Ratings are expressed as a code: The first two characters (letters) designate the typical application for that cartridge or "application indicator" BR = Breathing Air DR = Air/Gas Drying CL = Air/Gas Cleaning DD = Air/Gas Sweetening CM = Carbon Monoxide Removal CD = Carbon Dioxide Removal ND = Inert Gas Drying NG = Natural Gas Drying PA = Particulate Removal OY = Oxygen The numerical character (digits) designate the performance scale - with a range of 1 to 10 we'll call this the"Performance Rating". 1 = Poorest 5 = Standard 10 = Best
