The HB-20-1W cartridge filter is a nominally rated “Calypso Blue Premium Grade” filter manufactured by Harmsco Filtration. The filter is 4.5″ in diameter and has a particle retention rating of 1 micron. The filter is 20″ in length and has 26 sq. ft. of NSF 62 Certified Polyester Cellulose filter media. This filter is a double open end, (DOE) configuration. Filters are individually wrapped in cases of 4.