Scope

Figure 1. Setup of equipment for the particle size analysis of soils.

The particle size analysis of soils (Hydrometer) determines the distribution of particle sizes in soils down to the 0.001 mm particle size based on the rate at which they fall in a liquid. The procedure is based on Stoke’s law, which is a formula that expresses the rates of settling of spherical particles in a fluid: V=Cr2, where V is the velocity (in cm/sec), r is the particle’s radius (in cm), and C is a constant relating relative densities of fluid and particle, acceleration due to gravity, and the viscosity of the fluid.

The procedure uses a prepared, dispersed sample placed into a sedimentation cylinder. Hydrometers are then used to measure the specific gravity of the solution containing the suspended particles as they fall. Measurements of specific gravity with correction for temperature and the specific gravity of the particles involved determine the percentage of the sample which has not yet settled. Measurement of the elapsed time, adjusted for depth of measurement, temperature, and specific gravity of the soil, determines the maximum diameter of the particles still suspended at the effective depth.

The procedure involving the hydrometer analysis can be reported in conjunction with the mechanical sieve analysis (T 27). When particle sizes below the 0.075 mm (#200) sieve are not specified then the use of hydrometer analysis is not necessary. Any particle size determination below the 0.075 (#200) sieve should use the hydrometer procedure