Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2006
Publication Date: 7/17/2006
Citation: Nelson, S.O., Trabelsi, S. 2006. Dielectric spectroscopy of wheat from 10 mhz to 1.8 ghz. Measurement Science and Technology. 17:2294-2298. 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. The dielectric properties of grain and seed are highly correlated with their moisture content, so electronic instruments can be designed to measure grain and seed moisture content. Such grain and seed moisture meters have long been used for rapid determination of grain moisture content when it is traded and when it is prepared for 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. Higher frequencies than those currently employed by grain moisture meters are being considered for improving the reliability of such instruments. Therefore, new data are needed concerning the dielectric properties of grain and seed. Also, knowledge of the variation of these properties with frequency of the electric fields used, the moisture content of the grain, and its temperature are important in this research. In the new research reported, the dielectric properties of hard red winter wheat were measured by dielectric spectroscopy techniques over moisture ranges from 11 to 25 percent, frequencies from 10 MHz to 1.8 GHz, and temperatures between 5 and 95 degrees C. Thus, the variation of the dielectric properties of wheat with these three variables, frequency, moisture content, and temperature, was well determined, and relationships were discovered that will be useful in new research to improve the reliability of moisture sensing in grain. 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 consumers.
Technical Abstract: The dielectric properties (components of the complex permittivity relative to free space) of ground hard red winter wheat of 11 to 25 percent moisture content were determined by dielectric spectroscopy measurements with an open-ended coaxial-line probe and impedance analyzer over the frequency range from 10 MHz to 1.8 GHz at temperatures from 5 to 95 degrees C. Both the dielectric constant and dielectric loss factor, over the stated range of variables, decreased with increasing frequency and increased with increasing moisture content and temperature. Plots of the dielectric constant and loss factor in the complex permittivity plane revealed a linear relationship between the two permittivity components at frequencies above 1 GHz, but they showed nonlinearity at lower frequencies due to the increasing influence of ionic conduction as confirmed by Cole-Cole plots of the permittivity data.