Dielectric moisture measurement method is based on the fact, that permittivity Er of dry material and hard consisting particles is 2...5, air 1 and Er of free water is 80. Because of that the resulting Er of various materials is mainly determined by moisture content. The practical use of this method is complicated by the high and changing electrical losses of material under test. Despite of numerous dielectric mixture theories and formulas it is more accurate to use experimentally established dependences between Er and moisture content W.
In general the plots Er = f (W) are not linear due to the altering of water electrical properties in the material at different moisture contents because of the water-solid interaction processes. Inherent dependence is presnet between volumetric moisture content Wv and Er of various materials. Mass based moisture Wm derivates as Wm=Wv/d, where d is density of the material.
The measurement of Er of natural materials is a complicated technical problem. The proper measurement method, frequency, schematics and electrodes must be chosen in order to avoid the influence of conductivity to measurement results. Also water physical processes in materials must be understood and taken into account.
The older types of the electrical moisture meters are based on the conductivity measurement. The conductivity does really change with moisture content, but in much bigger scale conductivity depends on salt content of material. The best picture about material state gives simultaneous measurement of its permittivity and conductivity. The percometer is a suitable instrument for such studies.
Company Adek LLC has been active in dielectric research of materials since 1980s. Below are presented some samples of experimentally established dependences between Er of different materials and moisture content W. These dependences are used at moisture measurement.
Er=f(Wv) for 4 Estonian soiltypes, blue line is coarse sand
Er=f(Wm) of††Estonian softwood†timber†
Sample of a floor moisture chart, measured by percometer