Sedimentation methods (Hydrometer, pipette, and centrifugation) are used for quantitative determination of particle size distribution of particles smaller than 75µm.
Sedimentation methods are based on Stokes’ law that establishes the velocity at which particles settle in suspension under gravity. This law states that denser (larger) particles sink faster than less dense (smaller) particles when suspended in a liquid. The time rate of settlement provides a measure of the relative size of fine soil grains.
These methods are relatively time consuming and require a certain attention to sampling procedures to ensure correct sampling time and solution temperature. They require relatively large samples (10-20g for the pipette and 50g for the hydrometer). They can also give unreliable results for particles 1 µm and smaller because of the effect of Brownian motion on the rate of sedimentation.