The Hydrometer Test is used to determine the particle size distribution of fine-grained soils (silt and clay) based on sedimentation principles. Principle The test is based on Stokes’ Law, which states: Larger particles settle faster, while smaller particles remain suspended longer in water. So, by measuring density of suspension at different times, we can determine particle sizes. Applications • Classification of clayey & silty soils • Design of foundations • Permeability analysis • Settlement calculation • Used along with sieve analysis for full gradation Important Points • Used only for fine particles (< 0.075 mm) • Based on sedimentation, not sieving • Requires temperature & meniscus correction • Combined with sieve test for full soil analysis