Diving cylinder

Diving cylinders to be filled at a diving air compressor station

A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set or an emergency gas supply for surface-supplied diving. It provides gas to the diver through the demand valve of a diving regulator or the breathing loop of a diving rebreather.

Diving cylinders are usually manufactured from aluminium or steel alloys, and typically have an internal volume of between 3 and 18 litres (0.11 and 0.64 cu ft) and a maximum pressure rating from 200 to 300 bars (2,900 to 4,400 psi). The internal cylinder volume is also expressed as "water capacity" - the volume of water which could be contained by the cylinder. When pressurised, a cylinder carries a volume of gas greater than its water capacity because gas is compressible. 600 litres (21 cu ft) of gas at atmospheric pressure is compressed into a 3-litre cylinder when it is filled to 200 bar. Cylinders also come in smaller sizes, such as 0.2, 1.5 and 2 litres, however these are not generally used for breathing, instead being used for purposes such as surface marker buoy, drysuit and buoyancy compensator inflation.

Divers use gas cylinders above water for storage of oxygen for first aid treatment of diving disorders and as part of storage "banks" for diving air compressor stations, gas blending and surface supplied breathing gas. Similar cylinders are also used for many purposes not connected to diving. For these applications they are not diving cylinders. Breathing gas storage cylinders mounted externally on diving bells or diver submersibles operate under similar conditions to scuba cylinders and may also be considered diving cylinders.

Terminology

The term "diving cylinder" tends to be used by gas equipment engineers, manufacturers, support professionals, and divers speaking British English. "Scuba tank" or "diving tank" is more often used colloquially by non-professionals and native speakers of American English. The term "oxygen tank" is commonly used by non-divers when referring to diving cylinders; however, this is a misnomer. These cylinders typically contain (compressed atmospheric) breathing air, or an oxygen-enriched air mix. They rarely contain pure oxygen, except when used for rebreather diving, shallow decompression stops in technical diving or for in-water oxygen recompression therapy. Breathing pure oxygen at depths greater than 6 metres (20 ft), equivalent to a partial pressure of oxygen of 1.6 bars (1.6 atm), can result in oxygen toxicity, a highly dangerous condition that can trigger seizures and thus lead to drowning.

Diving cylinders have also been referred to as bottles or flasks, usually with one of the epithets scuba, diving, or bailout. Cylinders may also be called aqualungs, a genericized trademark derived from the Aqua-lung equipment made by the Aqua Lung/La Spirotechnique company, although that is more properly applied to an open circuit scuba set or open circuit diving regulator.

Diving cylinders may also be specified by their application, as in bailout cylinders, stage cylinders, deco cylinders, sidemount cylinders, pony cylinders, suit inflation cylinders, etc.

Parts of a cylinder

Two 12 litre steel cylinders connected by an isolation manifold and tank bands

The functional diving cylinder consists of the pressure vessel and a cylinder valve:

The pressure vessel

The pressure vessel is a seamless cylinder normally made of cold-extruded aluminium or forged steel. 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. Bill Stone designed and utilized epoxy based tank for exploration of the San Agustín and Sistema Huautla caves in Mexico to decrease the weight for dry sections and vertical passages.

Aluminium cylinders

An especially common cylinder available at tropical dive resorts is an "aluminium-80" which is an aluminium cylinder 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 pressure of 3,000 pounds per square inch (207 bar). Aluminium cylinders are also often used where divers carry many cylinders, such as in technical diving in water which is warm enough that the dive suit does not provide much buoyancy, because the greater buoyancy of aluminium cylinders reduces the amount of extra buoyancy the diver would need to achieve neutral buoyancy. They are also sometimes preferred when carried as "sidemount" or "sling" cylinders as the near neutral buoyancy allows them to hang comfortably along the sides of the diver's body, without disturbing trim, and they can be handed off to another diver or stage dropped with a minimal effect on buoyancy. Most aluminium cylinders are flat bottomed, allowing then to stand upright on a level surface, but some were manufactured with domed bottoms.

The aluminium alloys used for diving cylinders are 6061 and 6351. 6351 alloy is subject to sustained load cracking and cylinders manufactured of this alloy should be periodically eddy current tested according to national legislation and manufacturer's recommendations.

Manufacture

Aluminium cylinders are usually manufactured by cold extrusion of aluminium billets in a process which first presses the walls and base, then trims the top edge of the cylinder walls, followed by press forming the shoulder and neck. The final structural process is machining the neck outer surface and bore to size and cutting the neck threads and O-ring groove. The cylinder is then heat-treated, tested and stamped with the required permanent markings. Aluminium diving cylinders commonly have flat bases, which allows them to stand upright on horizontal surfaces, and which are relatively thick to allow for rough treatment and considerable wear. 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.

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  Section of die with billet inserted
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  Cold extrusion process
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  Extrusion product before trimming
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  Section after closure of the top end
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  Section showing machined areas of the neck in detail
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  Hydrostatic test

Steel cylinders

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. They also often have a lower mass than aluminium cylinders with the same gas capacity, due to considerably higher material strength, so the use of steel cylinders can result in both a lighter cylinder and less ballast required for the same gas capacity, a two way saving on overall dry weight carried by the diver.

Steel cylinders are more susceptible than aluminium to external corrosion, particularly in seawater, and may be galvanized or coated with corrosion barrier paints to resist corrosion damage. It is not difficult to monitor external corrosion, and repair the paint when damaged, and steel cylinders which are well maintained have a long service life, often longer than aluminium cylinders, as they are not susceptible to fatigue damage when filled within their safe working pressure limits.

Steel cylindera are manufactured with domed (convex) and dished (concave) bottoms. The dished profile allows them to stand upright on a horizontal surface, and is the standard shape for industrial cylinders. The cylinders used for emergency gas supply on diving bells are often this shape, and commonly have a water capacity of about 50 litres ("J"). Domed bottoms give a larger volume for the same cylinder mass, and are the standard for scuba cylinders up to 18 litres water capacity, though some concave bottomed cylinders have been marketed for scuba.

Manufacture

Steel alloys used for dive cylinder manufacture are authorised by the manufacturing standard. For example, The US standard DOT 3AA requires the use of open-hearth, basic oxygen, or electric steel of uniform quality. Approved alloys include 4130X, NE-8630, 9115, 9125, Carbon-boron and Intermediate manganese, with specified constituents, including manganese and carbon,and molybdenum, chromium, boron, nickel or zirconium.

Steel cylinders may be manufactured from steel plate discs, which are cold drawn to a cylindrical cup form, in two or three stages, and generally have a domed base if intended for the scuba market, so they cannot stand up by themselves. After forming the base and side walls, the top of the cylinder is trimmed to length, heated and hot spun to form the shoulder and close the neck. This process thickens the material of the shoulder. The cylinder is heat-treated by quenching and tempering to provide the best strength and toughness. 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. After inspection and hydrostatic testing they are stamped with the required permanent markings, followed by external coating with a corrosion barrier paint or hot dip galvanising.

Cylinder neck

The neck of the cylinder is internally threaded to fit a cylinder valve. There are several standards for neck threads, these include:

• Taper thread (17E), with a 12% taper right hand thread, standard Whitworth 55° form with a pitch of 14 threads per inch (5.5 threads per cm) and pitch diameter at the top thread of the cylinder of 18.036 millimetres (0.71 in). These connections are sealed using thread tape and torqued to between 120 and 150 newton-metres (89 and 111 lbf⋅ft) on steel cylinders, and between 75 and 140 N⋅m (55 and 103 lbf⋅ft) on aluminium cylinders.

