Special Issue: Modern Sensing Technologies

Vol. 90, April 2008, pp. 128-138

The Physical Basis of Dielectric Moisture Sensing

¹J. H. CHRISTIE and ²I. M. WOODHEAD

¹Streat Technology Ltd, 20 Pentre Tce, Christchurch, New Zealand

²Lincoln Ventures Ltd, Lincoln University, Christchurch, New Zealand

E-mail: christij@lvl.co.nz

 Received: 15 October 2007   /Accepted: 20 February 2008   /Published: 15 April 2008

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.

Keywords: Moisture content, Proton hopping, Dipole rotation

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