SR 50sqft Posi-Flo II Cartridge Filter Posi-Flo® II Filters remove particles so small you can't even see them. And, that includes gel-like particles from suntan and body oils, as well as sand, dirt and leaves. What's more, the filter element in Posi-Flo® II Filters holds more dirt than others of the same size. The secret is a balanced-flow hydraulic design that directs water equally over the entire surface area of the element, for greater dirt holding capacity before needing cleaning.