Parallel threads are made to several standards:

• M25x2 ISO parallel thread, which is sealed by an O-ring and torqued to 100 to 130 N⋅m (74 to 96 lbf⋅ft) on steel, and 95 to 130 N⋅m (70 to 96 lbf⋅ft) on aluminium cylinders;
• M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to 100 to 130 N⋅m (74 to 96 lbf⋅ft) on steel cylinders, and 85 to 100 N⋅m (63 to 74 lbf⋅ft) on aluminium cylinders;
• 3/4"x14 BSP parallel thread, which has a 55° Whitworth thread form, a pitch diameter of 25.279 millimetres (0.9952 in) and a pitch of 14 threads per inch (1.814 mm);
• 3/4"x14 NGS (NPSM) parallel thread, sealed by an O-ring, torqued to 40 to 50 N⋅m (30 to 37 lbf⋅ft) on aluminium cylinders, which has a 60° thread form, a pitch diameter of 0.9820 to 0.9873 in (24.94 to 25.08 mm), and a pitch of 14 threads per inch (5.5 threads per cm);
• 3/4"x16 UNF, sealed by an O-ring, torqued to 40 to 50 N⋅m (30 to 37 lbf⋅ft) on aluminium cylinders.

The 3/4"NGS and 3/4"BSP are very similar, having the same pitch and a pitch diameter that only differs by about 0.2 mm (0.008 in), but they are not compatible, as the thread forms are different.

All parallel thread valves are sealed using an O-ring at top of the neck thread which seals in a chamfer or step in the cylinder neck and against the flange of the valve.

The shoulder of the cylinder carries stamp markings providing required information about the cylinder.

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  Stamp markings on an American manufacture aluminum 40 cu ft 3000 psi cylinder
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  Stamp markings on an American manufacture aluminum 80 cu ft 3000 psi cylinder
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  Stamp markings on a British manufacture aluminium 12.2 l 232& bar cylinder
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  Stamp markings on an Italian manufacture steel 7 l 300 bar cylinder

The cylinder valve

Regulators with DIN- and yoke-valve.

The purpose of the cylinder valve or pillar valve is to control gas flow to and from the pressure vessel and to provide a connection with the regulator or filling hose. Cylinder valves are usually machined from brass and finished by a protective and decorative layer of chrome-plating. A metal or plastic dip tube or valve snorkel screwed into the bottom of the valve extends into the cylinder to reduce the risk of liquid or particulate contaminants in the cylinder getting into the gas passages when the cylinder is inverted, and blocking or jamming the regulator. Some of these dip tubes have a plain opening, but some have an integral filter.

Cylinder valves are classified by four basic aspects: The cylinder thread specification, the connection to the regulator, pressure rating, and other distinguishing features.

Cylinder thread variations

Cylinder threads may be in two basic configurations: Taper thread and parallel thread. These thread specifications are detailed in a previous section. The valve thread specification must exactly match the neck thread specification of the cylinder. Improperly matched neck threads can fail under pressure and can have fatal consequences if someone is hit by the flying valve or cylinder.

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  Draeger 300 bar taper thread DIN cylinder valve
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  A 232 bar DIN connection cylinder valve with M25x2 parallel thread cylinder connection

Connection to the regulator

Section view of A-clamp-, yoke- or INT-valve, showing the sealing surfaces, according to ISO 12209-3.

A rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Fluoroelastomer (e.g. viton) O-rings may be used with cylinders storing oxygen-rich gas mixtures to reduce the risk of fire. There are two basic types of cylinder valve to regulator connection in general use for Scuba cylinders containing air:

• A-clamp or yoke connectors - 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. The connection is officially described as connection CGA 850 yoke. The yoke is screwed down snug by hand (overtightening can make the yoke impossible to remove later without tools) and the seal is created by clamping the O-ring between the surfaces of the regulator and valve. When the valve is opened, cylinder pressure expands the O-ring against the outer surface of the O-ring groove in the valve. Insufficient clamping force may allow the pressure to extrude the O-ring between the valve and regulator faces, resulting in a leak. This type of connection 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 overpressurisation.

Section view of DIN-valve to regulator connection, showing the sealing surfaces, according to ISO 12209-2.

• DIN screw thread connectors - the regulator screws into the cylinder valve trapping the O-ring securely between the sealing face of the valve and the O-ring groove in the regulator. 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 diver traveling abroad with a DIN system may need to take an adaptor, either for connecting the DIN regulator to a rented cylinder, or for connecting an A-clamp filler hose to a DIN cylinder valve.

There are also cylinder valves for scuba cylinders containing gases other than air:

• 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 26×2 fitting on both the cylinder and the regulator. These are intended 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 were required in the European Union 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 that 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 previous system worked.
• An M 24x2 male thread cylinder valve was supplied with some Dräger semi-closed circuit recreational rebreathers (Dräger Ray) for use with nitrox mixtures. The regulator supplied with the rebreather had a compatible connection.

Pressure rating

Yoke valves are rated between 200 and 240 bar, and there does not appear to be any mechanical design detail preventing connection between any yoke fittings, though some older yoke clamps will not fit over the popular 232/240 bar combination DIN/yoke cylinder valve as the yoke is too narrow.

DIN valves are produced in 200 bar and 300 bar pressure ratings. The number of threads and the detail configuration of the connections is designed to prevent incompatible combinations of filler attachment or regulator attachment with the cylinder valve.

• 232 bar DIN (5-thread, G5/8) Outlet/Connector #13 to DIN 477 part 1 - (technically they are specified for cylinders with 300 bar test pressure)
• 300 bar DIN (7-thread, G5/8) Outlet/Connector #56 to DIN 477 part 5 - these are similar to 5-thread DIN fitting but are rated to 300 bar working pressures. (technically they are specified for cylinders with 450 bar test pressure) 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 (Occupational Safety and Health Administration and Compressed Gas Association).

Adaptors are available to allow connection of DIN regulators to yoke cylinder valves (A-clamp or yoke adaptor), and to connect yoke regulators to DIN cylinder valves. (plug adaptors and block adaptors) Plug adaptors are rated for 232/240 bar, and can only be used with valves which are designed to accept them. Block adaptors are generally rated for 200 bar, and can be used with almost any 200 bar DIN valve.

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  A yoke (A-clamp) to DIN adaptor allows connection of a DIN regulator to a Yoke cylinder valve
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  DIN plug adaptor
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  DIN valve with plug adaptor for yoke attachment fitted

Other distinguishing features

Plain valves

A 12 litre, 232 bar cylinder with DIN valve. The shoulder colour-coding is the old UK standard for compressed breathing air prior to 2006

"H"-valve with DIN connections

• The most commonly used cylinder valve type is the single outlet plain valve, sometimes known as a "K" valve, which allows connection of a single regulator, and has no reserve function. It simply opens to allow gas flow, or closes to shut it off. Several configurations are used, with options of DIN or A-clamp connection, and vertical or transverse spindle arrangements. The valve is operated by turning a knob, usually rubber or plastic, which affords a comfortable grip. Several turns are required to fully open the valves. Some DIN valves are convertible to A-clamp by use of an insert which is screwed into the opening.

• Y and H cylinder valves have two outlets, each with its own valve, allowing two regulators to be connected to the cylinder. If one regulator "freeflows", which is a common failure mode, or ices up, which can happen in water below about 5 °C, its valve can be closed and the cylinder breathed from the regulator connected to the other valve. The difference between an H-valve and a Y-valve is that the Y-valve body splits into two posts roughly 90° to each other and 45° from the vertical axis, looking like a Y, while an H-valve is usually assembled from a valve designed as part of a manifold system with an additional valve post connected to the manifold socket, with the valve posts parallel and vertical, which looks a bit like an H. Y-valves are also known as "slingshot valves" due to their appearance.

