Stage cylinder

A diving cylinder or diving gas cylinder is a gas cylinder used to store and transport high pressure gas used in diving operations. This may be breathing gas used with a scuba set, in which case the cylinder may also be referred to as a scuba cylinder, scuba tank or diving tank. When used for an emergency gas supply for surface supplied diving or scuba, it may be referred to as a bailout cylinder or bailout bottle. It may also be used for surface-supplied diving or as decompression gas . A diving cylinder may also be used to supply inflation gas for a dry suit or buoyancy compensator. Cylinders provide 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 when used on a scuba set are normally fitted with one of two common types of cylinder valve for filling and connection to the regulator. Other accessories such as manifolds, cylinder bands, protective nets and boots and carrying handles may be provided. Various configurations of harness may be used by the diver to carry a cylinder or cylinders while diving, depending on the application. Cylinders used for scuba typically have an internal volume (known as water capacity) of between and a maximum working pressure rating from . 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. Scuba divers may dive with a single cylinder, a pair of similar cylinders, or a main cylinder and a smaller "pony" cylinder, carried on the diver's back or clipped onto the harness at the side. Paired cylinders may be manifolded together or independent. In technical diving, more than two scuba cylinders may be needed.

When pressurised, a cylinder carries an equivalent volume of free gas greater than its water capacity, because the gas is compressed up to several hundred times atmospheric pressure. The selection of an appropriate set of diving cylinders for a diving operation is based on the amount of gas required to safely complete the dive. Diving cylinders are most commonly filled with air, but because the main components of air can cause problems when breathed underwater at higher ambient pressure, divers may choose to breathe from cylinders filled with mixtures of gases other than air. Many jurisdictions have regulations that govern the filling, recording of contents, and labelling for diving cylinders. Periodic testing and inspection of diving cylinders is often obligatory to ensure the safety of operators of filling stations. Pressurised diving cylinders are considered dangerous goods for commercial transportation, and regional and international standards for colouring and labelling may also apply.

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; however, this is a misnomer since 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 can result in oxygen toxicity.

Diving cylinders have also been referred to as bottles or flasks, usually preceded with the word scuba, diving, air, 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

The functional diving cylinder consists of a pressure vessel and a cylinder valve. There are usually one or more optional accessories depending on the specific application.

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. They are occasionally used when portability for accessing the dive site is critical, such as in cave diving. Composite cylinders certified to ISO-11119-2 or ISO-11119-3 may only be used for underwater applications if they are manufactured in accordance with the requirements for underwater use and are marked "UW". The pressure vessel comprises a cylindrical section of even wall thickness, with a thicker base at one end, and domed shoulder with a central neck to attach a cylinder valve or manifold at the other end.

Occasionally other materials may be used. Inconel has been used for non-magnetic and highly corrosion resistant oxygen compatible spherical high-pressure gas containers for the US Navy's Mk-15 and Mk-16 mixed gas rebreathers.

Aluminium

An especially common cylinder provided at tropical dive resorts is the "aluminium-S80" which is an aluminium cylinder design with an internal volume of rated to hold a nominal volume of of atmospheric pressure gas at its rated working pressure of . 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 them to stand upright on a level surface, but some were manufactured with domed bottoms. When in use, the cylinder valve and regulator add mass to the top of the cylinder, so the base tends to be relatively buoyant, and aluminium drop-cylinders tend to rest on the bottom in an inverted position if near neutral buoyancy. For the same reason they tend to hang at an angle when carried as sling cylinders unless constrained.

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. 6351 alloy has been superseded for new manufacture, but many old cylinders are still in service, and are still legal and considered safe if they pass the periodic hydrostatic, visual and eddy current tests required by regulation and as specified by the manufacturer. The number of cylinders that have failed catastrophically is in the order of 50 out of some 50 million manufactured. 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.

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, boring 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 and reduces excess buoyancy.

