DE filter (diatomaceous earth)
Also called: D.E. filter, diatomaceous earth filter, DE grids, DE powder.
A DE filter is a pool filter whose fabric-covered grids are coated with diatomaceous earth, a fine powder of fossilised algae skeletons poured in through the skimmer. That powder coating makes DE the finest of the three filter types. Every backwash flushes the powder out, so every backwash ends with a fresh charge.
Why it matters
DE gives the clearest water and charges for it in labour. Dose the recharge by filter area, from your filter's own manual: Pentair's FNS Plus manual works out to a flat 0.1 lb of DE per square foot, and some DE boxes print a heavier dose. Too much DE packs the grids. Once or twice a season the grids come out for a deep clean, and torn grids send powder into the pool. Pool DE is flux-calcined, which turns its silica into crystalline cristobalite, so wear a respirator (N95 or better) and pour it as a wet slurry. For an above-ground pool a DE filter is usually overkill.
In numbers
Pentair's FNS Plus manual tabulates 2.4 lb of DE for a 24 sq ft filter, 3.6 lb for 36, 4.8 lb for 48 and 6.0 lb for 60. A level 1 lb coffee can of DE weighs only half a pound, so measure by weight.