Submitted to: Measurement Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/20/2001
Publication Date: 11/6/2001
Citation: Trabelsi, S., Kraszewski, A.W., Nelson, S.O. 2001. Microwave Dielectric Sensing of Bulk Density of Granular Materials. Measurement Science and Technology 12:2191-2197. Interpretive Summary: When working with granular materials such as cereal grains and seeds and with other materials of a granular nature in the chemical, mining, construction, and pharmaceutical industries, it is often necessary to have information about the bulk density or weight per unit volume of the granular materials. Bulk density determinations are traditionally measured by weighing a known volume of material, which requires collecting a sample from flowing materials and weighing it. It would often be useful to be able to measure the bulk density of such granular materials as they are being conveyed. If the microwave dielectric properties of a granular material are measured, it is possible to determine the bulk density of that material directly, independent of its moisture content or its temperature. This paper describes a technique whereby the dielectric properties can be determined by microwave measurements and the bulk density of the material at the time of the measurements can be determined from those measured properties. Effectiveness of the method has been shown with samples of wheat, oats, corn and soybeans over wide ranges of moisture content and temperature. In laboratory tests, the bulk densities of wheat, corn and soybeans were determined by this technique with accuracies of about 1.5 %, which is sufficiently reliable for most applications. The technique may therefore be implemented following the development of practical microwave instruments for the automatic monitoring of the bulk density of granular materials, making this information available for more efficient management of various processes.
Technical Abstract: A nondestructive dielectric method for sensing bulk density of granular materials is presented. The bulk density is determined from measurement of the dielectric properties of these materials at a single microwave frequency without knowledge of their moisture content and temperature. Bulk density calibration equations are generated from a complex-plane representation of the dielectric properties normalized to bulk density. Effectiveness of the method is shown through measurement of the dielectric properties at 7 GHz for materials with significant compositional and structural differences, i.e., wheat, oats, corn, and soybeans, over wide ranges of moisture content and temperature. The standard error of calibration and the relative error calculated for each material indicate that the method is as accurate or better than, commonly used methods for on-line density determination. Because the density is expressed in terms of the relative complex permittivity, the method can be applied regardless of the measurement technique (transmission lines, resonant cavity, admittance or impedance).