Samir Trabelsi

Free-space measurement of dielectric properties of cereal grain and oilseed at microwave frequencies

Principles of dielectric property measurement by microwave free-space transmission measurements are presented, and the important sources of errors in such measurements are discussed. A system, including a vector network analyser, horn/lens antennas, holder for grain and oilseed samples and a radiation absorbing enclosure that was used for such measurements is described, and the techniques and procedures followed to obtain reliable permittivity data for wheat, shelled corn (maize) and soybeans are outlined. Data illustrating linear relationships between microwave attenuation and phase shift per unit sample thickness, each divided by the bulk density of the granular materials, and frequency and moisture content are presented graphically. The linear dependence of calculated permittivity components, dielectric constant and loss factor, on bulk density is also shown, and permittivity components for wheat, corn and soybeans are listed for reference at frequencies from 5 to 17 GHz at different densities and moisture levels at about 23 °C. Permittivity values are also listed for the same three commodities, adjusted to a medium density value through use of the Landau and Lifshitz, Looyenga dielectric mixture equation, for the total range of moisture contents at 10 GHz and at the same temperature.

Phase-shift ambiguity in microwave dielectric properties measurements

Phase measurements of the transmission coefficient are important when used for the dielectric characterization of materials. They are required for industrial material monitoring applications, where the phase is correlated with parameters such as moisture content and density, which need to be continuously determined, However, when the thickness of the material under test is greater than the wavelength in the material, a phase problem is encountered. Two methods are proposed to solve this problem. The first is based on the selection of the appropriate material thickness; the second requires the use of measurements at two frequencies. Advantages and limitations of both methods are discussed, and numerical validations are given for particulate materials.

A microwave method for on-line determination of bulk density and moisture content of particulate materials

A new method for simultaneous and independent on-line determination of bulk density and moisture content in particulate materials by measurements of the relative complex permittivity is proposed. The bulk density is determined, based on a representation in the complex plane of relative complex permittivity normalized to bulk density. For moisture content determination, a new density-independent function, exclusively dependent on the dielectric properties, is used. Results obtained from measurements on wheat over broad ranges of microwave frequencies, temperatures, densities, and moisture contents are presented.

Dielectric spectroscopy of watermelons for quality sensing

Dielectric properties of four small-sized watermelon cultivars, grown and harvested to provide a range of maturities, were measured with an open-ended coaxial-line probe and an impedance analyser over the frequency range from 10 MHz to 1.8 GHz. Probe measurements were made on the external surface of the melons and also on tissue samples from the edible internal tissue. Moisture content and soluble solids content (SSC) were measured for internal tissue samples, and SSC (sweetness) was used as the quality factor for correlation with the dielectric properties. Individual dielectric constant and loss factor correlations with SSC were low, but a high correlation was obtained between the SSC and permittivity from a complex-plane plot of dielectric constant and loss factor, each divided by SSC. However, SSC prediction from the dielectric properties by this relationship was not as high as expected (coefficient of determination about 0.4). Permittivity data (dielectric constant and loss factor) for the melons are presented graphically to show their relationships with frequency for the four melon cultivars and for external surface and internal tissue measurements. A dielectric relaxation for the external surface measurements, which may be attributable to a combination of bound water, Maxwell–Wagner, molecular cluster or ion-related effects, is also illustrated. Coefficients of determination for complex-plane plots, moisture content and SSC relationship, and penetration depth are also shown graphically. Further studies are needed for determining the practicality of sensing melon quality from their dielectric properties.

Density-independent functions for on-line microwave moisture meters: a general discussion

Density-independent calibration functions are a suitable solution for problems involving variation of bulk density in on-line determination of the moisture content in particulate materials by microwave techniques. Foundations of three of these functions are briefly reviewed and their frequency and temperature dependences are shown for wheat. Their effectivenesses for prediction of the moisture content are compared and optimum conditions for a cost-effective moisture meter are discussed.

Nondestructive microwave characterization for determining the bulk density and moisture content of shelled corn

A method based on nondestructive microwave characterization is used for simultaneous determination of the bulk density and moisture content of shelled corn. This method can be applied regardless of the measurement technique and thus considerably simplifies the calibration procedure. Calibration equations, both for the bulk density and for the moisture content, are given, together with the standard error of performance (SEP) at several frequencies in the range 11-18 GHz and three temperatures, 14, 24 and C. The bulk density, ranging from 695 to , can be determined with SEP in the range 11-. Neither the sample moisture content nor its temperature are required in order to determine the bulk density. The moisture content ranging from 9% to 19% on a wet basis, can be determined at each temperature without knowledge of the bulk density with SEP of less than 0.5% moisture content. Results of an error analysis of the measurements show that about half of the total uncertainties in bulk density and moisture content can be considered systematic errors and thus they are correctable.

Nondestructive sensing of physical properties of granular materials by microwave permittivity measurement

The dielectric properties of wheat, corn, and soybeans were determined by measuring the scattering transmission coefficient S21 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 the loss factor decreased with frequency and increased linearly with bulk density, moisture content, and temperature. Three different approaches are used to correlate the 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. The potential use of these relationships is shown for the development of indirect methods for the nondestructive and instantaneous determination of the physical properties of cereal grain and seed from measurements of their dielectric properties. Different sources of error in attenuation and phase shift measurement are discussed, and their effects on the accuracy for the determination of the relative complex permittivity and physical properties are investigated

Dielectric spectroscopy of wheat from 10 MHz to 1.8 GHz

The dielectric properties (components of the complex permittivity relative to free space) of ground hard red winter wheat of 11–25% moisture content were determined by dielectric spectroscopy measurements with an open-ended coaxial-line probe and impedance analyser over the frequency range from 10 MHz to 1.8 GHz at temperatures from 5 to 95 °C. Both the dielectric constant and dielectric loss factor, over the stated range of variables, decreased with increasing frequency and increased with increasing moisture content and temperature. Plots of the dielectric constant and loss factor in the complex permittivity plane revealed a linear relationship between the two permittivity components at frequencies above 1 GHz, but they showed nonlinearity at lower frequencies due to the increasing influence of ionic conduction as confirmed by Cole–Cole plots of the permittivity data.

Using cereal grain permittivity for sensing moisture content

A brief history of cereal grain moisture measurement by sensing the electrical properties of grain is presented. The basic principles are also described for using radio-frequency (RF) and microwave dielectric properties, or permittivity, of grain for sensing moisture through their correlation with moisture content. The development of density-independent functions of the permittivity is explained. The findings of recent research are summarized, which indicate that reliable density-independent moisture content determinations can be realized by measurements on grain at RF and microwave frequencies. Development of these techniques will provide useful instruments for on-line monitoring of moisture content in flowing grain to manage moisture in grain, prevent spoilage in storage and transport, improve processing, and provide information important for yield determinations in precision agriculture applications.

Wheat Permittivity Measurements in Free Space

Requirements for a free space microwave transmission measurernent system for determining the permittivity of grain are analyzed. Experimental verification of these requirements is providedfor two cultivars ofhard red winter wheat of various densities, moisture contents and temperatures in the frequency range from 10 to 18 GHz. Uncertainties in the dielectric constant determination are less than ±1%, and those for the loss factor are less than ,±.3%.