It’s well known that scuba diving tanks are the canisters that supply air to the diver underwater. But beyond performing that seemingly simple, singular function there is much variety. For a plain metal cylinder, scuba tanks have a surprising number of differences.
With the exception of certain unique designs, all tanks are made either of a steel alloy or aluminum. Technical diving, specialized sport diving and military applications will sometimes use a chrome-molybdenum alloy. Each type has its pros and cons.
Steel tanks were traditionally heavier, but much less prone to puncture or rupture, scratching or denting. Contemporary steel tanks are much closer in weight to aluminum ones, since they can be made thinner while retaining adequate strength. More importantly, the threads on the neck of a steel tank are much less likely to get stripped from repeated replacement of the regulator gear.
Aluminum tanks are lighter, though the difference underwater is minor. The difference becomes more important when considering how much effort is involved in transporting them between dives. Aluminum air tanks also frequently have a positive buoyancy that may need to be compensated for with added weights. That fact somewhat counteracts their lighter weight advantage.
Aluminum tanks are more common and usually less expensive than steel, but the latter tend to last longer. A properly maintained steel tank will last 30 years or more. Some dive shops won’t fill an aluminum tank over 15 years old.
Aluminum tanks are less durable in the presence of high heat. At 350F, the walls weaken to the point that – at standard compressed-air pressure – they can burst. Naturally, no tank should ever be exposed to such temperatures, but stranger things have happened.
Though a steel tank can rust in the form of iron oxide, aluminum can corrode too, producing aluminum oxide. Neither is a serious health risk, but both should be maintained periodically to eliminate any contaminants. Aluminum will require this less often.
The overwhelming majority of scuba tanks – aqua-lungs as they were known when first designed by Jacques Cousteau and Emil Gagnon in WWII – hold about 80 cubic feet of compressed air. Several larger models do exist, for very large divers or those who need an extra-large capacity for long dives.
Smaller tanks – sometimes called auxiliary tanks, but most frequently referred to as pony tanks – are used as a backup in case the primary fails or is emptied. Smaller tanks hold about 65 cubic feet of compressed air, but ponies typically will be from 10-40 cubic feet. Pony’s usually are secured by a separate strap.
Any tank, properly cared for, will last through many years of reliable service. Considering what depends on it, make the effort to research your purchase carefully.