Impedance spectroscopy method for testing moistened wheat crops

The authors presented the results of a study of the electrical and dielectric characteristics of wetted wheat grains by measuring their complex electrical resistance (impedance Z) in a wide frequency range (from 1 Hz to 100 MHz). The results of electrical impedance measurements of grains with surface or volumetric moisture content under different experimental conditions can provide useful information on the properties of the biological tissues of grain crops. These results can also be used to develop a new type of impedance sensor for testing grain quality and moisture content. The authors used well-dried wheat grains and grains saturated with moisture and saline as objects of research. A major problem in grain impedance measurements is the selection of a suitable electrode material to be placed on the end surfaces of the samples. The electrodes must ensure reliable contact with the grain and have a minimum transient resistance. The end surfaces of the pressed samples were reinforced with a protective dielectric ring to prevent transverse deformation. These contacts provided a transition resistance between 1-2 ohms. The authors have identified processes of accumulation of electric charges near the surface of metal electrodes at low frequencies and on internal grain structures, leading to an increase in the dielectric permittivity and dissipation factor. The behavior of the active and reactive components of the impedance at higher frequencies is determined by dielectric relaxation processes. The obtained impedance spectra were compared with the spectra of the most suitable equivalent electrical circuits. The radio components of the circuits provide information about the basic mechanisms of alternating electric current flow through the complex inhomogeneous structure of the grain. The authors found that moistening the grain with saline water enhances the process of accumulation of electric charges and affects the dispersion of the real and imaginary components of the impedance.

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