Hydrometer analysis is a widely used method of obtaining an estimate of the distribution of soil particle sizes from the No. 200 (0.075 mm) sieve to around 0.01 mm. The data are presented on a semilog plot of percent finer vs. particle diameters and may be combined with the data from a sieve analysis of the material retained (+) on the No.200 sieve. The principal value of the hydrometer analysis appears to be to obtain the clay fraction (generally accepted as the percent finer than 0.002 mm). The hydrometer analysis may also have value in identifying particle sizes < 0.02 mm in frost susceptibility checks for pavement subgrades. This test is done when more than 20% pass through No.200 sieve and 90% or more passes the No. 4 (4.75 mm) sieve.
The hydrometer analysis is based on Stokes’ Law, which gives the relationship among the velocity of fall of spheres in a fluid, the diameter of the sphere, the specific weights of the sphere and of the fluid, and the fluid viscosity. In equation form this relationship is