NIST Handbook 143
State Weights and Measures Laboratories Program Handbook

7.6 Technical Criteria for Moisture Laboratories - Grain and/or Commodities (OWM Technical Guide)

7.6.1 Scope

7.6.1.1 The purpose of this section is to specify the specific technical criteria needed to meaningfully assess the competence of a testing laboratory that performs moisture analyses.

7.6.1.2 This document may also be used as a guide by moisture laboratories in the development and implementation of their quality systems.

7.6.1.3 Laboratories may be recognized based on procedures used to determine moisture. These four procedural categories identify the method of moisture determination and the reference methods. The scope of recognition follows the categories as defined in Table 13. Laboratories may be recognized under two or more categories if they have the necessary equipment, skills, knowledge, measurement control and demonstrated competence to perform adequately in each category.

NOTE: The national laboratory for grain moisture in the United States is the Grain Inspection Packers and Stockyards Administration (GIPSA) formerly known as the Federal Grain Inspection Service (FGIS). The standard method recognized by the GIPSA laboratory is the Air Oven Method. Grain testing laboratories should reference the Air Oven Method and participate in interlaboratory comparisons to ensure comparability of measurements with the national laboratory.

Table 13. Moisture determination laboratory classifications
 

7.6.1.4 Laboratories should document the grains and/or commodities tested and maintain replicate results within ± 0.2 percent. The scope of recognition should indicate identified grains and/or commodities and the uncertainty limits. Uncertainty analysis should be completed according to the ISO Guide for the Expression of Uncertainty in Measurement.

7.6.1.5 Important definitions in this section follow:

Moisture content The loss in mass, expressed as a percentage, undergone by the product under the conditions specified in this document.

Air Oven A device which utilizes gravity convection or mechanical convection (forced-draft) to heat air in a chamber to its controlled temperature.

Vacuum Oven A device which utilizes a vacuum to alter the pressure within the oven chamber and maintain a temperature at that specific pressure.

Karl Fischer Titration A procedure that involves the simultaneous grinding of grain/commodity and extraction of the water with methanol and the subsequent titration of the extract with Karl Fisher reagent.

Basic Reference Method An ISO procedure (ISO Guide 711) utilizing oven and chemicals to determine moisture content of a product.

7.6.2 References

[1] United States Department of Agriculture, Grain Inspection Packers and Stockyards Administration, (GIPSA) Moisture Handbook, September, 1986.

[2] American Society for Testing and Materials (ASTM) E 145-68, Standard Specification for Gravity-Convection And Forced-Ventilation Ovens (reapproved 1987).

[3] Association of Official Analytical Chemists (AOAC), Official Methods of Analysis, 15th edition, Volume one and two, 1990.

[4] AOAC, Automatic Karl Fischer Titration of Moisture in Grain, Volume 64, No. 6, 1981.

[5] Anderson, J. E. "Storage of Cereal Grains and Their Products," Chapter 1, Moisture - Its Significance, Behavior, and Measurement. (Hunt, W. Haward and Pixton, S. W.).

[6] ISO 711, International Standard, Cereals and Cereal Products - Determination of Moisture Content (Basic Reference Method) Second edition, 1985.

[7] Scholz, Eugen "Karl Fischer Titration," 1984.

7.6.3 Statistical Process Control

7.6.3.1 An appropriate statistical process control system should be in place including procedures for measurement verification through participation in interlaboratory comparisons between the laboratory, GIPSA and other laboratories.

7.6.3.2 It is critical that testing results remain consistent with previous laboratory test results; repeated tests should remain within the laboratories documented uncertainties and be evaluated using statistical methods.

7.6.3.3 A laboratory should have an operating procedure to review data for trends and failures to indicate where failure has occurred. Records of corrective action, repair, or recalibration should also be maintained.

7.6.4 Accommodation and environment

7.6.4.1 To make adequate measurements, moisture determination laboratories should have a facility with adequate environmental controls appropriate for the laboratory classification.

7.6.4.2 Due to the extreme hygroscopic properties of ground grain (oven methods) and methanol (Karl Fischer chemical method), humidity and temperature should be controlled within the following parameters:

• Temperature: 20 °C to 23 °C, set point ±2 °C, maximum change of 1.0 °C/h; and

• Humidity: 30 percent to 55 percent.

NOTE: Sample exposure to the environment should be minimized throughout the collection, storage, and testing process.

7.6.4.3 To prevent contamination of test samples, from grain and/or commodity dust particles, adequate cleanliness should be maintained.

7.6.4.4 Adequate space should be available for sample storage, preparation and testing and be appropriately organized for efficient operation. Laboratory equipment should be logically placed to accommodate handling of samples, operation and maintenance of equipment and to prevent accidental sample substitution.

7.6.4.5 Laboratories should refrigerate storage areas for grain and/or commodities under test. Storage areas should be maintained between 1.5 °C and 7.0 °C.

7.6.4.6 Excess vibration may adversely affect the performance of precision analytical balances and mass comparators and should be prevented. Proximity to heavy machinery, railways, heavily traveled highways, or similar sources of known vibration is not recommended. Steps should be taken to reduce vibration to an acceptable level of stability with methods such as isolated foundations for balances or elimination of the source.

7.6.4.7 Laboratories should ensure that electrical disturbances which affect moisture results are kept at a minimum.

7.6.4.8 Undesirable effects due to static electricity should be controlled as needed by maintaining appropriate laboratory humidity or with anti-static deionizing radiation devices. The grounding of balances or operators, or the use of special conductive flooring may also be used.

