In situmeasurement of moisture in soil and similar substances by `fringe' capacitance

Abstract

The determination of moisture content (volume fraction) of soil and similar materials by measurements of capacitance at v.h.f. is studied, including in situ measurements made by using the `fringe' capacitance of a suitably designed probe electrode system. Results on samples of made up and natural soils with moisture contents from 0 to about 45% are shown to be independent of soil type within the limits of uncertainty resulting from variability of moisture distribution in the substances tested. There is a linear relationship between moisture content and the `fringe' capacitance from 0 to about 10% moisture. From 5 to about 45% the moisture content is a linear function of the logarithm of the `fringe' capacitance. Estimates of the numerical constants of these relationships, obtained by statistical procedures, enable calibration curves to be derived for which the variances and standard errors are calculated. Over most of the range of practical interest the standard error is approximately equal to 0.5% moisture content (volume basis). The relationships between real relative permittivity and moisture content are also derived. Brief directions for using the method, including the treatment of results of replicate tests in the field, are given.