File:Die with billet.png, alt=Diagram showing a steel die in section with an aluminium billet inderted, Section of die with billet inserted
File:Aluminium cylinder extrusion.gif, alt=Animation showing cold extrusion of the aluminium cylinder by pressing a rounded end cylindrical mandrel into the billet, with the aluminium extruding between the sides of the die and the mandrel to form a blind tube, Cold extrusion process
File:Cylinder open.png, alt= The blind tube of the cylinder after removal from the die. It consists of the base and walls of the cylinder, but is still open at the top, Extrusion product before trimming
File:Cylinder closed.png, alt= The cylinder has been closed at the top by further cold forming, and the neck is still closed, Section after closure of the top end
File:Cylinder machined with neck detail.png, alt= The cylinder neck has been machined, and the threaded hole for the cylinder valve is shown, Section showing machined areas of the neck in detail
File:Hydrostatic test.png, alt= The cylinder undergoes hydrostatic testing for quality control, Hydrostatic test

Steel

In cold water diving, where a person 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 cylinders 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.

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 and final inspection.

An alternative production method is backward extrusion of a heated steel billet, similar to the cold extrusion process for aluminium cylinders, followed by hot drawing and bottom forming to reduce wall thickness, and trimming of the top edge in preparation for shoulder and neck formation by hot spinning. The other processes are much the same for all production methods.

Cylinder neck

The ''neck'' of the cylinder is the part of the end which is shaped as a narrow concentric cylinder, and 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 . These connections are sealed using thread tape and torqued to between on steel cylinders, and between on aluminium cylinders.

Parallel threads are made to several standards:
* M25x2 ISO parallel thread, which is sealed by an O-ring and torqued to on steel, and on aluminium cylinders;
* M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to on steel cylinders, and on aluminium cylinders;
* 3/4"x14  BSP parallel thread, which has a 55° Whitworth thread form, a pitch diameter of 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 on aluminium cylinders, which has a 60° thread form, a pitch diameter of , and a pitch of 14 threads per inch (5.5 threads per cm);
* 3/4"x16  UNF, sealed by an O-ring, torqued to on aluminium cylinders.
*7/8"x14 UNF, sealed by an O-ring.
The 3/4"NGS and 3/4"BSP are very similar, having the same pitch and a pitch diameter that only differs by about , 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.

Permanent stamp markings

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

File:Permanent cylinder markings 2.gif, alt= Diagram of a cylinder shoulder with stamp marking: TC3ALM 207 DOT-3AL 3000 LS 10822 LUXFER 08(testing authority stamp)04 S040, Stamp markings on an American manufacture aluminum 40 cu ft 3000 psi cylinder
File:Permanent cylinder markings 3.gif, alt= Diagram of a cylinder shoulder with stamp marking: TC3ALM 207 DOT-3AL 3000 P1576 LUXFER 01(testing authority stamp)93 S80 and date stamps for 3 hydrostatic tests, Stamp markings on an American manufacture aluminum 80 cu ft 3000 psi cylinder
File:Permanent cylinder markings 4.gif, alt= Diagram of a cylinder shoulder with stamp marking: M25x2 ISO BS5045/3/B/S CP 23200kPa 11/92 SER NO P2699M LUXUK 2451 3V03 TP 34800kPa CAPACITY 12.2L TARE 16.3 kg, and three hydrostatic test dates, Stamp markings on a British-manufacture aluminium 12.2-litre 232 bar cylinder
File:Permanent cylinder markings 5.gif, alt= Diagram of a cylinder shoulder with stamp marking: FABER 96/9856 187 BS5045/1/CM/S TP 265 BAR AIR/LUCHT 06.96 CP 300BAR 15C 7.0L WT 9.9 kg, Stamp markings on an Italian manufacture steel 7-litre 300 bar cylinder

Universally required markings include:
*Identification of the manufacturer
*Manufacturing standard, which will identify the marerial specification
*Serial number
*Date of manufacture
*Charging pressure
*Capacity
*Mark of the accredited testing agency
*Date of each revalidation test
A variety of other markings may be required by national regulations, or may be optional.

The cylinder 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 thread specification, the connection to the regulator, pressure rating, and other distinguishing features. Standards relating to the specifications and manufacture of cylinder valves include ISO 10297 and CGA V-9 Standard for Gas Cylinder Valves. The other distinguishing features include outlet configuration, handedness and valve knob orientation, number of outlets and valves (1 or 2), shape of the valve body, presence of a reserve valve, manifold connections, and the presence of a bursting disk overpressure relief device.

