An investigation of the nature of the radio-frequency dielectric response in cereal grains and oilseeds with engineering implications for grain moisture meters

The goals for this research were: (1) improving the understanding of the dielectric relaxation processes that influence radio-frequency measurements of the dielectric characteristics of cereal grains and oilseeds, and (2) creating a more effective grain moisture measurement algorithm based on deeper understanding of those physical processes.; The significance of moisture content in cereal grains and oilseeds, the methods used for grain moisture measurement, and the need for improvements in the radio-frequency dielectric moisture method are reviewed. Background information is presented on dielectric characteristics, with emphasis on dielectric relaxation effects and grain moisture measurement.; The grain samples used in this study were obtained in the course of the Annual Moisture Calibration Survey conducted by the USDA-Grain Inspection, Packers and Stockyards Administration during 1998, 1999, and 2000. These grain samples represented all growing regions for the fifteen most economically significant cereal grain and oilseed crops in the United States and constituted the most extensive set of such samples ever subjected to dielectric characterization.; Dielectric characteristics were measured with three separate instrumentation systems to acquire data covering the frequency range from 100 Hz to 501 MHz. All three instrumentation systems were designed to measure the complex dielectric constant of grain samples and yielded results that were comparable in the overlapping frequency ranges.; Mathematical simulations of dielectric characteristics comparing measured dielectric results with theoretical dielectric relaxation behavior are described. The relative contributions of free and bound water and conductivity effects in the audio-frequency and radio-frequency ranges are quantified. An alternative explanation for the physical mechanism of conductivity in grains is proposed and its plausibility is justified.; The effectiveness of a particular dielectric mixture equation for normalizing the density of grain samples is evaluated and confirmed. An improved moisture measurement algorithm based on an optimum combination of measurement frequency, dielectric parameters, density correction, and temperature correction is proposed.; A unified calibration method for fifteen grain types is described, and its performance is shown to be significantly superior to that of current commercial instrumentation. Implications for the future of grain moisture measurement by the radio-frequency dielectric method are discussed.