A diving cylinder (the term which tends to be used by divers) or SCUBA tank (more often used colloquially by non-divers) is used to store and transport high pressure breathing gas as a component of an Aqua-Lung. It provides gas to the SCUBA diver through the demand valve of a diving regulator. Diving cylinders are typically filled in the range of 186 to 300 bar (2700 to 4400 psi, or 18.6 to 30.0 MPa) and have a volume of 1.5 to 18 litres or a gas carrying capacity of 850 to 4200 litres (24 to 120 ft³).

Divers use gas cylinders above water for many purposes including storage of gases for oxygen first aid treatment of diving disorders and as part of storage "banks" for diving air compressor stations. There are also used for many purposes not connected to diving.

Parts of a Diving Cylinder

The diving cylinder consists of several parts:

* The pressure vessel is normally made of cold-extruded aluminium or forged steel. An especially common cylinder available at tropical dive resorts is an aluminium cylinder rated to hold 80 ft³ of gas (at 1 bar) at its rated pressure of 3000 psi (i.e., its capacity is approximately 10 litres at 200 bar). Aluminium cylinders are used where divers carry many cylinders, such as in technical diving, because the greater buoyancy of aluminium cylinders reduces the extra buoyancy the diver would need to achieve neutral buoyancy. In cold water diving, where a diver wearing a highly buoyant thermally insulating dive suit has a large excess of buoyancy, steel cylinders are often used because they are denser than aluminium cylinders. Kevlar composite cylinders are used in fire fighting breathing apparatus and oxygen first aid equipment, but are rarely used for diving, due to their high positive buoyancy.

* The pillar valve is the point at which the pressure vessel connects to the diving regulator. The purpose of the pillar valve is to control gas flow to and from the pressure vessel and to form a seal with the regulator. Some counties insist that the pillar valve includes a burst disk, a type of pressure 'fuse', that will fail before the pressure vessel fails in the event of over pressurisation.

* A rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Halocarbon o-rings are used with cylinders storing oxygen-rich gas mixtures to reduce the risk of fire.

* Y pillar valves. Most pillar valves only have one output and one valve. A Y valve has two outputs and two valves allowing two regulators to be connected to the cylinder. If one regulator "freeflows", which is a common failure mode, its valve can be closed and the cylinder breathed from the regulator connected to the other valve.

* Reserve lever. Diving cylinders, until the 1970s, before pressure gauges on regulators came into common use, often used a mechanical reserve mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled a lever and finished the dive before the reserve was consumed.

Types of Pillar Valve

There are three types of Pillar Valve:

* A-clamp or yoke - the connection on the regulator surrounds the valve pillar and presses the output O-ring of the pillar valve against the input seat of the regulator. This type is simple, cheap and very widely used worldwide. It has a maximum pressure rating of 232 bar and the weakest part of the seal, the o-ring, is not well protected from over-pressurisation.

* 232 bar DIN (5-thread, metric M 25X2) - the regulator screws into the pillar valve trapping the O-ring securely. These are more reliable than A-clamps because the o-ring is well protected, but many countries do not use DIN fittings widely on compressors, or cylinders which have DIN fittings, so a European diver with a DIN system abroad in many places will need to take an adaptor.

* 300 bar DIN : (7-thread, metric M 25X2) - these are similar to 5-thread DIN fitting but are rated to 300 bar working pressures. The 300 bar pressures are common in European diving and in US cave diving, but their acceptance in U.S. sport diving has been hampered by the fact that United States Department of Transportation rules presently prohibit the transport of metal scuba cylinders on public roads with pressures above about 230 bar, even if the cylinders and air delivery systems have been rated for these pressures by the American agencies which oversee cylinder testing and equipment compatibility for SCUBA (OSHA and CGA).

The new European Norm EN 144-3:2003 introduced a new type of valve, similar to existing 232 bar or 300 bar DIN valves, however, with a metric M 26X2 fitting on both the cylinder and the regulator. These are to be used for breathing gas with oxygen content above that normally found in natural air in the Earth's atmosphere (i.e., 22% -100%). From August 2008, these shall be required for all diving equipment used with Nitrox or pure oxygen. The idea behind this new standard is to prevent a rich mixture being filled to a cylinder, which is not oxygen clean. However even with use of the new system there still remains nothing except human procedural care to ensure that a cylinder with a new valve remains oxygen-clean - which is exactly how the current system works.

Purposes of Diving Cylinders

Divers may carry more than one cylinder. In parts of the world where diving takes place in warm water and in good visibility, recreational divers usually carry only cylinder. An example of this type is coral reef diving where it is possible to do an interesting dive without going deep or needing long decompression. Where diving risks are higher, for example in parts of the world where the water is cold and visibility is low or when recreational divers do deeper or decompression diving, divers routinely carry more than one gas source. An example of this type is north European diving where the temperature is often less than 15°C/60°F and visibility less than 10m/33ft and many interesting dive sites are shipwrecks in deeper water on the sea bed.

Each cylinder may have a different purpose:

* primary breathing source - the cylinder intended for most of the dive,
* bail out or bale out - a cylinder used purely as an independent safety reserve,
* pony (bottle) - a small bail out.

