A simple interpolation formula for the sedimentation velocity of a particle, including the limiting cases of small and large Reynolds numbers is proposed. A comparison with other known similar formulas is given. It is shown that the adjustment of sedimentation law is important to describe the hydrocyclones separation curve for large particles. A way of representing the separation curve, which allows justify the assumptions underlying the model of Schubert-Neesse is shown. It is shown that the correction of formula for the sedimentation velocity significantly affects the estimate of the effective coefficient of turbulent diffusion in the apparatus. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.