Cylinder threads may be in two basic configurations: Taper thread and parallel thread. 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. The valve pressure rating must be compatible with the cylinder pressure rating.

Parallel threads are more tolerant of repeated removal and refitting of the valve for inspection and testing.

Accessories

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

Manifolds

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.

Cylinders may also be manifolded by a removable whip, commonly associated with dual outlet cylinder valves, and the on board emergency gas supply of a diving bell is usually manifolded by semi-permanent metal alloy pipes between the cylinder valves.

Valve cage

Also known as a manifold cage or regulator cage, this is a structure which can be clamped to the neck 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 (roll-off). 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 .

Cylinder boot

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 . 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 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.

Dust caps and plugs

These are used to cover the cylinder valve orifice when the cylinder is not in use to prevent dust, water or other materials from contaminating the orifice. They can also help prevent the O-ring of a yoke type valve from falling out. The plug may be vented so that the leakage of gas from the cylinder does not pressurise the plug, making it difficult to remove.

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 United States 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 , and the compact aluminum range have a working pressure of .
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 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 United States, 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.

Pressure monitoring

The internal pressure of a diving cylinder is measured at several stages during use. It is checked before filling, monitored during filling and checked when filling is completed. This can all be done with the pressure gauge on the filling equipment.

Pressure is also generally monitored by the diver. Firstly as a check of contents before use, then during use to ensure that there is enough left at all times to allow a safe completion of the dive, and often after a dive for purposes of record keeping and personal consumption rate calculation.

The pressure is also monitored during hydrostatic testing to ensure that the test is done to the correct pressure.

Most diving cylinders do not have a dedicated pressure gauge, but this is a standard feature on most diving regulators, and a requirement on all filling facilities.

There are two widespread standards for pressure measurement of diving gas. In the United States and perhaps a few other places the pressure is measured in pounds per square inch (psi), and the rest of the world uses bar. Sometimes gauges may be calibrated in other metric units, such as kilopascal (kPa) or megapascal (MPa), or in atmospheres (atm, or ATA), particularly gauges not actually used underwater.

Capacity

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 stamp 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 are effectively identical at two decimal places accuracy.

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.
*22 litres: Available in steel, 200 and 232bar,
*20 litres: Available in steel, 200 and 232bar,
*18 litres: Available in steel, 200 and 232 bar, used as single or twins for back gas.
*16 litres: Available in steel, 200 and 232bar, used as single or twins for back gas.
*15 litres: Available in steel, 200 and 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 or 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, 200, 232, 300 bar, used for back gas as singles and twins, and as bailout cylinders. Also a popular size for SCBA
*5.5 litres: Available in steel, 200 and 232 bar,
*5 litres: Available in steel, 200 bar, used for rebreathers
*4 litres: Available in steel, 200 bar, used for rebreathers and pony cylinders
*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
*1.5 litres: Available in steel, 200 and 232 bar, used for 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 when pressurized to . It has an internal volume of approximately .

Standard sizes by volume of gas stored

* Aluminum C100 is a large (13.l l), high-pressure () cylinder. Heavy at .
* Aluminum S80 is probably the most common 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 and working pressure of . 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.
* Aluminium C80 is the high-pressure equivalent, with a water capacity of 10.3 L and working pressure .
* Aluminum S40 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 and working pressure .
* Aluminum S63 (9.0 L) , 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 and HP80 (10.1 L) at are both more compact and lighter than the Aluminium S80 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.

Physical dimensions

Cylinders made from seamless steel and aluminium alloys are described here. The constraints on filament wound composite cylinders will differ:

There are a small number of standardised outside diameters as this is cost effective for manufacture, because most of the same tooling can be shared between cylinders of the same diameter and wall thickness. A limited number of standard diameters is also convenient for sharing accessories such as manifolds, boots and tank bands. Volume within a series with given outside diameter is controlled by wall thickness, which is consistent for material, pressure class, and design standard, and length, which is the basic variable for controlling volume within a series. Mass is determined by these factors and the density of the material. Steel cylinders are available in the following size classes, and possibly others:
* OD = 83mm, 0.8 to 1.8 litres
* OD = 100mm, 2.0 to 4.75 litres
* OD = 115mm, 2.5 to 5.0 litres
* OD = 140mm, 4.0 to 15.0 litres
* OD = 160mm, 6.0 to 16.0 litres
* OD = 171mm, 8.0 to 23.0 litres
* OD = 178mm, 8.0 to 35.0 litres
* OD = 204mm, 10.0 to 40.0 litres
* OD = 229mm, 20.0 to 50.0 litres
* OD = 267mm, 33.0 to 80.0 litres

