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, buoyancy compensator, decompression buoy, or lifting bag. Cylinders provide breathing gas to the diver by free-flow or through the demand valve of a diving regulator, or via the breathing loop of a diving rebreather. Diving cylinders are usually manufactured from aluminum or steel alloys, and when used on a scuba set are normally fitted with one of two common types of scuba 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 3 and 18 litres (0.11 and 0.64 ft3) and a maximum working pressure rating from 184 to 300 bars (2,670 to 4,350 psi). Cylinders are also available in smaller sizes, such as 0.5, 1.5 and 2 litres; however these are usually used for purposes such as inflation of surface marker buoys, dry suits, and buoyancy compensators rather than breathing. 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 to carry different gases. Larger cylinders, typically up to 50 litre capacity, are used as on-board emergency gas supply on diving bells. Large cylinders are also used for surface supply through a diver's umbilical, and may be manifolded together on a frame for transportation. The selection of an appropriate set of scuba cylinders for a diving operation is based on the estimated 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 labeling for diving cylinders. Periodic testing and inspection of diving cylinders is often obligatory to ensure the safety of operators of filling stations. Pressurized diving cylinders are considered dangerous goods for commercial transportation, and regional and international standards for colouring and labeling may also apply.
Terminology A "diving gas cylinder" can refer to any gas cylinder containing a diving gas. The terms "diving cylinder" or "scuba cylinder" tend 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 usually a misnomer since these cylinders typically contain compressed atmospheric breathing air, or an oxygen-enriched air mixture. They rarely contain pure oxygen, except when used for closed circuit 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) 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. Scuba 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, decocompression (deco) cylinders, sidemount cylinders, pony cylinders, suit inflation cylinders, etc. The same cylinder, rigged in the same way, may be used as a bailout cylinder, a decompression cylinder or a stage cylinder.
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 aluminum or forged steel. 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. 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". 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, and a few other military rebreathers.
Aluminium
Aluminium cylinders are popular as rental equipment at tropical dive resorts as they require less maintenance. They are also often used where divers carry several 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 aluminum cylinders reduces the amount of extra buoyancy the diver would need to achieve neutral buoyancy. They may also be preferred when carried as "side mount" 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. 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 aluminum drop-cylinders tend to rest on the bottom in an inverted position (with the base up) if near neutral buoyancy. For the same reason they tend to hang at an angle with the base up when carried as sling cylinders unless constrained or ballasted. Aluminum cylinders are usually manufactured by cold extrusion of aluminum 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 gas cylinder permanent markings. Although some aluminium cylinders were manufactured with domed bottoms, most have flat bases, allowing them to stand upright on a level surface. The flat bottoms are relatively thick to allow for rough treatment and wear, which makes them heavier than they need to be for strength, but the extra weight at the base reduces excess buoyancy and keeps the centre of gravity lower, which gives better balance in the water.
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. As a result, the use of steel cylinders can result in both a lighter cylinder and less ballast required for the same gas capacity, a two-fold 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. 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 either domed (convex) or 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 often have 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 cylinders with foot rings are made for industrial uses but are not legal for underwater use, as they corrode in the crevice between the foot ring and the cylinder, and cannot be effectively visually inspected in this area. 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, coated externally with a corrosion barrier paint or hot-dip galvanised, and then given a 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. A third method is to start with seamless steel tube of a suitable diameter and wall thickness, manufactured by a process such as the Mannesmann process, and to close both ends by the hot spinning process. When a neck opening is only required at one end, the base is spun first and dressed inside for a uniform smooth surface, then the process of closing the shoulder and forming the neck is the same as for the pressed plate method.
