Influence of Nonequilibrated Water on Microwave Dielectric Properties of Wheat and Related Errors In Moisture Sensing

Authors: TRABELSI, SAMIR - UNIVERSITY OF GEORGIA; Nelson, Stuart

Submitted to: IEEE Transactions on Instrumentation and Measurement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2006
Publication Date: 1/15/2007
Citation: Trabelsi, S., Nelson, S.O. 2007. Influence of Nonequilibrated Water on Microwave Dielectric Properties of Wheat and Related Errors In Moisture Sensing. IEEE Transactions on Instrumentation and Measurement. 56(1): 194-198.

Interpretive Summary: The electrical characteristics of grain known as dielectric properties are used by electronic grain moisture meters for rapid determination of moisture content. Moisture content is important in determining the potential for safe storage of grain, and the moisture content must be measured whenever a grain lot is sold. Recent research has shown that measurements on grain at microwave frequencies offer advantages for moisture testing over the lower radio frequencies currently used by commercial grain moisture meters. Experience has shown that rain or artificial wetting of grain causes errors in the moisture readings provided by grain moisture meters. These errors result because the dielectric properties of the grain are affected when moisture equilibrium in the grain kernels is upset by wetting or drying. Therefore, the effects of wetting grain samples on the microwave dielectric properties were studied to determine the extent of this influence. Samples of hard red winter wheat of 10.6 percent moisture were raised to about 14, 18 and 23 percent by spraying them with proper amounts of water. The dielectric properties were then tracked over the next six hours. Results showed that the microwave dielectric properties stabilized after two to four hours, depending upon the amount of water added. They also revealed information related to the degree of binding of water to constituents of the wheat kernels. The new information is useful to engineers developing new moisture meters and scientists interested in moisture equilibrium in grain kernels. It will therefore contribute to the goals of providing high quality and healthful products for consumers.

Technical Abstract: Dielectric properties of wheat samples, in which moisture equilibrium was upset by adding water, were tracked versus time in the frequency range between 5 GHz and 15 GHz at room temperature (23 degrees Celsius). Results show an initial drop in the dielectric constant and loss factor, which reflects the initial stages of water binding, followed by a plateau indicating the final binding level of the water molecules inside the wheat kernels. Complex-plane representation of the dielectric properties divided by bulk density at a given frequency compares well to that obtained for well equilibrated wheat samples over the same moisture range. The Cole-Cole diagram corresponding to each sample reveals a significant difference in the dielectric response at different times illustrating the different stages of water binding.