Reserve valves

A J-valve from c.1960

Draeger 200 bar cylinder valves with manifold and reserve lever

Draeger taper thread cylinder valve with reserve lever

• Reserve lever or "J-valve". Until the 1970s, when submersible pressure gauges on regulators came into common use, diving cylinders often used a mechanical reserve mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off by a spring loaded valve when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled down on a rod that ran along the side of the cylinder and which activated a lever to open a bypass valve. The diver would then finish the dive before the reserve (typically 300 pounds per square inch (21 bar)) was consumed. On occasion, divers would inadvertently trigger the mechanism while donning gear or performing a movement underwater and, not realizing that the reserve had already been accessed, could find themselves out of air at depth with no warning whatsoever. The J-valve got its name from being item "J" in one of the first scuba equipment manufacturer catalogs. The standard non-reserve yoke valve at the time was item "K", and is often still referred to as a K-valve. J-valves are still occasionally used by professional divers in zero visibility, where the submersible pressure gauge (SPG) can not be read. While the recreational diving industry has largely discontinued support and sales of the J-valve, the US Department of Defense, the US Navy, NOAA (the National Oceanographic and Atmospheric Administration) and OSHA (the national Occupational Health and Safety Adminsitration) all still allow or recommend the use of J-valves as an alternative to a bailout cylinder or as an alternative to a submersible pressure gauge. They are generally not available through recreational dive shops, but are still available from some manufacturers. They can be significantly more expensive than K-valves from the same manufacturer.
• Less common in the 1950s thru 1970s was an R-valve which was equipped with a restriction that caused breathing to become difficult as the cylinder neared exhaustion, but that would allow less restricted breathing if the diver began to ascend and the ambient water pressure lessened, providing a larger pressure differential over the orifice. It was never particularly popular because, were it necessary for the diver to descend (as is often necessary in cave and wreck diving, breathing would become progressively more difficult as the diver went deeper, eventually becoming impossible until the diver could ascent to a low enough ambient pressure.

Handed valves

Some cylinder valve models have axial spindles - in line with the cylinder axis, and are not handed. Standard side-spindle valves have the handwheel on the diver's right side when back-mounted. Side-spindle valves used with manifolds must be a handed pair - one with the handwheel to the right and the other with the handwheel to the left, but in all cases the valve is opened by turning the handwheel anticlockwise, and closed by turning it clockwise. This is the convention with almost all valves for all purposes. Left hand side-spindle valves are used by sidemount divers.

Bursting disk

Some national standards require that the cylinder valve includes a bursting disk, a pressure relief device that will release the gas before the cylinder fails in the event of overpressurization. If a bursting disk ruptures during a dive the entire contents of the cylinder will be lost in a very short time. The risk of this happening to a correctly rated disc, in good condition, on a correctly filled cylinder is very low.

Accessories

Additional components for convenience, protection or other functions, not directly required for the function as a pressure vessel.

Manifolds

Main article: Manifold (scuba)

Face sealed isolation manifold on twin 12 l steel cylinders. The plastic discs are records of the latest internal inspection

Draeger 200 bar cylinder manifold

Barrel seal isolation manifold

Left side cylinder valve for barrel seal manifold with blanking plug and DIN connection

A cylinder manifold is a tube which connects two cylinders together so that the contents of both can be supplied to one or more regulators. There are three commonly used configurations of manifold:

• The oldest type is a tube with a connector on each end which is attached to the cylinder valve outlet, and an outlet connection in the middle, to which the regulator is attached. A variation on this pattern includes a reserve valve at the outlet connector. The cylinders are isolated from the manifold when closed, and the manifold can be attached or disconnected while the cylinders are pressurised.
• More recently, manifolds have become available which connect the cylinders on the cylinder side of the valve, leaving the outlet connection of the cylinder valve available for connection of a regulator. This means that the connection cannot be made or broken while the cylinders are pressurised, as there is no valve to isolate the manifold from the interior of the cylinder. This apparent inconvenience allows a regulator to be connected to each cylinder, and isolated from the internal pressure independently, which allows a malfunctioning regulator on one cylinder to be isolated while still allowing the regulator on the other cylinder access to all the gas in both cylinders.
• These manifolds may be plain or may include an isolation valve in the manifold, which allows the contents of the cylinders to be isolated from each other. This allows the contents of one cylinder to be isolated and secured for the diver if a leak at the cylinder neck thread, manifold connection, or burst disk on the other cylinder causes its contents to be lost.

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. These manifolds can include a reserve valve, either in the main valve or at one cylinder. This system is mainly of historical interest.

Valve cage

Also known as a manifold cage or regulator cage, this is a structure which can be clamped to the neck or shoulder of the cylinder or manifolded cylinders to protect the valves and regulator first stages from impact and abrasion damage while in use and from rolling the valve closed by friction of the handwheel against an overhead. A valve cage is often made of stainless steel, and some designs can snag on obstructions.

Cylinder bands

Cylinder bands are straps, usually of stainless steel, which are used to clamp two cylinders together as a twin set. The cylinders may be manifolded or independent. It is usual to use a cylinder band near the top of the cylinder, just below the shoulders, and one lower down. The conventional distance between centrelines for bolting to a backplate is 11 inches (280 mm).

Cylinder boot

Twinned cylinders showing cylinder boots, nets and lower band

A cylinder boot is a hard rubber or plastic cover which fits over the base of a diving cylinder to protect the paint from abrasion and impact, to protect the surface the cylinder stands on from impact with the cylinder, and in the case of round bottomed cylinders, to allow the cylinder to stand upright on its base. Some boots have flats moulded into the plastic to reduce the tendency of the cylinder to roll on a flat surface. It is possible in some cases for water to be trapped between the boot and the cylinder, and if this is seawater and the paint under the boot is in poor condition, the surface of the cylinder may corrode in those areas. This can usually be avoided by rinsing in fresh water after use and storing in a dry place. The added hydrodynamic drag caused by a cylinder boot is trivial in comparison with the overall drag of the diver, but some boot styles may present a slightly increased risk of snagging on the environment.

Cylinder net

A cylinder net is a tubular net which is stretched over a cylinder and tied on at top and bottom. The function is to protect the paintwork from scratching, and on booted cylinders it also helps drain the surface between the boot and cylinder, which reduces corrosion problems under the boot. Mesh size is usually about 6 millimetres (0.24 in). 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. Occasionally sleeves made from other materials may be used to protect the cylinder.

Cylinder handle

A 15 litre, 232 bar cylinder with "Yoke" connection valve and cylinder handle

A cylinder handle may be fitted, usually clamped to the neck, to conveniently carry the cylinder. This can also increase the risk of snagging in an enclosed environment.

Cylinder pressure rating

The thickness of the cylinder walls is directly related to the working pressure, and this affects the buoyancy characteristics of the cylinder. A low-pressure cylinder will be more buoyant than a high-pressure cylinder with similar size and proportions of length to diameter and in the same alloy.

Working pressure

Scuba cylinders are technically all high-pressure gas containers, but within the industry in the US there are three nominal working pressure ratings (WP) in common use;

low pressure (2400 to 2640 psi — 165 to 182 bar),

standard (3000 psi — 207 bar), and

high pressure (3300 to 3500 psi — 227 to 241 bar).

