Keep moisture out of your Spare Air/HEED! Moisture causes rust and can compromise the integrity of your tank. Ambient air used by hand pumps contains varying amounts of moisture depending on temperature and relative humidity. Dry-Pac is a patented method of extracting that moisture before passing that air through the pump. This is the medium that's inside the Dry-Pac unit in the Hill Hand Pump. Dry-Pac medium is vacuum packed and can be stored indefinitely providing the vacuum remains intact.