Dielectric Reference Materials for Mathematically Modeling and Standardizing Grain Moisture Meters

Abstract

Dielectric-type instruments are the most widely used technology for grain moisture measurement. Measuring fundamental dielectric parameters is advantageous in terms of understanding the measurement and in transferring calibrations; however, few grain moisture meter models report such fundamental parameters. One reason for this is the difficulty in calibrating instruments to measure them. High molecular weight alcohols are commonly used as dielectric reference materials for calibrating research-type instruments. Grain moisture meters are, of course, designed to handle grain rather than liquids, so performing meaningful tests on alcohols is very difficult. Suitable granular dielectric reference materials are needed. This paper describes research using three alcohols (pentanol, hexanol, and decanol) and low-loss dielectrics (air and glass and plastic beads). Dielectric measurements were performed from 1 to 501 MHz using a HP-4291A Material/Impedance Analyzer and a transmission-line dielectric test cell. Mathematical models based on ABCD matrix methods were created to predict complex dielectric constants from measurements of complex reflection coefficient. The low-loss materials were successful for calibrating the system to measure dielectric constant in spite of the fact that the actual dielectric constants were unknown. Two characteristics were assumed: constant dielectric constant over the frequency range and constant (near zero) loss factor over the frequency range. These two constraints (along with test cell parameters defined by finite element analysis) permitted unambiguous determination of all parameters for the test cell mathematical model. The resulting test cell model accurately predicted dielectric characteristics of alcohols.