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United States Department of Agriculture

Agricultural Research Service

Title: Flow-Through Coaxial Sample Holder Design for Dielectric Properties Measurements from 1 to 350 Mhz

Authors
item Lawrence, Kurt
item Nelson, Stuart
item Bartley Jr, Philip - UNIV GA AGRIC ENGR DEPT

Submitted to: Institute of Electrical and Electronics Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 14, 1998
Publication Date: N/A

Interpretive Summary: Moisture content is the moist important factor for safe storage of grain. If moisture content can be reliably determined while the grain is moving through a combine or conveying equipment, the information required could be obtained on line. To sense moisture content of moving grain based on its electrical or dielectric properties, a flow-through sensor must be designed. Since there are factors other than moisture content that affect the dielectric properties of a material, techniques to eliminate these confounding effects are being explored. One such technique is to take dielectric properties measurements over a wide range of frequencies and use the information to improve the accuracy of the moisture determination. However, for these measurements, a sample holder, suitable for flowing grains, must be designed and tested. This article reports the design of a coaxial sample holder for dielectric properties measurements on flowing grain at frequencies from 1 to 350 MHz. Sophisticated electronic-analysis techniques were used to characterize the sample holder, and standard materials were used to calibrate and verify the dielectric measurements. Suitable accuracy was achieved so that on-line moisture sensing in grain can be developed to help in maintaining high grain quality for domestic use and for export.

Technical Abstract: A system for measuring the dielectric properties of cereal grains from 1 to 350 MHz with a coaxial sample holder is presented. A signal-flow graph model was used to determine the permittivity of several polar alcohols from the full two-port S-parameter measurements. At the lowest frequencies (1-25 MHz), where the phase measurements are less accurate, a lumped-parameter model was used to predict the dielectric loss factor values. The system was calibrated with measurements on air and decanol and verified with measurements on octanol, hexanol, and pentanol. The standard error for the polar alcohols used for verification was 2.3% for the dielectric constant and 7.6% for the dielectric loss factor. Although measurements were taken on static samples, the sample holder is designed to accommodate flowing grain.

Last Modified: 11/27/2014
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