New density-independent calibration function for microwave sensing of moisture content in particulate materials

Abstract

Microwave techniques have been considered for a long time for moisture sensing in many food processing and agriculture-related industries. They are suitable for on-line realtime monitoring and control. However, with particulate materials, bulk density fluctuations cause significant errors in moisture content determination. To overcome this shortcoming, densityindependent calibration functions are needed. In this paper, a new approach is presented in which both bulk density and moisture content are determined directly from measured microwave dielectric properties. A simple relationship between bulk density and the dielectric properties is identified, and a new density-independent function for moisture content prediction, exclusively dependent on the dielectric properties of the material under test (0 , 00), is proposed. The validity and applicability of this function are demonstrated with an extensive data set obtained from measurements on a granular material (wheat), over wide ranges of frequency (11-18 GHz), temperature (01 C-42 C), moisture content (10.6%-19.2%, wet basis), and bulk density (0.72-0.88 g/cm 3). Explicit calibration equations for moisture prediction at different frequencies and temperatures are provided. Although data obtained by a transmission microwave measurement technique were used, this new approach remains valid in general for other techniques, provided that 0 and 00 are determined accurately.

Use of grain and seed dielectric properties for moisture measurement

“…Density-independent functions of the dielectric properties for predicting moisture content from measured dielectric properties of wheat and corn were compared in several studies [39][40][41].…”

“…Principles of the density-independent moisture calibration function developed by Trabelsi [38][39] have been described previously for use in measuring both moisture content and bulk density of grain and seed [49][50][51], but a brief description is provided here. Measurements on a large number of samples of hard red winter wheat of different moisture contents, bulk densities, and temperatures are summarized in plots for measurements at 11.3 and 18.0 GHz in Fig.…”

Microwave sensing method for simultaneous and independent determination of bulk density and moisture content of shelled peanuts

“…A density-independent permittivity function was used successfully for moisture determination in cereal grain and oilseed. It is expressed in terms of ε′, ε″ and f a as follows [5]: (3) to (5) …”

“…It is based on a complex-plane representation [4] from which the bulk density can be determined from the dielectric properties independent of temperature and moisture content. Then moisture content is determined independent of bulk density by means a permittivity calibration function that is expressed in terms of measured dielectric properties [5]. Bulk density and moisture calibration equations are given at 24 o C and X-band microwave frequencies ranging from 8 GHz to 12 GHz.…”

Dielectric properties of agricultural products and some applications

“…The slope of the line increases with frequency and pivots about a point on the ε"/ρ = 0 axis that represents the ε'/ρ value for completely dry grain. The equation of the straight line provides a means for determining the bulk density of the grain independent of moisture content and temperature and also provides a basis for sensing moisture content independent of density and temperature from the measured permittivity at a single frequency (Trabelsi et al 1998). This microwave sensing technique should be useful for monitoring moisture content in moving grain and other particulate materials, since it offers a densityindependent method for sensing moisture content in granular and particulate materials.…”