A basis for indicating the resistance in a hydraulic system, equivalent to the height of a column of water that would cause the same resistance (100 feet of head equals 13 pounds per square inch).

A device that removes undissolved particles from water by recirculating the water through a porous substance (a filter medium or element).

Usually refers to powder-like substance such as diatomaceous earth or volcanic ash used to coat a septum-type filter. Can also be used as an aid to sand filters.

A device within a filter tank designed to entrap solids and conduct water to a manifold, collection header, pipe or similar conduit and return it to the pool or spa. A filter element usually consists of a septum and septum support, or a cartridge.

A finely graded material (such as sand, diatomaceous earth, polyester fabric, anthracite, etc.) that removes filterable particles from the water.

A chemical substance or compound that promotes the combination, agglomeration, or coagulation of suspended particles in water.

That portion of the total chlorine remaining in chlorinated water that is not combined with ammonia or nitrogen and will react chemically with undesirable or pathogenic organisms.