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. Cylinder thread may be in either of two basic configurations: Taper thread or parallel thread. Parallel threads are more tolerant of repeated removal and refitting of the valve for inspection and testing. The valve thread specification must exactly match the neck thread specification of the cylinder, as 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. There are several standards for scuba cylinder 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. Other taper thread standards for connection of valves to gas cylinder necks of current validity and historical use exist, and some are interchangeable, while others are not. The vary in nominal diameter, thread form, and taper angle. 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 aluminum 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 aluminum 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. 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 0.2 mm (0.008 in), but they are not compatible, as the thread forms are different. All parallel thread valves use an O-ring at the top of the neck thread which seals in a chamfer or step in the cylinder neck and against the flange of the valve. Large cylinders such as those used for bell emergency gas generally use 25E taper thread to "ISO 11363-1, Gas cylinders – 17E and 25E taper threads for connection of valves to gas cylinders – Part 1: Specifications".
Permanent markings
The shoulder of the cylinder carries stamp markings providing required information about the cylinder.
Universally required markings include:
Identification of the manufacturer Manufacturing standard, which will identify the material specification Serial number Date of manufacture and initial valiation Charging pressure Capacity Mark of the accredited testing agency Date of each re-validation 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, which could block or jam the regulator. 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 spindle 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.
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 or more cylinders together so that the contents of all can be supplied to one or more regulators or distribution systems. There are three commonly used configurations of scuba 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 the valves are 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 manifold 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 cages A valve cage, also known as a manifold cage or regulator cage, can be clamped to the neck of a single cylinder or to manifolded cylinders, shielding the valves and first-stage regulators from impact and abrasion damage during use, and preventing accidental valve closure caused by the handwheel rubbing against an overhead surface (roll-off). A valve cage is typically made of stainless steel.
Cylinder bands Cylinder bands, or tank 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 cylinders, just below the shoulders, and one lower down. The conventional distance between centre-lines for bolting to a backplate is 11 inches (280 mm).
Cylinder boots
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 handles
A cylinder handle may be fitted to a scuba cylinder, usually clamped to the neck, to conveniently carry the cylinder. This can also increase the risk of snagging in an enclosed environment. Handles that are clamped to the cylinder neck may be fixed or folding. Some may require the valve to be removed to allow fitting.
Dust caps and plugs These are used to cover he 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 during storage and transport. A screw-in 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
Working pressure and cylinder volume determine the capacity of the cylinder to store gas. Many of the physical characteristics of the cylinder are consequences of these factors. Two other pressures are also relevant to cylinder use: Test pressure and developed pressure.
Working pressure
Working pressure is the maximum pressure that the cylinder is designed to tolerate indefinitely at reference temperature under normal operating conditions. It is determined during design and takes into account the material's strength, operating temperature range, and the working lifeespan. Diving 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 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 aluminum.
Test pressure
Test pressure is the pressure used to validate that the cylinder is strong enough to withstand unintended occasional overpressurisation in service safely. It is also known as proof 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 pressure of the contents of a diving cylinder is used as an indication of the amount of gas contained by the cylinder. It is measured at several stages during filling. 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 in a scuba cylinder is also monitored by the diver during a dive. 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. Bell emergency gas supplies are monitored from the bell gas panel using pressure gauges and may also use an electronic contents gauge which transmits the pressures to a surface control panel. 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 they are a standard feature on most diving regulators, and a requirement on all filling facilities. An alternative method of monitoring cylinder pressure during a dive is by means of a wireless pressure transmitter on the regulator first stage monitored by an air-integrated dive computer. There are two widespread standards for pressure measurement of diving gas. In the United States the pressure is measured in pounds per square inch (psi), and most of 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.
Size The most common dimension considered is the capacity for gas storage, but linear dimensions, mass and buoyancy are also important when carried by the diver.
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 alternative term 'water capacity', abbreviated as WC which is often stamp marked on the cylinder shoulder. It is 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. These are representative examples of standard sizes by internal volume, for a larger range, the catalogues of the manufacturers may be consulted. The applications are typical, but not exclusive.
