|pictured: Sherwood Aluminum Compact Neutral 80 scuba tank|
A scuba tank, also called a scuba cylinder, is constructed out of either steel or aluminum. Aluminum tanks are more common and less expensive, but more prone to denting and wear. Most dive gear shops will only have aluminum tanks for rent, so it is good to become used to their size, weight, shape and buoyancy.
Steel scuba tanks are a little more expensive, but they are far more durable. It is said that a properly maintained steel tank will last longer than the diver who wears it. The caveat with a steel scuba tank is that it will rust and a steel tank user needs to periodically inspect their tank for rust, but rusting can be somewhat avoided with proper care.
Whether you get aluminum or steel, regardless of the actual physical tolerance of the tank, when you get your tank filled it will be filled to 3000 psi (see the Scuba Physics section for explanation). The capacity of your tank is measured in pressurized cubic feet; in other words how many cubic feet (or liters) of air can be stored in the tank at 3000psi? An average tank holds about 80 cubic feet of air, though tanks range in size from 65 to 100 cubic feet. Extra-large and extra-small scuba tanks are available in size from 120 cubic feet down to the tiny 6 cubic foot "pony tanks".
The size of scuba tank you need is decided by the amount of air you will need; the amount of air you need is determined by your size, the size of your lungs, your fitness level and other factors. If you are a teen or a petite woman you will be fine with a smaller tank. An average adult will take an average (80 cu.ft.) tank. Large men who breathe deeply might want the largest tank available.
An 80 cu.ft. cylinder at 3000 psi contains 80 cu. ft. of air, which is about the size of a small telephone booth. The air in a telephone booth at 14.7 psi (or 1 ATM, which is ambient sea-level air pressure) weighs about 6.5lbs. The air is compressed 204.08 times smaller to fit into the scuba cylinder.
3000 psi / 14.7 psi = 204.08
The air in the tank still weighs 6.5lbs, which is why you can feel the weight difference between an empty tank and a full one. You would feel the same difference lifting the telephone booth full of air, compared with the same booth containing a vacuum.
One kind of scuba cylinder becoming increasingly popular is the pony tank. A pony tank is a little extra tank which straps to your gear, which has separate staged regulators and sometimes gauges as well. In case your equipment malfunctions or you misjudge your air consumption the pony contains enough air to do a proper slow ascent to the surface. A pony tank is usually carried as an emergency backup air supply, not as a means of extending a dive.