7.6.5 Equipment and Reference materials

7.6.5.1 Standard Reference Materials

7.6.5.1.1 Category Ia and Ib laboratories - The grains and/or commodities are prepared for use as reference standards according to documented procedures for oven testing. During preparation, storage and distribution, care should be taken to ensure reference sample integrity.

7.6.5.1.2 Category IIa and IIb laboratories - Specified reagents are used according to manufacturers' instructions for the specific type of Karl Fischer titration equipment in use. Category IIb laboratories generally should use chemicals specified in International Standard ISO 711, Sec. 7. During sample preparation, the integrity of the reagents and samples is critical.

7.6.5.2 Primary Equipment - Oven methods

7.6.5.2.1 Category Ia laboratories should have appropriate equipment for the moisture determination of grain and/or commodity moisture: mechanical (forced draft) or gravity convection ovens, desiccator, mill, moisture dishes, and analytical balance. Reference GIPSA Moisture Handbook, Chap. 4 for equipment specifications.

NOTE: The use of a mechanical convection (forced draft) oven is strongly recommended. Tests have shown that ovens of the forced-draft type have more uniform temperature due to heat circulation, the temperature recovery is more rapid after insertion of samples and will accommodate more samples than gravity convection ovens.

NOTE: Laboratories should have temperature monitoring devices with suitable accuracy and traceability to record oven temperatures during moisture testing.

7.6.5.2.2 Category Ib laboratories should have appropriate equipment for the moisture determination of grain and/or commodities: vacuum oven desiccator, mill, moisture dishes, and analytical balance with the same requirements as class Ia laboratories. Reference AOAC, Official Methods of Test for equipment specifications.

7.6.5.3 Primary Equipment - Karl Fischer chemical analysis

7.6.5.3.1 Category IIa laboratories should have appropriate equipment for the moisture determination of grain and/or commodities: Karl Fischer Titration Assembly (manual or automatic with a stirrer), mill, centrifuge, and analytical balance. Reference manufacturers' instructions and Karl Fischer Titration, Eugen Scholz, 1984.

7.6.5.3.2 Category IIb laboratories should have appropriate equipment for the moisture determination of grain and/or commodities: analytical balance, apparatus for reducing pressure, mill, dishes, cup, drying tube, oven, air-drying train, and desiccator. Reference International Standard ISO 711 for equipment specifications.

7.6.5.4 Secondary Equipment

7.6.5.4.1 Laboratories should measure air temperature and relative humidity of the laboratory environment. Instruments are used in close proximity to the grain and/or commodity tested. Laboratory ovens should be equipped with calibrated, accurate and traceable thermometers (accurate within ± 0.5 ° C ) to ensure the accuracy of the oven temperature.

7.6.5.4.2 Samples should be stored in clean, air-tight and moisture-proof glass containers or plastic bags to prevent the loss or gain of moisture in the product. Plastic bags should have a thickness of at least 0.1 mm (4 mil, 0.004 in). A thickness of 0.15 mm (6 mil, 0.006 in) is preferred due to greater durability.

7.6.6 Measurement traceability and calibration

7.6.6.1 Since direct traceability to a standard (national measurement unit) is not possible for moisture measurements of the type described here, and the Federal standard for grain moisture is defined by comparison to GIPSA results, it is critical for the laboratory to participate in interlaboratory comparison programs to establish "comparability" of the results between the laboratories.

7.6.6.2 It is essential that moisture testing equipment that affects the accuracy of the moisture measurement (e.g., to include thermometers and balances) be periodically calibrated or verified for accuracy and traceability.

7.6.6.3 Balances used in determining moisture content should be periodically verified or calibrated prior to use.

7.6.6.4 Instruments used to monitor environmental conditions in the laboratory should be periodically recalibrated. Appropriate calibration intervals should be established and documented by the laboratory.

7.6.7 Moisture determination methods

7.6.7.1 The types of moisture determination methods employed, and equipment used, should be appropriate for the grains and/or commodities being tested based on the procedural category. Measurement methods should provide acceptable repeatability. A documented procedure for each method should be available.

7.6.7.2 Computer programs used in the moisture determination of grains and/or commodities should have passed software quality analysis. Computer programs may be tested, using standard data sets designed to magnify errors, as an effective way of showing that program errors do not effect some measurements but cause others to be incorrect. Computer programs should be documented in detail. Documentation should include technical references that provide the basis for the algorithm, the moisture equations, and the data set used to test the program for errors.

7.6.8 Handling of grain and/or commodities for moisture determination

7.6.8.1 The laboratory should have documented procedures to ensure that all samples are properly and uniquely marked upon receipt to include: the type of sample and a nonrepetitive identification number to ensure adequate tracking of samples in laboratory and field.

7.6.8.2 The laboratory should document appropriate procedures for proper cleaning and storage of samples to ensure integrity.

7.6.8.3 Documented procedures should be maintained that ensure that subportions of the sample under test are representative of the whole.

NOTE: Care should be taken during sample collection and handling to prevent contamination of the sample with body oil or moisture, and to protect the sample from extreme environmental moisture and temperature.

7.6.9 Test Reports

7.6.9.1 Reports when necessary, should include: moisture determination results, sample type, condition and identification number, type of moisture determination used to ascertain the results, and the precision of measurement (repeatability between the dishes or duplicate runs.)

7.6.9.2 Environmental parameters measured during the test should be provided on reports. These include laboratory temperature and relative humidity.

7.6.9.3 Information regarding cleaning methods (if performed) should be provided on the test report.