US made aluminum cylinders usually have a standard working pressure of 3,000 pounds per square inch (210 bar), and the compact aluminum range have a working pressure of 3,300 pounds per square inch (230 bar). Some steel cylinders manufactured to US standards are permitted to exceed the nominal working pressure by 10% and this is indicated by a '+' symbol. This extra pressure allowance is dependent on the cylinder passing the appropriate higher standard periodical hydrostatic test.

Those parts of the world using the metric system usually refer to the cylinder pressure directly in bar but would generally use "high pressure" to refer to a 300 bars (4,400 psi) working pressure cylinder, which can not be used with a yoke connector on the regulator. 232 bar is a very popular working pressure for scuba cylinders in both steel and aluminium.

Test pressure

Hydrostatic test pressure (TP) is specified by the manufacturing standard. This is usually 1.5 × working pressure, or in the USA, 1.67 × working pressure.

Developed pressure

Cylinder working pressure is specified at a reference temperature, usually 15 °C or 20 °C. and cylinders also have a specified maximum safe working temperature, often 65 °C. The actual pressure in the cylinder will vary with temperature, as described by the gas laws, but this is acceptable in terms of the standards provided that the developed pressure when corrected to the reference temperature does not exceed the specified working pressure stamped on the cylinder. This allows cylinders to be safely and legally filled to a pressure that is higher than the specified working pressure when the filling temperature is greater than the reference temperature, but not more than 65 °C, provided that the filling pressure does not exceed the developed pressure for that temperature, and cylinders filled according to this provision will be at the correct working pressure when cooled to the reference temperature.

Cylinder capacity

12 litre and 3 litre steel diving cylinders: Typical Primary and Pony sizes

There are two commonly used conventions for describing the capacity of a diving cylinder. One is based on the internal volume of the cylinder. The other is based on nominal volume of gas stored.

Internal volume

The internal volume is commonly quoted in most countries using the metric system. This information is required by ISO 13769 to be stamped on the cylinder shoulder. It can be measured easily by filling the cylinder with fresh water. This has resulted in the term 'water capacity', abbreviated as WC which is often marked on the cylinder shoulder. It's almost always expressed as a volume in litres, but sometimes as mass of the water in kg. Fresh water has a density close to one kilogram per litre so the numerical values will be similar.

Standard sizes by internal volume

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. The applications are typical, but not exclusive.

• 18 litres: Available in steel, 232 bar, used as single or twins for back gas.
• 15 litres: Available in steel, 232 bar, used as single or twins for back gas
• 12.2 litres: Available in steel 232, 300 bar and aluminium 232 bar, used as single or twins for back gas
• 12 litres: Available in steel 200, 232, 300 bar, and aluminium 232 bar, used as single or twins for back gas
• 11 litres: Available in aluminium, 200, 232 bar used as single, twins for back gas or sidemount
• 10.2 litres: Available in aluminium, 232 bar, used as single or twins for back gas
• 10 litres: Available in steel, 200, 232 and 300 bar, used as single or twins for back gas, and for bailout
• 9.4 litres: Available in aluminium, 232 bar, used for back gas or as slings
• 8 litres: Available in steel, 200 bar, used for Semi-closed rebreathers
• 7 litres: Available in steel, 200, 232 and 300 bar, and aluminium 232 bar, back gas as singles and twins, and as bailout cylinders. A popular size for SCBA
• 6 litres: Available in steel, 300 bar, used for back gas as singles and twins, and as bailout cylinders. Also a popular size for SCBA
• 5 litres: Available in steel, 200 bar, used for rebreathers
• 4 litres: Available in steel, 200 bar, used for rebreathers
• 3 litres: Available in steel, 200 bar, used for rebreathers and pony cylinders
• 2 litres: Available in steel, 200 bar, used for rebreathers, pony cylinders, and suit inflation
• 0.5 litres: Available in steel and aluminium, 200 bar, used for buoyancy compensator and surface marker buoy inflation

Nominal volume of gas stored

The nominal volume of gas stored is commonly quoted as the cylinder capacity in the USA. It is a measure of the volume of gas that can be released from the full cylinder at atmospheric pressure. Terms used for the capacity include 'free gas volume' or 'free gas equivalent'. It depends on the internal volume and the working pressure of a cylinder. If the working pressure is higher, the cylinder will store more gas for the same volume.

The nominal working pressure is not necessarily the same as the actual working pressure used. Some steel cylinders manufactured to US standards are permitted to exceed the nominal working pressure by 10% and this is indicated by a '+' symbol. This extra pressure allowance is dependent on the cylinder passing the appropriate periodical hydrostatic test and is not necessarily valid for US cylinders exported to countries with differing standards. The nominal gas content of these cylinders is based on the 10% higher pressure.

For example, common Aluminum 80 (Al80) cylinder is an aluminum cylinder which has a nominal 'free gas' capacity of 80 cubic feet (2,300 L) when pressurized to 3,000 pounds per square inch (210 bar). It has an internal volume of 10.94 litres (0.386 cu ft).

Standard sizes by volume of gas stored

• Aluminum 80 is probably the most ubiquitous cylinder, used by resorts in many parts of the world for back gas, but also popular as a sling cylinder for decompression gas, and as side-mount cylinder in fresh water, as it has nearly neutral buoyancy. These cylinders have an internal volume of approximately 11 litres (0.39 cu ft) and working pressure of 3,000 pounds per square inch (210 bar). They are also sometimes used as manifolded twins for back mount, but in this application the diver needs more ballast weights than with most steel cylinders of equivalent capacity.
• Aluminum 40 is a popular cylinder for side-mount and sling mount bailout and decompression gas for moderate depths, as it is small diameter and nearly neutral buoyancy, which makes it relatively unobtrusive for this mounting style. Internal volume is approximately 5.5 litres (0.19 cu ft) and working pressure 3,000 pounds per square inch (210 bar).
• Aluminum 63 and steel HP65 (8.2 l) are smaller and lighter than the Al80, but have a lower capacity, and are suitable for smaller divers or shorter dives.
• Steel LP80 2,640 pounds per square inch (182 bar) and HP80 (10.1 l) at 3,442 pounds per square inch (237.3 bar) are both more compact and lighter than the Aluminium 80 and are both negatively buoyant, which reduces the amount of ballast weight required by the diver
• Steel HP119 (14.8 l), HP120 (15.3 l) and HP130 (16.0 l) cylinders provide larger amounts of gas for nitrox or technical diving.

Applications and configurations of diving cylinders

Technical diver with decompression gases in side mounted stage cylinders.

Divers may carry one cylinder or multiples, depending on the requirements of the dive. Where diving takes place in low risk areas, where the diver may safely make a free ascent, or where a buddy is available to provide an alternative air supply in an emergency, recreational divers usually carry only one cylinder. Where diving risks are higher, for example where the visibility is low or when recreational divers do deeper or decompression diving, and particularly when diving under an overhead, divers routinely carry more than one gas source.

Diving cylinders may serve different purposes. One or two cylinders may be used as a primary breathing source which is intended to be breathed from for most of the dive. A smaller cylinder carried in addition to a larger cylinder is called a "pony bottle". A cylinder to be used purely as an independent safety reserve is called a "bailout bottle" or Emergency Gas Supply (EGS). A pony bottle is commonly used as a bailout bottle, but this would depend on the time required to surface.

