Author
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Nelson, Stuart |
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Submitted to: Institute of Electrical and Electronics Engineers
Publication Type: Proceedings Publication Acceptance Date: 4/24/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Electrical characteristics, called dielectric properties, of cereal grains are important because of their usefulness in sensing moisture content, and they are used universally in practical grain moisture meters. The relationships between these dielectric properties of bulk grain samples and the properties of the grain kernels are useful in mathematical modeling of grain in connection with research on improving the reliability of sensing moisture content at radio and microwave frequencies. In some related studies on powdered coal and limestone materials, which were conducted for OSHA in connection with coal-mine safety applications, dielectric properties of coal and limestone and their mixtures were measured. Two dielectric mixture equations were tested for their accuracy in calculating the dielectric properties of a coal-limestone mixture from the properties of the two pure materials. An equation that relates the cube roots of the dielectric properties of the mixture to the cube roots of the dielectric properties of the constituents of the mixture was found to provide very accurate results. The dielectric constant of the mixture was calculated with errors of less than one-half of one percent when compared with the measured dielectric properties of the mixture, confirming that this mixture equation is very useful when working with materials of relatively low dielectric constants like grains and coal materials. The findings verify the usefulness of the mixture equation as a tool in modeling powdered and granular dielectrics for practical applications such as moisture sensing in grain. Technical Abstract: The permittivities of pulverized samples of coal and limestone were measured over a range of bulk densities at 11.7 GHz and 20 degrees C, and were used, along with particle densities, determined by pycnometer measurements, and the Landau & Lifshitz, Looyenga dielectric mixture equation to determine the solid material permittivities. Subsequently similar measurements were taken on a 35%-65% percent coal-limestone mixture, and the Landau & Lifshitz, Looyenga equation and the Complex Refractive Index dielectric mixture equation were used to calculate the permittivities of powdered coal, limestone, and the 35%-65% mixture at various bulk densities. Results compared to the measured permittivities showed that the Landau & Lifshitz, Looyenga equation provided much better estimates than the other equation, with errors of 0.15% in predicting the dielectric constants of powdered coal,1.4% in predicting the dielectric constants of powdered limestone, and 0.5% in predicting the dielectric constants of the coal-limestone mixture. |
