Number Distribution

A number distribution is a particle‑size distribution weighted by particle count, not by surface or volume. It emphasises fines because many small particles contribute more ‘counts’ than a few large ones. In filtration, number‑weighted PSDs are useful when retention of fine particles drives product quality (e.g., clarity or bacteria removal), but they can misrepresent hydraulic behaviour because pressure drop and cake permeability correlate more closely with surface‑ or volume‑weighted measures. Good practice examines multiple PSD metrics—number, surface and volume distributions; D10/D50/D90; mode and width—while considering shape and agglomeration state. Measurement technique matters: optical counters inherently return number distributions, laser diffraction typically reports volume distributions, and image analysis can be converted between bases. When specifying media, align mean‑flow pore size to the mode of the effective distribution reaching the filter after pumps and mixers, and validate with plant turbidity/particle counts to ensure the chosen cut‑off meets specification.