Scuba diving is one of the few sports that has a built-in limit: You can dive only as long as you have air in your tanks. But compressed air is a critical life-support supply, so although you cannot safely use the air compressor in your garage to refill your tanks, there are several options that divers have for keeping their cylinders full and their dives frequent.
Scuba tanks come in different sizes and materials, including aluminum and steel. Tanks rated for recreational use typically are compressed so that between 2,000 and 2,500 pounds per square inch of air is contained within them. Any compressor rated to fill a diving cylinder must meet this pressure requirement and must also be able to fill the tank with pure, dry air. Any impurities in the air will be concentrated under the pressure of depth and could lead to injury or death for a diver breathing contaminated air.
The safest bet for refilling a diving cylinder is to take it to a certified dive shop. Most dive shops have special equipment that will fill tanks with purified air, and their equipment can compensate for pressure problems related to the heat expansion of the tank as it undergoes rapid pressurization. Make sure your dive shop insists on refilling tanks only for certified divers, and check their inspection records to make sure they follow safe protocol.
For divers who can afford the expense, there are personal refilling stations available for purchase from scuba-supply companies; they can cost more than $1,000. These stations provide for purified air under appropriate pressures, and should be regularly inspected annually by a certified technician.