|Bartley Jr, Philip|
Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/1997
Publication Date: N/A
Citation: Interpretive Summary: The electrical or dielectric properties of grains are physical characteristics of the material that can be used to sense moisture content. There are several factors that affect the dielectric properties of grain. These include the measurement frequency and temperature, and the grain moisture content, bulk density, and possibly ionic conductivity. Normally, ,the dielectric properties are collected on static samples. However, recent research suggests that sensing moisture content on flowing grains can be improved with information of the grain's dielectric properties at multiple measurement frequencies. This article presents the results of dielectric property measurements on hard red winter wheat ranging in moisture content from 9% to 21% with varying bulk densities, in a flow-through sample holder. The measurements were taken on stationary samples at room temperature over a wide range of frequencies and will be beneficial in the development of an improved dielectric moisture sensor for flowing grains. Such an instrument would help improve the quality of grain in the market, both domestic and foreign, by providing more accurate information on grain moisture content.
Technical Abstract: A system for measuring the dielectric properties of cereal grains from 1 to 350 MHz with a coaxial sample holder is presented. Several polar alcohols were used to calibrate and verify permittivity measurements obtained with a signal-flow graph model from the full two-port S-parameter measurements. At the lowest frequencies (1-25 MHz), where the phase measurements are less saccurate, a lumped parameter model was used for the dielectric loss factor measurements. 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. Dielectric properties measurements at 25 degrees C were taken on four hard red winter wheat cultivars ranging in moisture content from about 9% to 21% with bulk densities varying from 0.66 to 0.83 g/cc. Most of the data agreed with measurements reported in the literature.