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

• "travel gas" is used during the descent and ascent. It is typically air or nitrox with an oxygen content between 21% and 40%. Travel gas is needed when the bottom gas is hypoxic and therefore is unsafe to breathe in shallow water.
• "bottom gas" is only breathed at depth. It is typically a helium-based gas which is low in oxygen (below 21%) or hypoxic (below 17%).
• "deco gas" is used at the decompression stops and is generally one or more nitrox mixes with a high oxygen content, or pure oxygen, to accelerate decompression.
• a "stage" is a cylinder holding reserve, travel or deco gas. They are usually carried "side slung", clipped on either side of the diver to the harness of the backplate and wing or buoyancy compensator, rather than on the back, and may be left on the distance line to be picked up for use on return (stage dropped). Commonly divers use aluminium stage cylinders, particularly in fresh water, because they are nearly neutrally buoyant in water and can be removed underwater with less effect on the diver's overall buoyancy.

For safety, divers sometimes carry an additional independent scuba cylinder with its own regulator to mitigate out-of-air emergencies should the primary breathing gas supply fail. For much common recreational diving where a controlled emergency swimming ascent is acceptably safe, this extra equipment is not needed or used. This extra cylinder is known as a bail-out cylinder, and may be carried in several ways, and can be any size that can hold enough gas to get the diver safely back to the surface.

Open-circuit scuba

For open-circuit scuba divers, there are several options for the combined cylinder and regulator system:

15 litre, 232 bar, A-clamp single cylinder open circuit scuba set

• Single cylinder consists of a single large cylinder, usually back mounted, with one first-stage regulator, and usually two second-stage regulators. This configuration is simple and cheap but it has only a single breathing gas supply: it has no redundancy in case of failure. If the cylinder or first-stage regulator fails, the diver is totally out of air and faces a life-threatening emergency. Recreational diver training agencies train divers to rely on a buddy to assist them in this situation. The skill of gas sharing is trained on most entry level scuba courses. This equipment configuration, although common with entry-level divers and used for most sport diving, is not recommended by training agencies for any dive where decompression stops are needed, or where there is an overhead environment (wreck diving, cave diving, or ice diving) as it provides no functional redundancy.

• Single cylinder with dual regulators consists of a single large back mounted cylinder, with two first-stage regulators, each with a second-stage regulator. This system is used for diving where cold water makes the risk of regulator freezing high and functional redundancy is required. It is common in continental Europe, especially Germany. 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. However, it is hard to reach the valves, so there may be some reliance on the dive buddy to help close the valve of the free-flowing regulator quickly.

• Main cylinder plus a small independent cylinder: this configuration uses a larger, back mounted main cylinder along with an independent smaller cylinder, often called a "pony" or "bailout cylinder". The diver has two independent systems, but the total 'breathing system' is now heavier, and more expensive to buy and maintain.
  • The pony is typically a 2 to 5 litre cylinder. Its capacity determines the depth of dive and decompression duration for which it provides protection. Ponies may be fixed to the diver's buoyancy compensator (BC) or main cylinder behind the diver's back, or can be clipped to the harness at the diver's side or chest or carried as a sling cylinder. Ponies provide an accepted and reliable emergency gas supply but require that the diver is trained to use them.
  • Another type of small independant air source is a hand-held cylinder filled with about 85 litres (3.0 cu ft) of free air with a diving regulator directly attached, such as the Spare Air. This source provides only a few breaths of gas at depth and is most suitable as a shallow water bailout.

7 litre, 232 bar, DIN pillar valve independent twin set. The left cylinder shows manufacturer markings. The right cylinder shows test stamps

• Independent twin sets or independent doubles consists of two independent cylinders and two regulators, each with a submersible pressure gauge. This system is heavier, more expensive to buy and maintain and more expensive to fill than a single cylinder set. The diver must swap demand valves during the dive to preserve a sufficient reserve of gas in each cylinder. If this is not done, then if a cylinder should fail the diver may end up having an inadequate reserve. Independent twin sets do not work well with air-integrated computers as they usually only monitor one cylinder. 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 cylinders may be mounted as a twin set on the diver's back, or alternatively can be carried in a sidemount configuration where penetration of wrecks or caves requires it, and where the cylinder valves are in easy reach.

• Plain manifolded twin sets, or manifolded doubles with a single regulator, consist of two back mounted cylinders with their pillar valves connected by a manifold but only one regulator is attached to the manifold. This makes it relatively simple and cheap but means there is no redundant functionality to the breathing system, only a double gas supply. This arrangement was fairly common in the early days of scuba when low pressure cylinders were manifolded to provide a larger air supply than was possible from the available single cylinders. It is still in use for large capacity bailout sets for deep commercial diving.

Isolation manifolded twin 12 litre, 232 bar scuba set with two A-clamp pillar valves and two regulators

• Isolation manifolded twin sets or manifolded doubles with two regulators, consist of two back mounted cylinders with their pillar valves connected by a manifold, with a valve in the manifold that can be closed to isolate the two pillar valves. In the event of a problem with one cylinder the diver may close the isolator valve to preserve gas in the cylinder which has not failed. The advantages of this configuration include: a larger gas supply than from a single cylinder; automatic balancing of the gas supply between the two cylinders; thus, no requirement to constantly change regulators underwater during the dive; and in most failure situations, the diver may close a valve to a failed regulator or isolate a cylinder and may retain access to all the remaining gas in both the tanks. 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. This configuration of cylinders is often used in technical diving.

Long 9.2 litre aluminium cylinder rigged for sling mounting

• Sling cylinders are a configuration of independent cylinders used for technical diving. They are independent cylinders with their own regulators and are carried clipped to the harness at the side of the diver. Their purpose may be to carry either stage, travel, decompression, or bailout gas while the back mounted cylinder(s) carry bottom gas. Stage cylinders carry gas to extend bottom time, travel gas is used to reach a depth where bottom gas may be safely used if it is hypoxic at the surface, and decompression gas is gas intended to be used during decompression to accelerate the elimination of inert gases. Bailout gas is an emergency supply intended to be used to surface if the main gas supply is lost.

Sidemount cylinder set with regulators fitted.

• Side-mount cylinders are cylinders clipped to the harness at the diver's sides which carry bottom gas when the diver does not carry back mount cylinders. They may be used in conjunction with other side mounted stage, travel and/or decompression cylinders where necessary. Skilled side-mount divers may carry as many as three cylinders on each side. This configuration was developed for access through tight restrictions in caves. Side mounting is primarily used for technical diving, but is also sometimes used for recreational diving, when a single cylinder may be carried, complete with secondary second stage (octopus) regulator, in a configuration sometimes referred to as monkey diving.

Rebreathers

Main article: Rebreather diving

Two 3 litre, 232 bar, DIN valved cylinders inside an Inspiration electronically controlled closed circuit diving rebreather.

Diving cylinders are used in rebreather diving in two roles:

• As part of the rebreather itself. The rebreather must have at least one source of fresh gas stored in a cylinder; many have two and some have more cylinders. Due to the lower gas consumption of rebreathers, these cylinders typically are smaller than those used for equivalent open-circuit dives. Rebreathers may use internal cylinders, or may also be supplied from "off-board" cylinders, which are not directly plumbed into the rebreather, but connected to it by a flexible hose and coupling and usually carried side slung.

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

• Rebreather divers also often carry a external bailout system if the internal diluent cylinder is too small for safe use for bailout for the planned dive. The bailout system is one or more independent breathing gas sources for use if the rebreather should fail:
  • Open-circuit: One or more open circuit scuba sets. The number of open-circuit bailout sets, their capacity and the breathing gases they contain depend on the depth and decompression needs of the dive. So on a deep, technical rebreather dive, the diver will need a bail out "bottom" gas and a bailout "decompression" gas(es). On such a dive, it is usually the capacity and duration of the bailout sets that limits the depth and duration of the dive - not the capacity of the rebreather.
  • Closed-circuit: A second rebreather containing one or more independent diving cylinders for its gas supply. Using another rebreather as a bail-out is possible but uncommon. Although the long duration of rebreathers seems compelling for bail-out, rebreathers are relatively bulky, complex, vulnerable to damage and require more time to start breathing from, than easy-to-use, instantly available, robust and reliable open-circuit equipment.

