Abstract
To perform fast and portable grain moisture measurements under field conditions, a novel moisture sensor was designed, which consisted of a coaxial waveguide, a circular waveguide, and an isolation layer. The electromagnetic characteristics of the sensor were simulated and measured. The analytical model, which represented the relationship between the reflection coefficient of the sensor and the complex permittivity of grain, was established by using the mode matching method. The reflection coefficient of the sensor was measured by using an ultra-wideband (UWB) radar module, and the moisture content of grains was calculated from the complex permittivity by using density-independent model. To verify the performance of the proposed method, wheat, rough rice, and barley were taken as examples. The measured results in the range from 1.0% to 26.0%, wet basis, agreed well with the reference values (R2 was more than 0.99), and the maximum absolute errors for wheat, rough rice, and barley were 1.1%, 1.0%, and 1.4%, respectively. In addition, the effect of isolation layer was discussed. Both the simulation results and the experimental results showed that the isolation layer improved the stability of sensor.
Highlights
Accurate determination of moisture content of grains is of great importance in grain trading, transportation, storage, and processing
The maximum absolute error of the wheat reduced to 0.6%, of the rough rice reduced to 0.5%, and of the barley reduced to 0.7%
It was necessary to determine the optimal frequency for each grain type
Summary
Accurate determination of moisture content of grains is of great importance in grain trading, transportation, storage, and processing. There are many problems in applying the open-ended coaxial waveguide directly to the grain moisture measurement field. The size of the grain kernel is too large for the small sensing region of the open-ended coaxial probe. Small size of the sensing region is the primary problem in applying the open-ended coaxial probe to the field of grain moisture measurement. The improved open-ended coaxial probe can be suitable for the field of grain moisture measurement. One is an infinitely large grain sample is filled in a half-space to avoid reflection signals affecting the measurement This is impractical for the grains which have small dielectric constants and low loss factors. Addition, the paper discussed the influence the pressure of the sensor on the accuracy of the proposed method
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