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item Nelson, Stuart

Submitted to: Transactions of the ASAE
Publication Type: Proceedings
Publication Acceptance Date: 7/6/2005
Publication Date: 7/17/2005
Citation: Trabelsi, S., Nelson, S.O. 2005. Microwave dielectric properties of cereal grain and oilseed. In: Proceedings of the American Society of Agricultural Engineers, St. Joseph, MI. Paper no.056165.

Interpretive Summary: Dielectric properties of materials are electrical properties that determine how materials interact with electric fields such as those of high-frequency and microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens and lower radio-frequency dielectric heating equipment. Dielectric properties are also important in low power applications, such as the rapid measurement of moisture content in grain and oilseeds. Electronic moisture meters use radio-frequency electric fields to sense moisture content in products such as grain, because the dielectric properties are highly correlated with the moisture content of the grain. Moisture content is the most important characteristic of cereal grains and oilseeds affecting their suitability for harvesting, storage, transport, and processing. It is also an important factor affecting the price paid for the commodity. 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 from its dielectric properties independent of changes in packing density. More basic data are needed on the microwave dielectric properties of cereal grains and oilseeds. Such data have been measured and are reported in this paper for wheat, corn, barley, oats, soybeans and canola. These data will be useful in determining relationships between the dielectric properties and other variables, such as density and moisture content, and in modeling the dielectric behavior of such grain and oilseeds. Therefore, they have value in further research on improving the utility and accuracy of microwave measurements for determination of moisture content. These advances will encourage the commercial development of the technique for practical use and provide new tools for managing moisture content and quality of grain and oilseed and their products for consumers.

Technical Abstract: Measurement of the dielectric properties of moist granular materials such as cereal grain and oilseed is essential for understanding their electrical behavior and the development of nondestructive methods for determining physical characteristics such as moisture content and bulk density. For better modeling of these materials and effective use of indirect characterization methods, the dielectric properties have to be measured accurately. Dielectric data for moist granular materials are limited in the literature, particularly those measured above 2.45 GHz. In this study, dielectric properties of cereal grains and oilseeds were measured with the same free-space measurement setup between 2 and 18 GHz at 23 degrees Celsius. For better accuracy, two horn/lens antennas providing a focused beam were used, multiple reflections were minimized by selecting appropriate sample thickness, and time-domain gating was applied to filter out undesirable post-calibration mismatches. For each sample of given moisture content, dielectric properties measurements were performed at three different densities. The moisture levels were those of interest to the grain and oilseed industry.