Sensors & Transducers (Apr 2008)
The Physical Basis of Dielectric Moisture Sensing
Abstract
Moisture content sensors for hygroscopic solids below saturation most commonly detect low frequency (<10 kHz) conductivity, or permittivity at microwave frequencies (0.1 – 10 GHz), with calibration being done empirically. Two physical processes are responsible for the moisture-dependent admittivity in these frequency ranges. At low frequencies ionic hopping between the absorbed water molecules gives rise to the “logarithmic” dependence of conductivity on moisture content that is a generic characteristic of hygroscopic solids. At higher frequencies the admittivity is dominated by the dipole response of the absorbed water molecules.
