Grain-size analysis by settling techniques presupposes free sinking of particles. Settling convection is a current system set up by different densities of clouds in a settling suspension. Settling convection tends to augment the sinking velocity of sand grains released at the surface, especially in samples with a narrow range in size and also for particles of less than 0.1-0.05 mm. Hence, the usefulness of various types of sedimentation analyzers based on sample introduction from above is restricted to sizes above 50 microns. Settling convection also renders ineffective counter-current methods (elutriators) in water. Elutriation in air is inapplicable for lutum particles that have been wet, hence for most natural sediments except eolian deposits.

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