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Title: Models for the Microwave Dielectric Properties of Grain and Seed

item Nelson, Stuart
item Trabelsi, Samir

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2010
Publication Date: 4/14/2011
Citation: Nelson, S.O., Trabelsi, S. 2011. Models for the microwave dielectric properties of grain and seed. Transactions of the ASABE. 54(2):549-553.

Interpretive Summary: Permittivities or dielectric properties of materials are electrical characteristics that determine how materials interact with electric fields. For example, these properties determine how rapidly the material will heat in a microwave oven. The dielectric properties of grain and seed are highly correlated with their moisture content, so electronic instruments have been designed to sense these properties for rapid measurement of grain and seed moisture content. Such grain and seed moisture meters have long been used for determing grain moisture content when it is traded and when it is put into storage. Improvements in grain moisture meters and grain moisture monitoring equipment are desirable, because moisture content is such an important factor in determining safe storability of grain and seed. Currently used moisture meters use frequencies between 1 and 20 MHz for sensing the dielectric properties and providing moisture contents of grain and seed. Microwave frequencies well above those in current use have shown promise for improving the reliability of such instruments. Therefore, there is need for data on the microwave dielectric properties of grain and seed for use in developing microwave moisture meters. Such dielectric properties of several kinds of cereal grain and oilseeds have been measured over a range of moisture contents and microwave frequencies. This paper reports the development of mathematical models for calculating the dielectric properties of wheat, corn, barley, grain sorghum, soybeans, canola, and peanuts in the frequency range from 5 to 15 GHz. The new models will be useful for scientists and engineers working on development of microwave moisture meters and other microwave applications involving grain and seed. The models will also be incorporated into dielectric properties data on grain and seed in the ASABE Standards for ready reference. Microwave measurements can be used for reliable measurements of grain and seed moisture content in static and flowing grain, so they offer promise for improved monitoring of moisture in granular materials. These advances will aid the commercial development of new techniques for practical use and provide new tools for managing moisture content and quality of grain and oilseed and their products for both producers and consumers.

Technical Abstract: Based on microwave dielectric properties of wheat, corn, barley, oats, grain sorghum, soybeans, canola, shelled peanuts and pod peanuts measured over ranges of frequency and moisture content, models are developed for predicting the dielectric constant and loss factor of these commodities. Nearly linear relationships between dielectric properties and log of frequency or frequency and moisture content permit multiple linear regression models to be developed for predicting dielectric constants and loss factors for each kind of grain and seed at 23 'C. The models predict dielectric constants with standard errors of 1% to 2% with some up to about 4% for grain and seed at frequencies in the range from 5 to 15 GHz over given moisture ranges. Loss factors are predicted with standard errors of a few percent, but much larger errors can naturally result when loss factors are very small.