A.W. Kraszewski

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.

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%.

Microwave Monitoring of Moisture Content in Grain - Further Considerations

Variations and fluctuations of the bulk density of grain have been recognized as a major factor limiting the accuracy of moisture content measurements by dielectric methods. Experimental results of attenuation and phase shift measurements as a function of moisture content and density at 9.4 GHz for six kinds of grain are presented , and technical means to eliminate or limit the density effect are discussed.

Moisture content determination in grain by measuring microwave parameters

A concept of moisture content determination from the measurement of the attenuation and phase shift of an electromagnetic wave transmitted through a layer of wet material is explored in the paper. Experimental results for wheat of 10 to 19% moisture content (wet basis) at temperatures from to C at the frequency of 16.8 GHz are used to illustrate the procedure which provides a density-independent and temperature-compensated moisture content determination.

Microwave dielectric sensing of bulk density of granular materials

A nondestructive dielectric method for sensing bulk density of granular materials is presented. The bulk density is determined from measurement of the dielectric properties of these materials at a single microwave frequency without knowledge of their moisture content and temperature. Bulk density calibration equations are generated from a complex-plane representation of the dielectric properties normalized with respect to bulk density. The effectiveness of the method is shown through measurement of the dielectric properties at 7 GHz for materials with significant compositional and structural differences, i.e. wheat, oats, corn and soybeans, over wide ranges of moisture content and temperature. The standard error of calibration and the relative error calculated for each material indicate that the method is as accurate as or better than commonly used methods for on-line density determination. Because the density is expressed in terms of the relative complex permittivity, the method can be applied regardless of the measurement technique (using transmission lines, a resonant cavity, admittance or impedance).

Solid and Particulate Material Permittivity Relationships

The complex permittivities of solid and pulverized samples of two plastics, Rexolite 1422 and Kynar, were measured at frequencies of2 .45 GHz, 11.5 GHz, and 22.0 GHz at 25°C by the short-circuited wave guide technique. Several dielectric mixture equations and extrapolation of functions of the real and imaginary parts of the permittivity that are linear with bulk density were then used to estimate the permittivities at solid-material densitiesfrom measurements on the pulverized samples. For these materials, the best estimates of the permittivities were provided by extrapolations that are consistent with the Complex Refractive Index and Landau and Lifshitz, Looyenga mixture equations. The Boucher mixture equation often gave values very close to the Landau and Lifshitz, Looyenga equation, and theB ruggeman-Hanai, Rayleigh, and Lichtenecker mixture equations gave increasingly larger permittivity estimates in that order.

DIELECTRIC PROPERTIES OF MATERIALS AND MEASUREMENT TECHNIQUES

ABSTRACT The dielectric properties of materials are defined, and the nature of their dependence on moisture content, frequency of the applied electric field, temperature of the material, and density of particulate materials is discussed. Dielectric properties of liquid water are summarized. The influence of moisture content, frequency, temperature, and density on the values of the dielectric constant and loss factor, real and imaginary parts of the relative complex permittivity, respectively, are illustrated for several different kinds of materials. Examples include dielectric properties for the cereal grains corn and wheat, pecans, and pulverized coal. Techniques for the measurement of dielectric properties of materials at high frequencies and microwave frequencies useful for dielectric heating and drying applications are discussed briefly, and numerous publications describing these methods in greater detail are cited for reference.

Microwave Dielectric Properties of Shelled, Yellow-Dent Field Corn

Dielectric properties of three hybrids of shelled, yellow-dent field corn were measured at different bulk densities and moisture contents in a Styrofoam container located between two horn antennas infree space. Relative complex permittivities were determined from measurements of attenuation and phase shift at frequencies from 11 to 18 GHz and at temperatures from 4 to 45 °C. The grain moisture content ranged from about 9% to 19%, wet basis.

Microwave Aquametry: An Effective Tool for Nondestructive Moisture Sensing

Moisture content in solid, granular and pulverized materials is one of the most important material parameters during production, trading, processing and storage of those materials. Recent advances in application of microwave measuring techniques to nondestructive determination of moisture content are reviewed, with a special emphasis being put on a newly developed concept of a density-independent calibration. It is concluded that those techniques provide accurate, fast and nondestructive means for moisture content testing in such materials and satisfy requirements of automated industrial processes, scientific laboratories, material mass storage, personnel safety and long-term transport.

Regression Analysis of Microwave Spectra for Temperature-Compensated and Density-Independent Determination of Wheat Moisture Content

Partial least-squares regression (PLSR) was used to generate wheat moisture content predictive models from eight-frequency microwave attenuation (A) and phase (P) spectra in the 10.36 to 18.0 GHz range, as obtained by a free-space technique with a 10.4 cm thick sample. Spectra (n = 379) were measured for a set of grain samples that had been treated to span the agriculturally practical ranges of moisture content (M) (10.6 to 19.2% g/gwet), temperature (K) (–1 to 42°C), and bulk density (D) (0.72 to 0.88 g/mL). The sample– property space formed by M, K, and D was used to prune redundant samples and select representative subsets for calibration (n = 279), cross-validation (n = 40 segments), and testing (n = 31). Twelve model types are reported and vary from attenuation or phase alone to the combination of attenuation, phase, temperature, and density (i.e., APKD). For optimization of each PLSR model, the raw spectral, temperature, and density data were preprocessed with variable ratios, mathematical transformations, and/or variable scaling. The lowest moisture prediction errors were for temperature-and density-corrected models with variables AKD or APKD; these produced root-mean-square cross-validation and prediction errors (RMSECV and RMSEP) of 0.19 to 0.20% in moisture content units. The more practical unifrequency models, APK at 15.2 GHz, and AK at 18.0 GHz, yielded RMSECV values of 0.21% and 0.35%, respectively. Addition of temperature to dielectric data always substantially reduced the model error. However, the multiplicative effect of density is well corrected by using the ratio A/P, or partly corrected by using the features in the attenuation spectra. Data trends suggest that dual-frequency PK models might benefit from a wider frequency range, and unifrequency AK models might be better at frequencies higher than 18.0 GHz. The results presented make it possible to evaluate a wide variety of instrumental configurations that might be proposed to suit particular engineering criteria such as measurement accuracy, range of operating conditions, and hardware complexity.