Equivalent spherical diameter

Of note, the ISO standards providing guidance for performing particle size determination by static and dynamic image analysis (respectively ISO 13322-1 and 13322-2) [4][5] recommend to define particle size by a combination of 3 primary measurements, namely the area-equivalent diameter, the maximum Feret diameter, and the minimum Feret diameter.

Of note, the equivalent sieve diameter can be significantly smaller than the area-equivalent diameter obtained by optical methods, as particles can pass the sieve apertures in an orientation corresponding to their smallest projection surface.

[6] In addition, the favored mean particle size for laser diffraction results is the D[4,3] or De Brouckere mean diameter, which is typically applied to measurement techniques where the measured signal is proportional to the volume of the particles.

This corresponds to the diameter of a sphere with the same translational diffusion coefficient D as the particle, in the same fluid and under the same conditions.

The relationship between the diffusion coefficient D and the HDD is  defined by the Stokes–Einstein equation: where Particle size analysis techniques based on gravitational or centrifugal sedimentation (e.g., hydrometer technique used for soil texture[7]) are based on Stokes’ law, and consist in calculating the size of particles from the speed at which they settle in a liquid.