Location: Quality & Safety Assessment ResearchTitle: Measuring the complex permittivity of thin grain samples by the free-space transmission technique Author
Submitted to: Instrumentation and Measurement Technology Conference Record
Publication Type: Proceedings
Publication Acceptance Date: 3/9/2012
Publication Date: 5/13/2012
Citation: Roelvink, J.T., Trabelsi, S. 2012. Measuring the complex permittivity of thin grain samples by the free-space transmission technique. Instrumentation and Measurement Technology Conference Record. p. 310-313. Interpretive Summary: Dielectric properties, or permittivities, of materials are electrical characteristics that determine how they 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 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 determination of grain moisture content when it is traded and going 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 grain moisture meters use frequencies in the range from 1 to 20 MHz for their peration. Research on microwave measurements of the grain and seed permittivities and their use for sensing moisture content has revealed important advantages of the microwave frequencies for improving accuracy of moisture determination and monitoring moisture in flowing grain. The microwave technique determines the complex permittivity (dielectric properties) from measurement of the attenuation and phase shift of microwaves traversing a layer of grain or seed. Certain restrictions on the thickness of the grain layer have been necessary for successful determination of moisture content for materials of very low and very high moisture levels. The research reported in this paper overcomes those restrictions on layer thickness for different moisture level materials. A method is presented for numerically solving a higher-order model for the free-space transmission equation used to determine the complex permittivity of thin grain samples. The ambiguity in the phase measurement has been accounted for by comparing several numerical solutions at two frequencies. Results for the permittivity of grain at five moisture contents for two sample thicknesses are presented. Results show that the permittivity of relatively thin grain samples calculated by using the higher-order model agrees very well with results calculated by using the approximate model that has been used previously. The advantage of using relatively thin grain samples is that a wider range of grain moisture contents can be reliably measured for a fixed sample thickness. 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 the benefit of both producers and consumers.
Technical Abstract: In this paper, a numerical method for solving a higherorder model that relates the measured transmission coefficient to the permittivity of a material is used to determine the permittivity of thin grain samples. A method for resolving the phase ambiguity of the transmission coefficient is presented. Results calculated by using the higher-order model for the permittivity of grain over a range of moisture contents and frequencies are presented and compared to those calculated by using an approximate model. The advantage of using relatively thin grain samples is that a wider range of grain moisture contents can be reliably measured for a fixed sample thickness.