for one year, December 1976.

Section 1 - PURPOSE AND SCOPE

1.1 This standard is to provide a uniform method for determining the moisture content of unground samples of agricultural seeds. Only those seeds are included for which documented comparisons with the Karl Fischer method are available. These techniques should become common practice and their use referenced in all technical presentations where moisture content determinations have been a factor. Deviations from these methods should be reported.

1.2 S'pecifying sampling procedures is not within the scope of this recommendation. It will be assumed that the portion used for analysis is representative of the sample as a whole.

Section 2 - APPARATUS

2.1 Moisture dishes. Moisture dishes should be made of heavy gauge aluminum that does not dent readily. The dishes should be provided with tightly fitting covers. Both the dish and its cover should be identified by the same number. Before using, dry the moisture dish for one hour at the drying temperature to be used and obtain the tare weight. Other similar containers may be used provided they are tarred before and after drying and their use is mentioned in referring to the moisture measurement techniques used.

2.2 Desiccator. Desiccator should be airtight and should contain activated alumina or other equally suitable desiccant. Silica gel and anhydrous calcium are not suitable desiccant.

2.3 Oven. Oven may be of gravity-convection or mechanical convection (forced-draft) type. It should be well insulated, maintain a reasonably uniform temperature throughout the chamber and maintain specified temperature at shelf level. A properly ventilated oven, equipped with removable perforated or wire shelves and a suitable thermometer accurate to within 0.5 deg C, is required to insure uniformity of heating, ovens should be in operation for several hours prior to the drying operation.

2.4 Balance. An analytical balance should be used for all weighings in making moisture content determinations. Weighings should be made to the nearest 0.01 9 or about 0.001 of the total sample weight.

Section 3 - PROCEDURE

3.1 Place a minimum of 15 9 of a representative portion of the unground sample in each of two or more tarred moisture dishes. For high-moisture content seeds (over 25 %), use 100 9 portions instead of 15 9 portions for the determination. Weigh the covered dishes and contents. Subtruct the weight of each dish from the total weight of the portion. Uncover the dishes and place them with their covers in the oven. The oven temperature and heating period depend on the seed as shown in Table 1.

3.2 The dishes should be placed with the bulb of the oven thermometer as close to them as possible. At the end of the heating period, cover the dishes as soon as possible and place them in a desiccator. Weigh the dishes when they reach room temperature. Calculate the percentage of moisture by dividing the loss in weight due to heating by the weight of the original sample and multiply by 100. Replace determinations should check within 0.2 percent moisture.

(cited from "AGRICULTURAL ENGINEERS YEARBOOK" p.417, (1977))

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