FREQUENCY, MOISTURE, TEMPERATURE, AND DENSITY-DEPENDENT DIELECTRIC PROPERTIES OF WHEAT STRAW

The dielectric constant (epsilon') and dielectric loss factor (epsilon ''), components of the complex permittivity relative to free space, of wheat straw samples at moisture content of 10.4% to 20.3% wet basis (w.b) were determined with a parallel-plate electrode assembly over the frequency range from I to 1000 kHz, bulk density range from 47.2 to 108.1 kg m(-3), and temperature range from 5 degrees C to 40 degrees C. Obvious negative and linear relationships between logarithmic permittivities (epsilon' and epsilon '') and logarithmic frequency were found below about 500 kHz. Over the investigated ranges of the variables, both epsilon' and epsilon '' increased almost linearly with increasing temperature and increased obviously with an increase in moisture content and bulk density at higher levels. The relationship between the dielectric properties and moisture content, temperature, and bulk density at some selected frequencies could be described by third-degree polynomial equations. Analysis of variance showed that moisture content, temperature, and bulk density were all significant factors of these equations at a significance level of 0.0001. The equations were verified to be accurate in calculating the dielectric properties of wheat straw from a given moisture content, temperature, and bulk density at the selected frequency. This study offers useful information for developing moisture sensors of wheat straw from dielectric properties.