This part of ISO 13318 describes a method for determining the particle size distribution of homogeneous particulate material using centrifugal sedimentation in a liquid. Solids concentrations are determined by the attenuation of an X-ray beam. The resulting signal enables conversion to a particle size distribution. The method of determining the particle size distribution described in this standard is applicable to powders which can be dispersed in liquids or powders which are present in slurry form. The typical particle size range for analysis is from 0,1 µm to 5 µm. The method is applicable to powders in which all particles have the same effective density, chemical composition and comparable shapes. Materials possessing elements with an atomic number greater than about 12 can be expected to produce adequate X-ray opacity. Particles should not undergo chemical or physical change in the suspension liquid. It is necessary that the particles have a higher density than that of the liquid.