Scuba diving tanks come in a variety of capacities, styles and contents. The standard mixture is nothing more than ordinary compressed air, containing 78% nitrogen, 21% oxygen just as exists in the atmosphere near sea level. Deep dives and other specialty applications may use a higher concentration of oxygen, add helium or have other exotic mixtures. Nitrox tanks, just as one example, have a slightly higher concentration of oxygen. That difference helps reduce the possibility of decompression sickness, since less nitrogen gets stored in the body during breathing underwater. On the flip side, oxygen in too high concentration is toxic, and conditions in the tank and on the dive have to be carefully controlled.
Scuba Tank Pressure
All tanks are filled to around 3,000 psi (pounds per square inch, about 204 atm), since regulators are designed to take air at that pressure and reduce it in steps. There are differences between tank pressures, but regulators can compensate within the common range. So-called ‘high-pressure’ 3,442 psi tanks are rapidly becoming much more common.
In the standard 2-stage, open-circuit design, the mixture is first reduced to 10 atm (147 lbs per square inch). As the diver descends, breathing normally through the mouth, the regulator delivers air at the ambient pressure of the surrounding water.
Higher pressure tanks put more strain on regulator and valve parts, seals, etc. so factor this into any selection.
Even main supply tanks range everywhere from small capacity 65′s to over 130. The number refers to the cubic feet of air in the tank. This makes for a wide range of possible dive times. But if you typically come up with a tank that is half-full you’re carrying more weight than necessary, both on the tank and from its contents. Larger capacity tanks weigh more and air has weight. Eighty cubic feet of air – the standard amount – compressed or not, weighs 6.5lbs (2.95 kg). That’s a not-insignificant percentage of the total, since the average steel tank will weigh about 28lbs (12.7 kg) empty. That makes it easy to tell the difference between a completely empty tank and one completely full.
Scuba Tank Length
Lengths vary from about 21 inches (53.3 cm) to 26 inches (66 cm) for the ‘Standard AL80′ to 30 inches (76 cm) for the high capacity 130′s. A longer tank may require the selection of a different buoyancy compensator. Most tanks are around 7 1/4 inches (18.4 cm) in diameter, but larger ones do exist. Be careful though . Many storage racks on boats won’t accommodate the larger diameter tank, so make sure you check the specifications of your boat before buying a tank (if you have a boat:).
Scuba Tank Materials (The Buoyancy Factor)
Buoyancy is a factor when selecting a tank. Aluminum tanks will move from negatively buoyant (sinking) to positively buoyant (floating) as they’re emptied. That makes it more difficult for the diver to maintain neutral buoyancy, which is every diver’s challenge under the surface. This means that a diver who is properly weighted at the beginning of a dive, will not be nearer the end. Since maintaining neutral buoyancy is a dynamic challenge for divers, it’s not helpful for the tank to make the task more difficult.
Steel tanks, by contrast, change from negative to neutrally buoyant as they’re emptied. That makes it possible for the diver to begin the dive with less weight, and to more easily maintain the proper buoyancy throughout.
As tank technology develops, the differences are becoming less pronounced, but it’s wise to examine each tank carefully.
Price is always a factor, of course. Aluminum tanks are still, on average, cheaper than steel, but here again the difference is shrinking. Differences among manufacturers, extra features and other factors can easily swamp any inherent price advantage of one versus the other.
Naturally, if you keep a steel tank twice as long, the lifetime price difference may shift. However, as technology today changes much more rapidly than in the past decades, it becomes increasingly difficult to look ahead 20 or 30 years and decide whether a ‘life-time’ purchase is a worth the effort and money.