Hydrometer Anaysis

It is assumed that Stokes’ law can be applied to a mass of dispersed soil particles of various shapes and sizes. Larger particles settle more rapidly than the smaller ones. The hydrometer analysis is an application of Stokes’ law that per- mits the calculation of the grain size distribution in silts and clays, where the soil particles are given the sizes of equivalent spherical particles. The density of a soil-water suspension depends upon the concentration and specific gravity of the soil particles. If the suspension is allowed to stand,

the particles will gradually settle out of the suspen- sion, and the density will be decreased. The hydrometer is the instrument used to measure the density of the suspension at a known depth below the surface. The density measurement, together with knowledge of specific gravity of the soil particles, determines the percentage of dispersed soil particles in suspension at the time and depth of measurement. Stokes’ law is used to calculate the maximum equivalent particle diameter for the material in suspension at this depth and for the elapsed time of settlement. A series of density measurements at known depth of suspension and at known times of settlement gives the percentages of particles finer than the diameters given by Stokes’ law. Thus the series of readings will reflect the amount of different sizes of particles in the fine-grained soils.

Apparatus

Figure shows some of the items that you willneed to perform a hydrometer analysis. Another item you will need is an ASTM hydrometer (fig. 1).

ASTM hydrometers are graduated by the manufacturer to read in either specific gravity or in grams per liter and are calibrated at a standard temperature of 68Ð'oF (20Ð'oC). Other needed items are a dispersion cup and stirrer,