Soil particle concentrations and size analysis using a dielectric method

Limitations of traditional methods for particle-size analysis warrant the i nvestigation of new techniques. An alternative method based on the differen ce between the dielectric constant of soil solids (approximate to 4) and di spersing solution (approximate to 81) was developed. We determined changes in suspended sediment concentrations (C) using a coaxial probe placed on th e surface of a dispersed soil suspension by monitoring changes in the appar ent dielectric constant with time following complete mixing. A single-point calibration for each sample was obtained using the known initial concentra tion. A refractive index (n) model of the suspension dielectric properties gave the slope of a C vs. n curve for changes in silt-size (0.002-0.05 mm) particles. A magnetic stirring rod was used to homogenize the dispersion, a nd temperature changes were minimized given the rapid measurement time. Usi ng the dielectric method. particle-size distributions were measured on a 1- to 2-g sample with 400-s settling time because the effective depth of meas urement was only 1.5 mm. Wet sieving was used to remove the sand fraction. Comparisons between the silt and clay fractions obtained using the dielectr ic and pipette methods were in agreement. The combination of speed, automat ion, small sample size, and nearly continuous data should be balanced again st the higher cost of the equipment necessary for the dielectric method.