Micron rating
Also called: microns, filter micron rating, µm.
A micron rating is the particle size a pool filter is said to trap, in microns (µm). The ladder quoted everywhere, sand 20–40, cartridge 10–20 and DE 2–5, is a dealer convention, not a manufacturer specification. Treat a micron rating as a rough ranking, not a measurement.
Why it matters
The ranking is real: DE is the finest of the three and sand is the coarsest. The numbers are shakier. Hayward's own pool-filter landing page and its 28-page ProGrid D.E. owner's manual mention microns zero times, and no maker publishes a tested micron figure for DE. What the makers do publish is filter area in square feet. More area means more surface catching dirt, longer between cleanings and less strain on the pump, so when you compare two filters of the same type, area is the number to read. Decide between filter types on labour, water use and running cost instead.