Divers doing technical diving often carry different gases, each in a separate cylinder, for each phase of the dive:

* travel gas - a cylinder holding gas for use during the descent - typically a nitrox with a medium oxygen partial pressure
* bottom gas - a cylinder holding gas for use at depth - typically a helium-based gas with a low oxygen partial pressure
* stage - a cylinder holding gas for use at the decompression stop - typically nitrox with a high oxygen partial pressure

Rebreathers also use internal cylinders:

* oxygen rebreathers have an oxygen cylinder
* semi-closed circuit rebreathers have a "diluent" cylinder, which often contains air, nitrox or a helium based gas
* closed circuit rebreathers have an oxygen cylinder and a "diluent" cylinder, which often contains air, nitrox or a helium based gas

Breathing Capacity

A commonly asked question is "What is the underwater duration of a particular cylinder?" There are two parts to this answer:

1. What is the cylinder's capacity to store gas?
Two features of the cylinder determine its gas carrying capacity:

* working gas pressure : this normally ranges between 200 bar/3000 psi and 300 bar/4400 psi
* internal volume : this normally ranges between 3 litres and 18 litres

To calculate the quantity of gas:

 quantity of gas = volume x pressure

So, a 3 litre, 300 bar cylinder can carry up to 900 litres (33 ft³) of gas.

2. How much gas does the diver consume?
There are three factors at work here:

* breathing rate or respiratory minute volume (RMV), in litres per minute (lpm), of the diver. In normal conditions this will be between 10 and 25 lpm. At times of high work rate or panic, breathing rates can rise to 100 lpm.
* ambient pressure: the depth of the dive determines this. The ambient pressure at the surface is 1 bar / 14.7 psi. For every 10 metres/33 feet in salt water the diver descends, the pressure increases by 1 bar / 14.7 psi.
* time

To calculate the quantity of gas consumed:

 gas consumed = breathing rate x pressure x time

Thus, a diver with a breathing rate of 20 lpm will consume at 30 meters (4 bar) the equivalent of 80 lpm at 1 bar (80 lpm at the surface). If this diver only had the 3 litre 300 bar cylinder to breathe from (900 litres at 1 bar), the gas in the cylinder would be exhausted after a little over 900/80 = about 11 minutes.

Filling tanks

Tanks should only be filled with air from diving air compressors or with other breathing gases using gas blending techniques. Both these services should be provided by reliable suppliers such as dive shops. Breathing industrial compressed gases can be lethal because the high pressure increases the effect of any impurities in them.

Special precautions need to be taken with gases other than air:

* oxygen in high concentrations is a major cause of fire and rust.
* oxygen should be very carefully transferred from one tank to another and only ever stored in tanks that are certified and labeled for oxygen use.
* gas mixtures containing proportions of oxygen other than 21% could be extremely dangerous to divers who are unaware of the proportion of oxygen in them. All cylinders should be labeled with their composition.

Contaminated air at depth can be fatal. Common contaminants are: carbon monoxide a by-product of combustion, carbon dioxide a product of metabolism, oil and lubricants from the compressor.

The blast caused by a sudden release of the gas pressure inside a diving cylinder makes them very dangerous if mismanaged. The greatest risk of explosion exists at filling time and comes from thinning of the walls of the pressure vessel due to corrosion. Another cause of failure is damage or corrosion of the threads and neck of the cylinder where the pillar valve is screwed in. Aluminium cylinders have been observed occasionally to fail explosively, fragmenting the cylinder wall. Steel cylinders usually remain mostly intact, and tend to fail at the neck.

Keeping the cylinder slightly pressurized at all times reduces the possibility of contaminating the inside of the cylinder with corrosive agents, such as sea water, or toxic material, such as oils, poisonous gases, fungi or bacteria.

Manufacture and Testing

Most countries require tanks to be checked on a regular basis, see gas cylinder. This usually consists of an internal visual inspection and a hydrostatic test. In the United States, a visual inspection is required every year, and a hydrostatic every five years. In European Union countries a visual inspection is required every 2.5 years, and a hydrostatic every five years. In Norway a hydrostatic (including a visual inspection) is required 3 years after production date, then every 2 years.

Legislation in Australia requires that cylinders are hydrostatically tested every twelve months, regardless. A hydrostatic test involves pressurising the cylinder to its test pressure and measuring its volume before and after the test. A permanent increase in volume above the tolerated level means the cylinder fails the test and should be destroyed.

When a cylinder is manufactured, its specification, including Working Pressure, Test Pressure, Data of Manufacture, Capacity and Weight are stamped on the cylinder. On testing, the test date, or the test expiry date in some countries such as Germany, is punched into the neck of the tank for easy verification at fill time.

Most compressor operators check these details before filling the cylinder and may refuse to fill non-standard or out-of-test cylinders.

Tank (Cylinder) Tips

Buying a SCUBA tank or cylinder may be a good idea if you dive frequently and travel to your dive sites by car. There are two basic choices: steel or aluminum. Both have advantages and disadvantages. Steel cylinders are very durable, lighter and smaller than their aluminum counterparts but can corrode and may require more maintenance. Aluminum 80 tanks are common at dive destinations and are the tanks many students learn to dive with. They are less expensive than steel cylinders. Regardless of the material you choose, several sizes are available. If you are considering purchasing a tank, this is a good opportunity to get one suited to your physical size. Smaller people can be much more comfortable diving with smaller tanks which can lead to improved weighting and buoyancy control. Your dealer will be happy to help you find the right tank for your needs. Remember that SCUBA equipment works as a system so don't hesitate to take your existing gear to the shop and try everything out together.

If you own your own cylinder, you have the additional responsibility of having it visually inspected once per year for internal and external corrosion or damage. Your dealer will use a special light to look all the way into the tank to inspect it inside and out. The dealer will also replace o-rings and any necessary valve parts. Every five years your tank will require a mandatory hydrostatic test. A current inspection sticker is required for tank fills.

Once you have your new SCUBA cylinder: