Filter Purity Levels 30 parts per million de-ioniser filter: Purifies ordinary tap water to 30 parts per million (30ppm). Water at this level of purity will provide good spot-free results when hand washing vehicles. The green resin media inside the cartridge changes colour to purple to indicate when the media is becoming exhausted. 0 parts per million de-ioniser filter: Purifies ordinary tap water to 0 parts per million (0ppm). Water at this higher level of purity will provide excellent spot-free results when hand washing vehicles. The blue resin media inside the cartridge changes colour to brown to indicate when the media is becoming exhausted. In hard water areas we recommend the use of a green (30ppm) in-line filter prior to the blue filter in order to prolong filter life.