Wall thickness varies depending on location, material and practical considerations. The sides of the cylindrical section are sufficient to withstand the stresses of a large number of cycles to test pressure, with an allowance for a small amount of material loss due to general corrosion and minor local damage due to abrasion and normal wear and tear of use, and a limited depth of local damage due to pit and line corrosion and physical damage. The amount of damage and material loss allowed is compatible with the visual inspection rejection criteria. Steel cylinders are designed for test stresses to be below the fatigue limit for the alloy. The wall thickness is roughly proportional to diameter for a given test pressure and material strength – if the diameter is double, the basic wall thickness will also double. The cylindrical section has the lowest wall thickness, and it is consistent within manufacturing tolerances for the entire cylindrical section.

End thickness allows for considerably more wear and tear and corrosion on the bottom of the cylinder, and the shoulder is made thicker to allow for the variabilities inherent in the manufacturing process for closing the end, and for any stress raisers due to the process of permanent stamp marking. To a large extent bottom thickness distribution of a steel cylinder and shoulder thickness of all metal cylinders are influenced by the manufacturing process, and may be thicker than strictly necessary for strength and corrosion tolerance.

Buoyancy characteristics

The density of a cylinder is concentrated in the ends, which are relatively thick walled and have a lower enclosed volume per unit mass. The details vary depending on the specification, but this tendency is common to both steel and aluminium cylinders, and is more extreme in flat or dished ends. As a consequence, long narrow cylinders are less dense than short wide cylinders for the same material and the same end configuration, while for the same internal volume, a short wide cylinder is heavier than a long narrow cylinder.

Buoyancy of a diving cylinder is only of practical relevance in combination with the attached cylinder valve, scuba regulator and regulator accessories, as it will not be used underwater without them. These accessories are attached to the top of the cylinder, and both decrease the buoyancy of the combined unit and move the centre of gravity towards the top (valved end).

Back mounted cylinder sets are generally not removed during a dive, and the buoyancy characteristics can be allowed for at the start of the dive, by ensuring that the diver has sufficient reserve buoyancy to float with the cylinders full, and sufficient ballast to remain submerged when the cylinders are all empty. The buoyancy compensator must be sufficient to provide some positive buoyancy at all depths with full cylinders. Adjustments to ballasting can compensate for other buoyancy variables. Inability to remain comfortably immersed at the shallowest decompression stop can lead to incomplete decompression and increased risk of decompression sickness.

The change in buoyancy of a 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 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, as they are likely to be detached for staging or handed off when relatively full. This is less likely to be a problem for a solo diver's bailout set, as there will be fewer occasions to remove it during a dive. Side-mount sets for tight penetrations are expected to be swung forward or detached to pass through tight constrictions, and should not grossly affect trim or buoyancy during these maneuvers.

A major manufacturer of steel cylinders, Faber, claim that their steel cylinders are neutral or slightly negative when empty, but do not specify which pressure rating this refers to, or whether this takes into account the cylinder valve.

Applications and configurations

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 cylinder" 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 and can be removed underwater with less effect on the diver's overall buoyancy.
* "Suit inflation gas" may be taken from a breathing gas cylinder or may be supplied from a small independent cylinder.

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:

* 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 independent air source is a hand-held cylinder filled with about 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.

* 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 only work well with air-integrated computers which can monitor two or more cylinders. 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.

* 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 isolation 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.

* 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 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.

* 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

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 an 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

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

Diving bells are required to carry an onboard 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.

Suit inflation cylinders

Suit inflation gas may be carried in a small independent cylinder. Sometimes argon is used for superior insulation properties. This must be clearly labelled and may also need to be colour coded to avoid inadvertent use as a breathing gas, which could be fatal as argon is an asphyxiant.

Other uses of compressed gas cylinders in diving operations

Divers also use gas cylinders above water for storage of oxygen for first aid treatment of