Author
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Nelson, Stuart |
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Submitted to: IEEE Instrumentation & Measurement Society
Publication Type: Proceedings Publication Acceptance Date: 5/1/2003 Publication Date: 5/19/2003 Citation: NELSON, S.O. DENSITY-PERMITTIVITY RELATIONSHIPS FOR GRANULAR AND POWDERED MATERIALS. INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS INSTRUMENTATION AND MEASUREMENTS/2003 TOTURIAL PROGRAM, MOISTURE AND DENSITY SENSING IN GRANULAR SOLIDS THROUGH MICROWAVE DIELECTRIC MEASUREMENTS, IEEE CONFERENCE, VAIL COLORADO. 2003. p. 9-22. Interpretive Summary: The dielectric properties of cereal grains and oilseeds are used in the determination of moisture content by electrical and electronic moisture meters. The dielectric properties of materials are those electrical characteristics that determine the interaction of the material and electric fields that can be used to sense moisture content instantaneously in properly designed moisture meters. However, the packing, or bulk density, of such granular materials interferes with accurate moisture measurement if differences in packing are not taken into account. This paper provides a detailed discussion of the influence of bulk density on the dielectric properties of granular and powdered materials. It also summarizes results of research to define the relationships between the dielectric properties and the bulk density of such materials as wheat, wheat flour, and pulverized coal, limestone, and some plastics. A recommended equation for correcting the dielectric properties for changes in the bulk density of such materials is presented. This information will be useful to those designing moisture meters for grain and seed and other granular or pulverized materials to provide more reliable moisture information for the management of grain and seed harvesting, storage, marketing and processing operations. Consequently, maintenance of better quality in agricultural grain and oilseed crops and products from those crops will benefit both producers and consumers. Technical Abstract: Both the use of linear relationships between functions of the permittivities of granular and powdered materials and their bulk densities and the use of dielectric mixture equations can be helpful in predicting the permittivities of these materials at some bulk density other than the bulk density at which the permittivities are measured or otherwise known. The linear relationship between the square root of the dielectric constant and bulk density of the granular or powdered material is consistent with the Complex Refractive Index Mixture equation. The linearity of the cube root of the dielectric constant with bulk density is consistent with the Landau and Lifshitz, Looyenga Mixture equation. Therefore, these equations can be used with the complex relative permittivity values to closely estimate the permittivities of granular and powdered materials at any density, when the permittivity of the material is known, or measured reliably, at any given bulk density. The density of the solid material from which the particulate materials are derived must be known to determine the volume fraction required for the computation, where the volume fraction of the solid particles in the air particle mixture is provided by the ratio of the bulk density of the air-particle mixture to the density of the solid material. For materials such as cereal grains, as demonstrated for wheat and flour, coal and limestone, and for plastics with properties similar to Rexolite 1422, and Kynar, the Complex Refractive Index and Landau and Lifshitz, Looyenga Mixture equations provided more reliable estimates of permittivities than most of the other dielectric mixture equations tested, which included also the Bottcher, Bruggeman-Hanai, Rayleigh, or Maxwell Garnett, and Lichtenecker equations. The Bottcher equation often gave values very close to those of the Landau and Lifshitz, Looyenga equation. The Bruggeman-Hanai, Rayleigh and Lichtenecker mixture equations gave estimated permittivities of increasing magnitude in the order listed for the types of materials tested, which had relatively low dielectric constants ranging between 2 and 8 at microwave frequencies. For materials of higher permittivity ranges, further testing is necessary to determine the mixture equation of choice. For the materials tested, the Landau and Lifshitz, Looyenga (LLL) equation performed better in most, but not all instances, than the Complex Refractive Index Mixture equation. Therefore, the LLL mixture equation is recommended for correcting permittivity values of granular and powdered materials for changes in bulk density. |