Surface supplied diver emergency gas supply

Commercial surface supplied diver wearing a single bailout cylinder plumbed into the helmet bailout block

Surface supplied divers are usually required to carry an emergency gas supply sufficient to allow them to return to a place of safety if the main gas supply fails. The usual configuration is a back mounted single cylinder supported by the diver's safety harness, with first stage regulator connected by a low pressure hose to a bailout block, which may be mounted on the side of the helmet or band-mask or on the harness to supply a lightweight full-face mask. Where the capacity of a single cylinder in insufficient, plain manifolded twins or a rebreather may be used. For closed bell bounce and saturation dives the bailout set must be compact enough to allow the diver to pass through the bottom hatch of the bell. This sets a limit on the size of cylinders that can be used.

Emergency gas supply on diving bells

Exterior view of closed bell used for saturation diving showing emergency gas supply cylinders

Diving bells are required to carry an on-board supply of breathing gas for use in emergencies. The cylinders are mounted externally as there is insufficient space inside. They are fully immersed in the water during bell operations, and may be considered diving cylinders.

Gas calculations

Further information: Scuba gas planning

It is necessary to know the approximate length of time that a diver can breathe from a given cylinder so that a safe dive profile can be planned.

There are two parts to this problem: The cylinder and the diver.

The cylinder's capacity to store gas

Two features of the cylinder determine its gas carrying capacity:

• cylinder gas pressure : when filled this normally ranges between 200 and 300 bars (2,900 and 4,400 psi), but the actual value should be measured for a real situation, as the cylinder may not be full.
• internal volume : this normally ranges between 3 litres and 18 litres for single cylinders.

To calculate the quantity of gas:

Volume of gas at atmospheric pressure = (cylinder volume) x (cylinder pressure) / (atmospheric pressure)

In those parts of the world using the metric system the calculation is relatively simple as atmospheric pressure may be approximated as 1 bar, So a 12 litre cylinder at 232 bar would hold almost 12 × 232 / 1 = 2,784 litres (98.3 cu ft) of air at atmospheric pressure (also known as free air).

In the US the capacity of a diving cylinder is specified directly in cubic feet of free air at the nominal working pressure, as the calculation from internal volume and working pressure is relatively tedious in imperial units. For examle, in the US and in many diving resorts in other countries, one might find aluminum cylinders of US manufacture with an internal capacity of 0.39 cubic feet (11 L) filled to a working pressure of 3,000 psi (210 bar); Taking atmospheric pressure as 14.7 psi, this gives 0.39 × 3000 / 14.7 = 80 ft³ These cylinders are described as "80 cubic foot cylinders", (the common "aluminum 80").

Up to about 200 bar the ideal gas law remains valid and the relationship between the pressure, size of the cylinder and gas contained in the cylinder is linear; at higher pressures there is proportionally less gas in the cylinder. A 3 litre cylinder filled to 300 bar will only carry contain 810 litres (29 cu ft) of atmospheric pressure gas and not the 900 litres expected from the ideal gas law.

Diver gas consumption

There are three main factors to consider:

• the rate at which the diver consumes gas, specified as surface air consumption (SAC) or respiratory minute volume (RMV) of the diver. In normal conditions this will be between 10 and 25 litres per minute (L/min) for divers who are not working hard. At times of extreme high work rate, breathing rates can rise to 95 litres per minute. For IMCA commercial diving gas planning purposes, a working breathing rate of 40 litres per minute is used, whilst a figure of 50 litres per minute is used for emergencies. RMV is controlled by blood CO₂ levels, and is usually independent of oxygen partial pressures, so does not change with depth. The very large range of possible rates of gas consumption results in a significant uncertainty of how long the supply will last, and a conservative approach is required for safety where an immediate access to an alternative breathing gas source is not possible. Scuba divers are expected to monitor the remaining gas pressure sufficiently often that they are aware of how much is still available at all times during a dive.
• ambient pressure: the depth of the dive determines this. The ambient pressure at the surface is 1 bar (15 psi) at sea level. For every 10 metres (33 ft) in seawater the diver descends, the pressure increases by 1 bar (15 psi). As a diver goes deeper, the breathing gas is delivered at a pressure equal to ambient water pressure, and the amount of gas used is proportional to the pressure. Thus, it requires twice as much mass of gas to fill the diver's lungs at 10 metres (33 ft) as it does at the surface, and three times as much at 20 metres (66 ft). The mass consumption of breathing gas by the diver is similarly affected.
• time at each depth. (usually approximated as time at each depth range)

To calculate the quantity of gas consumed:

gas consumed = surface air consumption × time × ambient pressure

Metric examples:

A diver with a RMV of 20 L/min at 30 msw (4 bar), will consume 20 x 4 x 1 = 80 L/min surface equivalent.

A diver with a RMV of 40 L/min at 50 msw (6 bar) for 10 minutes will consume 40 x 6 x 10 = 2400 litres of free air – the full capacity of a 12 litre 200 bar cylinder.

Imperial examples:

A diver with a SAC of 0.5 cfm (cubic feet per minute) at 100 fsw (4 ata) will consume 0.5 x 4 x 1 = 2 cfm surface equivalent.

A diver with a SAC of 1 cfm at 231 fsw (8 ata) for 10 minutes will consume 1 x 8 x 10 = 80 ft³ of free air – the full capacity of an 80 ft³ cylinder

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.

Breathing gas endurance

The amount of time that a diver can breathe from a cylinder is also known as air or gas endurance.

Absolute maximum breathing duration (BT) for a given depth can be calculated as

BT = available air / rate of consumption

which, using the ideal gas law, is

BT = (available cylinder pressure × cylinder volume) / (rate of air consumption at surface) × (ambient pressure)

This may be written as

(1) BT = (P_C-P_A)*V_C/(SAC*P_A)

with

BT = Breathing Time

P_C = Cylinder Pressure

V_C = Cylinder internal volume

P_A = Ambient Pressure

SAC = Surface air consumption

in any consistent system of units.

Ambient pressure (P_A) is the surrounding water pressure at a given depth and is made up of the sum of the hydrostatic pressure and the air pressure at the surface. It is calculated as

(2) P_A = D*g*? + atmospheric pressure

with

D = depth

g = Standard gravity

? = water density

in a consistent system of units

For metric units, this formula can be approximated by

(3) P_A = D/10 + 1

with depth in m and pressure in bar

Ambient pressure is deducted from cylinder pressure, as the quantity of air represented by P_A can in practice not be used for breathing by the diver as it required to balance the ambient pressure of the water.

This formula neglects the cracking pressure required to open both first and second stages of the regulator, and pressure drop due to flow restrictions in the regulator,both of which are variable depending on the design and adjustment of the regulator, and flow rate, which depends on the breathing pattern of the diver and the gas in use. These factors are not easily estimated, so the calculated value for breathing duration will be more than the real value.

