Every compressor filter cartridge has a limited life. Once the molecular sieve is saturated, moisture gets through and the air no longer meets the breathing-air standard. The calculator estimates how many hours a cartridge lasts on your compressor and how many cylinders it fills in that time.
The method is the one the makers use: the air reaches the filter saturated with water and a little warmer than the room, and the sieve takes up about a fifth of its own weight in water. Room temperature and tower pressure decide the rest. Enter the cartridge price as well to get the cost per fill.