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Title: NONDESTRUCTIVE SENSING OF GRANULAR MATERIAL PHYSICAL PROPERTIES BY MICROWAVE PERMITTIVITY MEASUREMENT

Author
item TRABELSI, SAMIR - UNIV OF GEORGIA
item Nelson, Stuart

Submitted to: IEEE Transactions on Instrumentation and Measurement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2005
Publication Date: 6/15/2006
Citation: Trabelsi, S., Nelson, S.O. 2006. Nondestructive sensing of granular material of physical properties by microwave permittivity measurement. IEEE Transactions on Instrumentation and Measurement 55(3):953-963..

Interpretive Summary: The moisture content of cereal grains and oilseed crops are the most important factors determining their suitability for harvest and safe storage. If moisture contents are too high, damage to kernels can result in harvesting and the commodities must be dried to a safe storage level to avoid spoilage losses. Thus, moisture content also affects the price paid for these commodities. Standard methods of determining moisture content are too tedious and time-consuming for use in the trade. Therefore, instruments have been developed for rapid moisture content estimation. These moisture meters sense the electrical characteristics of grain and seed samples that are well related to the amount of moisture in the sample, and so they can be calibrated to read moisture content directly in a few seconds. There is need for better reliability and accuracy in sensing moisture content through the electrical, or dielectric, properties of the grain and seed. Commonly used grain moisture meters use frequencies in the range from 1 to 20 MHz for sensing moisture content. Research with much higher frequencies, in the microwave range, has shown some advantages in using microwaves to determine moisture content. The degree of packing of granular materials affects their dielectric properties, which are sensed by the moisture meters. Thus variations in packing can cause errors in moisture determination unless these changes in bulk density of the samples are taken into account. Microwave measurements can automatically eliminate errors due to changes in density. This paper describes in detail the techniques of the microwave measurements that can determine moisture content independent of bulk density and discusses the errors associated with such measurements so that microwave instruments might be designed for grain and seed moisture testing with higher accuracy and reliability. The technique also appears to be applicable to measurements in moving grain and could thus be used to monitor flowing grain and provide better moisture information for management decisions necessary in harvesting and handling of grain and oilseed crops.

Technical Abstract: Dielectric properties of wheat, corn, and soybeans were determined from measurement of the scattering transmission coefficient in free space at frequencies between 2 and 13 GHz. For better accuracy, a pair of horn/lens antennas was used, and time domain gating was applied to the main response. Variations of the dielectric properties with frequency and physical properties such as bulk density, moisture content, and temperature were investigated. Both the dielectric constant and loss factor decreased with frequency and increased linearly with bulk density, moisture content, and temperature. Three different approaches are used to correlate measured dielectric properties and physical properties. Explicit relationships between the dielectric properties and the different physical properties are given at a midrange frequency of 7.0 GHz as an example. Potential use of these relationships is shown for the development of indirect methods for nondestructive and instantaneous determination of physical properties of cereal grain and seed from measurements of their dielectric properties. Different sources of errors in attenuation and phase shift measurement are discussed and their effects on the accuracy for determination of the relative complex permittivity and physical properties are investigated.