Capacitive throughput sensor for plant materials – Effects of frequency and moisture content

Abstract

The empirically determined material moisture content and frequency are among the main parameters that affect the dielectric properties of plant material. When using a capacitive sensor to measure the throughput of plant materials, an unexpected change in the measured material moisture content can result in significant error. This problem can be minimised by using a suitable electric field frequency. Thus, we studied the effect of various frequencies on capacitive throughput sensor behaviour.A novel laboratory capacitor was developed, and two sets of experiments using balsa blocks as plant material were performed. We confirmed that the dependence of sensor output voltage on the measured material moisture content could be described by S-shaped curves with two breakpoints between three sections. The first and third sections were characterised by a low influence of material moisture content on the capacitive throughput sensor output signal, whereas the moisture content had a larger influence in the second section. The shapes of the calculated curves were influenced by both the frequency of the electric field and the level of sensor filling. The sensor was less sensitive to moisture content changes when lower frequencies were employed and when the fill level was lower.