The grain size determination unit is equipped with a SediGraph 5100 analyzer combined with a MasterTech 51 automatic sampler that allows determination of particle size in different types of sediment or particulate matter. Determination is carried out by the sedimentation method following Stokes law. The SediGraph 5100 uses a low energy collimating X-Ray beam to mesure the sedimentation rate of each particle. The best results are obtained for the clay, silt or fine sand fractions. The whole system is automatized, so that the samples are introduced into the mixing chamber and the analytical process is programmed from the computer atached. All parameters, including the Reynolds number, are determinated automatically, thus eliminating the errors caused by manual introduction comomonly used in other systems.