Particle Size Measurement: The Equivalent Spherical Diameter

In particle size analysis, it is commonplace to analyse data for anisodiametric particles as if the particles were spheres. Expressions are given herein for the equivalent spherical diameters (ESD) for discs, rods and the more general oblate and prolate spheroids. The dependence of the ESD on the true major dimension and axial ratio differs with the physical basis of measurement, and identity of the ESD values from different methods cannot be expected. A comparison between any two sets of experimental data leads to estimation of the minor particle dimension and a quantitative measure of the particle non-sphericity.