However, in normal diving usage, a reserve is always factored in. The reserve is a proportion of the cylinder pressure which a diver will not plan to use other than in case of emergency. The reserve may be a quarter or a third of the cylinder pressure or it may be a fixed pressure, common examples are 50 bar and 500 psi. The formula above is then modified to give the usable breathing duration as

(4) BT = (P_C-P_R)*V_C/(SAC*P_A)

where P_R is the reserve pres

For example, (using the first formula (1) for absolute maximum breathing time), a diver at a depth of 15 meters in water with an average density of 1020 kg/m³ (typical seawater), who breathes at a rate of 20 litres per minute, using a dive cylinder of 18 litres pressurized at 200 bars, can breathe for a period of 72 minutes before the cylinder pressure falls so low as to prevent inhalation. In some open circuit scuba systems this can happen quite suddenly, from a normal breath to the next abnormal breath, a breath which may not be fully drawn. (There is never any difficulty exhaling). The suddenness of this effect depends on the design of the regulator and the internal volume of the cylinder. In such circumstances there remains air under pressure in the cylinder, but the diver is unable to breathe it. Some of it can be breathed if the diver ascends, as the ambient pressure is reduced, and even without ascent, in some systems a bit of air from the cylinder is available to inflate BCDs even after it no longer has pressure enough to open the demand valve.

Using the same conditions and a reserve of 50 bar, the formula (4) for usable breathing time is follows:

Ambient pressure = water pressure + atmospheric pressure = 15 msw/10 bar per msw + 1 = 2.5 bar

Usable pressure = fill pressure - reserve pressure = 200 bar - 50 bar = 150 bar

Usable air = usable pressure * cylinder capacity = 150 bar * 18 litres per bar = 2700 litres

Rate of consumption = surface air consumption * ambient pressure = 20 litres per minute per bar * 2.5 bar = 50 litres/min

Usable breathing time = 2700 litres / 50 litres per min = 54 minutes

This would give a dive time of 54 min at 15 m before reaching the reserve of 50 bar.

Reserves

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. The reserve is designed to provide gas for longer than planned decompression stops or to provide time to resolve underwater emergencies.

The size of the reserve depends upon the risks involved during the dive. A deep or decompression dive warrants a greater reserve than a shallow or a no stop dive. In recreational diving for example, it is recommended that the diver plans to surface with a reserve remaining in the cylinder of 500 psi, 50 bar or 25% of the initial capacity, depending of the teaching of the diver training organisation. This is because recreational divers practicing within "no-decompression" limits can normally make a direct ascent in an emergency. On technical dives where a direct ascent is either impossible (due to overhead obstructions) or dangerous (due to the requirement to make decompression stops), divers plan larger margins of safety. The simplest method uses the rule of thirds: one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.

Some training agencies teach the concept of minimum gas, rock bottom gas management or critical pressures which allows a diver to calculate an acceptable reserve to get two divers to the surface in an emergency from any point in the planned dive profile.

Professional divers may be required by legislation or industry codes of practice to carry sufficient reserve gas to enable them to reach a place of safety, such as the surface, or a diving bell, based on the planned dive profile. This reserve gas is usually required to be carried as an independent emergency gas supply (EGS), also known as a bailout cylinder, set or bottle. This usually also applies to professional divers using surface-supplied diving equipment.

Weight of gas consumed

The density of air at sea level and 15 °C is approximately 1.225 kg/m³. Most full-sized diving cylinders used for open circuit scuba hold more than 2 kilograms (4.4 lb) of air when full, and as the air is used, the buoyancy of the cylinder increases by the weight removed. The decrease in external volume of the cylinder due to reduction of internal pressure is relatively small, and can be ignored for practical purposes.

As an example, a 12-litre cylinder may be filled to 230 bar before a dive, and be breathed down to 30 bar before surfacing, using 2,400 litres or 2.4 m³ of free air. The mass of gas used during the dive will depend on the mixture - if we assume air, it will be approximately 2.9 kilograms (6.4 lb).

The loss of the weight of the gas taken from the cylinder makes the cylinder and diver more buoyant. This can be a problem if the diver is unable to remain neutrally buoyant towards the end of the dive because most of the gas has been breathed from the cylinder. The buoyancy change due to gas usage from back mounted cylinders is easily compensated by carrying sufficient diving weights to provide neutral buoyancy with empty cylinders at the end of a dive, and using the buoyancy compensator to neutralise the excess weight until the gas has been used.

The change in buoyancy of diving cylinder during the dive can be more problematic with side-mounted cylinders, and the actual buoyancy at any point during the dive is a consideration with any cylinder that may be separated from the diver for any reason. Cylinders which will be stage-dropped or handed off to another diver should not change the diver's buoyancy beyond what can be compensated using their buoyancy compensator. Cylinders with approximately neutral buoyancy when full generally require the least compensation when detached.

Table showing the buoyancy of diving cylinders in water when empty and full of air

Cylinder specification			Air capacity		Weight in air		Buoyancy in water
Material	Volume (litre)	Pressure (bar)	Volume (litre)	Weight (kg)	Empty (kg)	Full (kg)	Empty (kg)	Full (kg)
Steel	12	200	2400	3.0	16.0	19.0	-1.2	-4.2
	15	200	3000	3.8	20.0	23.8	-1.4	-5.2
	16 (XS 130)	230	3680	4.4	19.5	23.9	-0.9	-5.3
	2x7	200	2800	3.4	19.5	23.0	-2.2	-5.6
	8	300	2400	2.9	13.0	16.0	-3.6	-6.5
	10	300	3000	3.6	17.0	20.8	-4.2	-7.8
	2x4	300	2400	2.9	15.0	18.0	-4.1	-7.0
	2x6	300	3600	4.4	21.0	25.6	-5.2	-9.6
Aluminium	9 (AL 63)	207	1863	2.3	12.2	13.5	+1.8	-0.5
	11 (AL 80)	207	2277	2.8	14.4	17.2	+1.7	-1.1
	13 (AL100)	207	2584	3.2	17.1	20.3	+1.4	-1.8
Assumes 1 litre of air at atmospheric pressure and 15 °C weighs 1.225 g. Cylinder, valve and manifold weights will vary depending on model, so actual values will value accordingly.

Filling cylinders

Dive shop scuba filling station

Small stationary HP compressor installation

See also: Gas blending for scuba diving and Diving air compressor

Diving cylinders should only be filled with suitably filtered air from diving air compressors or with other breathing gases using gas blending techniques. In some jurisdictions, suppliers of breathing gases are required by legislation to periodically test the quality of compressed air produced by their equipment and to display the test results for public information. The standards for industrial gas purity and filling equipment and procedures may allow some contaminants at levels unsafe for breathing, and their use in breathing gas mixtures at high pressure could be harmful or fatal.

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 cylinder to another and only ever stored in containers that are cleaned 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.
• cylinders containing a high oxygen content must be cleaned for the use of oxygen and their valves lubricated only with oxygen service grease to reduce the chance of combustion.

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

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

Contamination by water during filling may be due two two causes. Inadequate filtration and drying of the compressed air can introduce small quantities of fresh water condensate, and failing to clear the cylinder valve orifice of water which may have dripped from wet dive gear, which can allow contamination by fresh or seawater. Both cause corrosion, but seawater contamination can cause a cylinder to corrode rapidly to the extent that it may be unsafe or condemned after even a fairly short period. This problem is exacerbated in hot climates, where chemical reactions are faster, and is more prevalent where filling staff are badly trained or overworked.

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 while filling, but cylinders have also been known to burst when overheated. The cause of failure can range from reduced wall thickness or deep pitting due to internal corrosion, neck thread failure due to incompatible valve threads, or cracking due to fatigue, sustained high stresses, or overheating effects in aluminum.

