The change in concentration or density of a moderately dilute (1−6% mass fraction) suspension with time is measured at known depths, using optical or X-ray sensing. X-ray gravitational sedimentation (XRS) is well established in several industries. Sedimentation methods are based on the application of Stokes' Law, which describes the terminal velocity for an isolated sphere settling in a viscous fluid under the influence of a gravitational field. Stokes' Law is valid only if the Reynolds number (Re) does not exceed about 0.25 (ISO 13317-1:2001). Using a density value of 3.2 g/cm3 for portland cement, the largest (equivalent spherical) diameter that can be sized accurately with XRS is about 95 µm in isopropanol (IPA) at 25 ºC. Particles larger than this will settle much more slowly than predicted by Stokes' Law. The upper size limit can be increased by using a higher viscosity fluid. Irregularly shaped particles should settle according to their equivalent spherical volume at low Re values. For fines, the effect of Brownian motion exerts a significant influence on settling at diameters below about 1 µm in water and about 0.7 µm in IPA. Convection currents in the settling suspension may further limit the lower size range. Parameters required for XRS are solid and liquid phase density, and liquid phase viscosity. The particles must remain stable against agglomeration during settling or the measured PSD will be skewed toward larger sizes.