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United States Department of Agriculture

Agricultural Research Service

Title: Microwave Sensing of Moisture in Granular Solids

item Nelson, Stuart
item Trabelsi, Samir - UNIV OF GEORGIA

Submitted to: International Symposium on Humidity and Moisture
Publication Type: Proceedings
Publication Acceptance Date: September 16, 2002
Publication Date: September 30, 2002

Interpretive Summary: Moisture content of granular solids is an important characteristic of such materials in the agricultural, food, pharmaceutical, construction, and materials processing industries. It is the most important characteristic of cereal grains affecting their suitability for harvesting, storage, transport, and processing. It is also an important factor affecting the price paid for the grain. Therefore, moisture content must be determined whenever grain is traded. If moisture content is too high at time of harvest, the grain kernels can be damaged in the mechanical harvesting process, leaving them more susceptible to infection by fungi. If they are stored at moisture contents too high for the prevailing environment, they can spoil because of the action of microorganisms, and the value is degraded or completely lost for human and animal consumption. Electronic moisture meters have been developed and used for many years to provide quick moisture determinations. However, instruments to monitor grain moisture content in moving grain for on-line applications have not been entirely satisfactory, because changes in the grain packing or grain temperature can give large errors in moisture content. Research on microwave measurements for sensing grain moisture content has provided means for determining moisture content independent of changes in packing and compensated for grain temperature. The general principles involved in using microwave measurements for sensing grain moisture are presented in this paper. The technique is illustrated with data on the microwave dielectric properties (electrical characteristics) of wheat, and it is shown that moisture content can be reliably sensed through measurement of these dielectric properties and use of moisture calibration equations that depend only on the dielectric properties. Thus, the measurement is independent of bulk density of the grain (degree of packing) and it can be compensated for variations in grain temperature. Further, research with corn, wheat, soybeans and oats has shown that a single moisture calibration equation works well for all grains, promising a universal calibration, which should encourage the commercial development of the technique for practical use with grain and seed and granular materials in other industries as well.

Technical Abstract: The importance of cereal grain moisture content in determining time of harvest and in preserving grain quality is described. Techniques for moisture measurement, including electronic moisture meters, are also discussed briefly, and the need for better moisture sensing techniques for modern agricultural on-line moisture monitoring is described. Principles of microwave free-space measurements involving attenuation and phase determination are explained, and density-independent functions of the grain permittivity are presented that permit reliable moisture sensing applicable to moving grain in which bulk density variation occurs. A new moisture calibration function, based on complex-plane plots of the dielectric constant and loss factor, each divided by density, is described, which promises a universal calibration for different types of grain and seed. This important advantage should encourage commercial development for practical use, and the principles developed should be applicable to granular solids in other industries as well as agriculture.

Last Modified: 4/18/2015
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