Hydro-Logic? Chlorashield membranes remove 100% of chloramine and are great for city water. Rated at 1 GPM down to 1 micron. They are made in the USA and have a low pressure drop combined with exceptionally high dirt holding capacity. HGC728772 is for Evolution-RO systems (20 in x 2.5 in) and should be replaced after 2,500 gallons. HGC741668 is for TallBoy systems (20 in x 2.75 in) and should be replaced after 6,000 gallons. HGC741640 is for PreEvolution sysytems (10 in x 4.5 in) and should be replaced after 7,500 gallons. HGC741700 is for smallBoy/Stealth-RO systems (9.75 in x 2.75 in) and should be replaced after 1,500 gallons when used with Stealth-RO systems and 3,000 gallons when used in smallBoy systems.