Cylinders used for scuba typically have an internal volume (known as water capacity) of between 3 and 18 litres (0.11 and 0.64 cu ft) and a maximum working pressure rating from 184 to 300 bars (2,670 to 4,350 psi).

Cylinders are also available in smaller sizes, such as 0.5, 1.5 and 2 litres, however these are usually used for purposes such as inflation of surface marker buoys, dry suits and buoyancy compensators rather than breathing.

Pressurized diving cylinders are considered dangerous goods for commercial transportation, and regional and international standards for colouring and labeling may also apply.

The term "diving cylinder" tends to be used by gas equipment engineers, manufacturers, support professionals, and divers speaking British English.

[5] Filament wound composite cylinders are used in fire fighting breathing apparatus and oxygen first aid equipment because of their low weight, but are rarely used for diving, due to their high positive buoyancy.

An especially common rental cylinder provided at tropical dive resorts is the "aluminium-S80" which is an aluminum cylinder design with an internal volume of 0.39 cubic feet (11.0 L) rated to hold a nominal volume of 80 cubic feet (2,300 L) of atmospheric pressure gas at its rated working pressure of 3,000 pounds per square inch (207 bar).

A larger number have failed the eddy current test and visual inspection of neck threads, or have leaked and been removed from service without harm to anyone.

This makes them heavier than they need to be for strength, but the extra weight at the base also helps keep the centre of gravity low which gives better balance in the water and reduces excess buoyancy.

The cylinders are machined to provide the neck thread and o-ring seat (if applicable), then chemically cleaned or shot-blasted inside and out to remove mill-scale.

[41] A relatively uncommon manifold system is a connection which screws directly into the neck threads of both cylinders, and has a single valve to release gas to a connector for a regulator.

Some divers will not use boots or nets as they can snag more easily than a bare cylinder and constitute an entrapment hazard in some environments such as caves and the interior of wrecks.

In the United States and perhaps[citation needed] a few other places the pressure is measured in pounds per square inch (psi), and the rest of the world uses bar.

[27] These are representative examples, for a larger range, the on-line catalogues of the manufacturers such as Faber, Pressed Steel, Luxfer, and Catalina may be consulted.

Steel cylinders are available in the following size classes, and possibly others:[55] Wall thickness varies depending on location, material, pressure rating and practical considerations.

The advantage is that a regulator failure can be solved underwater to bring the dive to a controlled conclusion without buddy breathing or gas sharing.

The complexity of switching regulators periodically to ensure both cylinders are evenly used may be offset by the redundancy of two entirely separate breathing gas supplies.

The disadvantages are that the manifold is another potential point of failure, and there is a danger of losing all gas from both cylinders if the isolation valve cannot be closed when a problem occurs.

There are three main factors to consider: To calculate the quantity of gas consumed: Metric examples: Imperial examples: Keeping this in mind, it is not hard to see why technical divers who do long deep dives require multiple cylinders or rebreathers, and commercial divers normally use surface-supplied diving equipment, and only carry scuba as an emergency gas supply.

It is strongly recommended by diver training organisations and codes of practice that a portion of the usable gas of the cylinder be held aside as a safety reserve.

High-pressure storage is commonly used when blending nitrox, heliox and trimix diving gases, and for oxygen for rebreathers and decompression gas.

However, immersion for cooling can also increase the risk of water contaminating the valve orifice of a completely depressurized tank and being blown into the cylinder during filling.

[106][107] Unless the cylinder walls are examined by ultrasonic methods, the interior must be visually inspected using sufficient illumination to identify any damage and defects, particularly corrosion.

This is more likely with taper thread valves, and when it happens most of the energy of the compressed gas is released within a second, and can accelerate the cylinder to speeds which can cause severe injury or damage to the surroundings.

Selecting the Proper Shipping Name (well known by the abbreviation PSN) is a way to help ensure that the dangerous goods offered for transport accurately represent the hazards.

"[136] International Civil Aviation Organization (ICAO) Technical Instructions for the Safe Transport of Dangerous Goods by Air states that provided that pressure in diving cylinders is less than 200 kilopascals (2 bar; 29 psi), these can be carried as checked in or carry-on baggage.

Transportation of pressurised diving gas cylinders with a combined water capacity of less than 1000 litres on a vehicle for personal use is exempt from ADR.

[138][141][142] Transport of gas cylinders in a vehicle, for commercial purposes, must follow basic legal safety requirements and, unless specifically exempted, must comply with ADR.

[138][141] Diving gases, including compressed air, oxygen, nitrox, heliox, trimix, helium and argon, are non-toxic, non flammable, and may be oxidizer or asphyxiant, and are rated in Transport category 3.

[144] A cylinder containing 200 kPa (29.0 psig/43.8 psia) or greater at 20 °C (68 °F) of non-flammable, nonpoisonous compressed gas, and being transported for commercial purposes is classified as HAZMAT (hazardous materials) in terms of 49 CFR 173.115(b) (1).

[145] Cylinders manufactured to DOT standards or special permits (exemptions)issued by the Pipeline and Hazardous Materials Safety Administration and filled to the authorized working pressure are legal for commercial transport in the USA under the provisions and conditions of the regulations.