Inspection and testing

Condemned diving cylinders set aside for metal recycling

See also: Gas cylinder and Hydrostatic test

Most countries require diving cylinders to be checked on a regular basis. This usually consists of an internal visual inspection and a hydrostatic test. The inspection and testing requirements for scuba cylinders may be very different from the requirements for other compressed gas containers due to the more corrosive environment.

• In the United States, an annual visual inspection is not required by the USA DOT, though they do require a hydrostatic test every five years. The visual inspection requirement is a diving industry standard based on observations made during a review by the National Underwater Accident Data Center.
• In European Union countries a visual inspection is required every 2.5 years, and a hydrostatic test every five years.
• In Norway a hydrostatic test (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.
• In South Africa a hydrostatic test is required every 4 years, and visual inspection every year. Eddy current testing of neck threads must be done according to the manufacturer's recommendations.

Water jacket hydrostatic test test diagram

A hydrostatic test involves pressurising the cylinder to its test pressure (usually 5/3 or 3/2 of the working 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 must be permanently removed from service.

An inspection includes external and internal inspection for damage, corrosion, and correct colour and markings. The failure criteria vary according to the published standards of the relevant authority, but may include inspection for bulges, overheating, dents, gouges, electrical arc scars, pitting, line corrosion, general corrosion, cracks, thread damage, defacing of permanent markings, and colour coding.

When a cylinder is manufactured, its specification, including manufacturer, working pressure, test pressure, date of manufacture, capacity and weight are stamped on the cylinder.

After a cylinder passes the test, the test date, (or the test expiry date in some countries such as Germany), is punched into the shoulder of the cylinder for easy verification at fill time. There is an international standard for the stamp format.

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

Safety

Before any cylinder is filled, verification of testing dates and a visual examination for external damage and corrosion are required by law in some jurisdictions, and are prudent even if not legally required at other places. Test dates can be checked by looking at the visual inspection sticker and the hydro-test date is stamped on top of the cylinder.

Before use the user should verify the contents of the cylinder and check the function of the cylinder valve. Pressure and gas mixture are critical information for the diver, and the valve should open freely without sticking or leaks from the spindle seals. Breathing gas bled from a cylinder may be checked for smell. If the gas does not smell right it should not be used. Breathing gas should be almost free of smell, though a very slight aroma of the compressor lubricant is fairly common. No smell of combustion products or volatile hydrocarbons should be discernible. Failure to recognize that the gas was not on or that cylinder was empty by divers conducting a pre-dive evaluation has been noted.

Cylinders should not be left standing unattended unless secured so that they can not fall in reasonably foreseeable circumstances as an impact could damage the cylinder valve mechanism, and conceivably fracture the valve at the neck threads. 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.

A neatly assembled setup, with regulators, gauges, and delicate computers stowed inside the BCD, or clipped where they will not be walked on, and stowed under the boat bench or secured to a rack, is the practice of a competent diver.

As the scuba set is a life support system, no unauthorised person should touch a diver's gear, even to move it, without their knowledge and approval.

Full cylinders should not be exposed to temperatures above 65 °C and cylinders should not be filled to pressures greater than the developed pressure appropriate to the certified working pressure of the cylinder.

Cylinders should be clearly labelled with their current contents. A generic "Nitrox" or "Trimix" label will alert the user that the contents may not be air, and must be analysed before use. In some parts of the world a label is required specifically indicating that the contents are air, and in other places a colour code without additional labels indicates by default that the contents are air.

In a fire, the pressure in a gas cylinder rises in direct proportion to its absolute temperature. If the internal pressure exceeds the mechanical limitations of the cylinder and there are no means to safely vent the pressurized gas to the atmosphere, the vessel will fail mechanically. If the vessel contents are ignitable or a contaminant is present this event may result in an explosion.

Accidents

The major diving accident and fatality research studies that have been conducted globally including work by the Divers Alert Network, the Diving Incident Monitoring Study, and Project Stickybeak have each identified cases where the mortality was associated with the diving cylinder.

Some recorded accidents associated with diving cylinders:

• Valve ejected due to mix up with valve threads 3/4"NPSM and 3/4"BSP(F) caused damage to a dive shop compressor room.
• Dive Instructor leg nearly amputated by ejected valve white attempting to remove valve from pressurised cylinder.
• Valve ejected during filling due to thread failure, sank dive boat. Bursting disks in cylinder valves replaced by solid bolts.
• Filling hose failure severely injured operator when hose hit his face. Wound exposed jaw bone. 14 stitches to close the wound.

Cases of lateral epicondylitis have been reported caused by the handling of diving cylinders.

Long term storage

Breathing quality gases do not normally deteriorate during storage in steel or aluminium cylinders. Provided there is insufficient water content to promote internal corrosion, the stored gas will remain unchanged for years if stored at temperatures within the allowed working range for the cylinder, usually below 65 °C. If there is any doubt, a check of oxygen fraction will indicate whether the gas has changed (the other components are inert). Any unusual smells would be an indication that the cylinder or gas was contaminated at the time of filling. However some authorities recommend releasing most of the contents and storing cylinders with a small positive pressure.

Gas cylinder colour-coding and labeling

A contents label for oxygen usage (UK)

A steel 15 litre cylinder with net and boot and a bare 12 litre aluminium cylinder. Both are labeled for Nitrox use. The aluninium cylinder also displays a triangular label specifying the date of the most recent internal inspection and an oval label recording the most recent neck thread eddy current test.

Worldwide

In many recreational diving settings where air and nitrox are the widely used gases, nitrox cylinders are colour-coded with a green stripe on yellow background. The normal colour of aluminium diving cylinders is their natural silver. Steel diving cylinders are often painted, to reduce corrosion, mainly yellow or white to increase visibility. In some industrial cylinder identification colour tables, yellow shoulders means chlorine and more generally within Europe it refers to cylinders with toxic and/or corrosive contents; but this is of no significance in scuba since gas fittings would not be compatible.

Cylinders that are used for partial pressure gas blending with pure oxygen may also be required to display an "oxygen service certificate" label indicating they have been prepared for use with high partial pressures and gas fractions of oxygen.

European Union

In the European Union gas cylinders may be colour-coded according to EN 1098-3. The "shoulder" is the domed top of the cylinder between the parallel section and the pillar valve. For mixed gases, the colours can be either bands or "quarters".

• Air has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) "quartered" shoulders.
• Heliox has either a white (RAL 9010) top and brown (RAL 8008) band on the shoulder, or white (RAL 9010) and brown (RAL 8008) "quartered" shoulders.
• Nitrox, like Air, has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) "quartered" shoulders.
• Pure oxygen has a white shoulder (RAL 9010).
• Pure helium has a brown shoulder (RAL 9008).
• Trimix has a white, black and brown segmented shoulder.

These breathing gas cylinders must also be labeled with their contents. The label should state the type of breathing gas contained by the cylinder.

South Africa

Scuba cylinders are required to comply with the colours and markings specified in SANS 10019:2006.

• Cylinder colour is Golden yellow with a French grey shoulder.
• Cylinders containing gases other than air or medical oxygen must have a transparent adhesive label stuck on below the shoulder with the word NITROX or TRIMIX in green and the composition of the gas listed.
• Cylinders containing medical oxygen must be black with a white shoulder.

References

• CEN. EN 1089-2:2002 Transportable gas Cylinders, Part 2 - Precautionary labels Superseded by EN ISO 7225:2007.
• CEN. EN 1089-3:2004 Transportable gas Cylinders, Part 3 - Colour coding Current standard.

External links

Media related to Diving cylinders at Wikimedia Commons