Tanks and Valves

A scuba tank (also called a scuba cylinder) holds your breathing gas. When choosing tanks (to rent or to buy), you’ll want to know something about the types of tanks, their capacity, rated pressure, valves, and inspection and maintenance standards.

Aluminum vs. Steel

Tanks for scuba use are made from aluminum or steel. The lightweight carbon-fiber tanks used by fire-fighters are not suitable for diving (they are very buoyant, and easier to damage). Tanks in the USA are described by the number of cubic feet of gas they hold when full; for example an AL63 tank is made of aluminum and holds 63 cubic feet when full. In other countries tanks are often rated by their “water capacity” (the amount of water they can hold, unpressurized); a 12-liter tank, for instance, is very close in capacity to an AL80.

Aluminum tanks are very common. Most rental tanks, whether from dive shops or charter boats, are aluminum. Steel tanks are rarer, but they are stronger than aluminum, and many steel tanks are designed to hold higher pressures, allowing you to carry more gas on your dive in roughly the same-sized tank.

Tank Characteristics

Tank size is of course important. Aluminum tanks are available to purchase in a wide range of sizes, from less than 10 cubic feet (serving as emergency-use pony bottles) to 100 cubic feet. Steel tanks range widely in size as well, up to 120 cubic feet capacity. The most common rental tank in the USA is the aluminum eighty (AL80), which actually contains about 78 cubic feet when full.

Temperature plays a role when determining capacity. As a tank (and the gas in it) warms up, its pressure rises. As the tank and gas cool, the pressure drops. A newly-filled tank which is warm to the touch might show a pressure of 3000 psi (pounds per square inch) but that same tank in cold water might read 2800 psi. Tank manufacturers specify the temperature and pressure at which the tank will be full. For example, most AL80s are intended to be filled to 3000 psi at 70 degrees Fahrenheit.

Steel tanks are often designed as High-Pressure (HP) or Low-Pressure (LP). A common HP tank rating is 3342 psi. A common LP rating is 2400 psi. Aluminum tanks are usually designed for 3000 psi. Tanks should not be overfilled, as this may cause stresses which render the tank unsafe.

Aluminum tanks are buoyant when empty, while empty steel tanks are still slightly heavier than water. Both types are negatively buoyant when full, steel by several pounds more than aluminum. This means that you can remove some weight from your BC or belt when switching from aluminum to steel tanks.

Some tanks have flat bottoms. Those with rounded bottoms are fitted with plastic boots which allow them to stand upright. Some divers add protective sleeves of plastic mesh to protect the paint on their tanks.

Tanks of course vary in size, but most scuba cylinders are between 7 and 8 inches in diameter. Precise dimensions can be found on each manufacturer’s web site, where you may also find information on the other markings on a tank. These vary somewhat but will include a serial number, rated fill pressure, the applicable manufacturing standards, and the date of manufacture.

Tank Valves

Today’s tank valves are either yoke or DIN (Deutsche Industrie Norm, the German manufacturing standards). Yoke valves are very common in the USA and on aluminum tanks. DIN valves are most often seen on steel tanks. The DIN connection between valve and regulator first stage is more secure, and is ideal for high-pressure tanks. An older valve design, known as the J-valve, incorporates a small gas reserve which can be used by pulling a lever built into the valve.

Opposite the valve’s handwheel you’ll find a short protrusion which contains the burst disk. This metal disk is designed to fail at a pressure several hundred psi higher than the tank’s rated fill pressure. If the tank is accidentally overfilled, the burst disk should fail and vent the tank noisily but without the massive damage that would be caused by a tank rupture. Tanks left baking the sun in the back of a car have also been known to heat up enough to raise their pressure to the burst disk point.

Both yoke and DIN valves use an o-ring to seal the connection between the valve and the regulator first stage. Inside the tank, the valve connects to a short metal tube called a dip tube. In the event that a tank is contaminated with water, metal flakes, or other solid matter, the dip tube prevents those contaminants from being taken up into the regulator, even when the tank is inverted.

Inspection and Maintenance

Scuba tanks must pass an annual visual inspection. The tank is drained, the valve removed, and the inside and outside are inspected for signs of damage, corrosion, or cracking. This requires removing any decals that you’ve applied to the tank. Any internal contaminants, corrosion, or rust is removed and the tank thoroughly dried before reassembly and filling. Following a satisfactory inspection, a dated sticker is applied to show that the tank passed. A tank whose inspection sticker is more than a year old should not be used.

Tanks must also pass a hydrostatic test every five years. The hydro test involves filling the tank with water and then pressurizing it to a point where the metal flexes. When the pressure is released, the tank must return to within a very small fraction of its original volume. When a tank passes a hydro test, the test date is stamped into the metal near the top. You should not use a tank whose stamped hydro date is more than five years old. If a tank fails this test, the metal is fatigued and the tank is not safe to use. Failed tanks are condemned.

Using a Tank

Before setting up your gear, it’s wise to crack open the valve to insure that any dust or water on the valve surface gets blown off. This also gives you a chance to sniff the gas flow. What comes out of a scuba tank should never have any discernable odor.

And you’ll want to inspect the valve’s o-ring. These get worn with use, and will eventually leak if not replaced. Often you’ll only see the leak once you’re underwater.

Air does have weight, of course: a cubic foot of air weighs about .08 pounds. So the air in a full AL80 weighs about six pounds. You breathe that are out into the water while diving. If you’re finishing up a dive with 500 psi in your tank, it now weighs five pounds less than it did when full. This is important to take into account when judging your weighting needs.

And please don’t assume that your tank is indestructible. Aluminum tanks in particular are soft enough that a severe blow can gouge or dent them enough to make them fail an inspection. Minor nicks and scratches, on the other hand, are of no concern. Finally, it’s important to never allow the tank to get fully empty. This can allow contaminants (dust, water) to enter the tank.