50 litres: Available in steel, 200 and 300 bar, a common size for bell onboard emergency gas. 22 litres: Available in steel, 200 and 232 bar, Occasionally used for back gas. 20 litres: Available in steel, 200 and 232 bar, Occasionally used for back gas. 18 litres: Available in steel, 200 and 232 bar, used as singles or occasionally twins for back gas. 16 litres: Available in steel, 200 and 232 bar, used as single or twins for back gas. 15 litres: Available in steel, 200 and 232 bar, used as singles or twins for back gas 12.2 litres: Available in steel 232, 300 bar and aluminium 232 bar, used as singles or twins for back gas 12 litres: Available in steel 200, 232, 300 bar, and aluminium 232 bar, used as singles 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, sidemount and back gas by smaller people 7 litres: Available in steel, 200, 232 and 300 bar, and aluminium 232 bar, back gas as singles and twins, and as bailout cylinders. 6 litres: Available in steel, 200, 232, 300 bar, used for back gas as singles and twins, and as bailout cylinders. 5.5 litres: Available in steel, 200 and 232 bar, used for bailout cylinders. 5 litres: Available in steel, 200 bar, used for rebreathers and bailout cylinders 4 litres: Available in steel, 200 bar, used for rebreathers and bailout cylinders 3 litres: Available in steel, 200 bar, used for rebreathers and bailout cylinders 2 litres: Available in steel, 200 bar, used for rebreathers, bailout 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 0.1 litres: Available in Aluminium: Used for decompression buoy inflation
Nominal volume 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 in the same internal volume. The actual working pressure in use is not necessarily the same as the nominal working pressure stamped on the cylinder. 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, a steel cylinder manufactured to the DOT 3AA standard, with a rated working pressure of 2,400 pounds per square inch (170 bar) with a '+' mark can be legally filled to 2,640 pounds per square inch (182 bar) provided it has passed the more stringent hydrostatic test procedure required for revalidation of the '+' rating. If this cylinder is rated as 80 cubic feet (2,300 L), that will be the volume of gas at atmospheric pressure that it can hold at the '+' rated pressure. At the nominal working pressure of 2400 pounds per square inch it will only hold 72.7 cubic feet of atmospheric pressure air. As a counterexample, the 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), as there is no '+' rating applicable to aluminium cylinders. It has an internal volume of approximately 11 litres (0.39 ft3). Standard sizes by volume of gas stored: This system of size specification is usually used in the US.
Aluminum C100 is a large (13.l l), high-pressure (3,300 pounds per square inch (228 bar)) cylinder. Heavy at 42.0 pounds (19.1 kg). 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 11 litres (0.39 ft3) and working pressure of 3,000 pounds per square inch (207 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. Aluminium C80 is the high-pressure equivalent, with a water capacity of 10.3 L and working pressure 3,300 pounds per square inch (228 bar). Aluminum S63 (9.0 L) 3,000 pounds per square inch (207 bar), 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. 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 5.8 litres (0.20 ft3) and working pressure 3,000 pounds per square inch (207 bar). Aluminium S30 (4.3 L) 3,000 pounds per square inch (207 bar), Aluminium S19 (2.7 L), 3,000 pounds per square inch (207 bar), Aluminium S13 (1.9 L), 3,000 pounds per square inch (207 bar), Steel LP80 2,640 pounds per square inch (182 bar) and HP80 (10.1 L) at 3,442 pounds per square inch (237 bar) 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.
Linear dimensions Linear dimensions capture the outside diameter (OD), wall thickness, and end thickness of a bare cylinder, as the sizes of valves and other accessories vary. 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 a given outside diameter is controlled by wall thickness, which is consistent for material, pressure class, 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, among others:
OD = 83 mm, 0.8 to 1.8 litres OD = 100 mm, 2.0 to 4.75 litres OD = 115 mm, 2.5 to 5.0 litres OD = 140 mm, 4.0 to 15.0 litres OD = 160 mm, 6.0 to 16.0 litres OD = 171 mm, 8.0 to 23.0 litres OD = 178 mm, 8.0 to 35.0 litres OD = 204 mm, 10.0 to 40.0 litres OD = 229 mm, 20.0 to 50.0 litres OD = 267 mm, 33.0 to 80.0 litres Wall thickness varies depending on location, material, pressure rating, and practical considerations. The walls of the cylindrical section are designed to withstand the stresses from a large number of cycles to test pressure, with allowances for minor material loss from general corrosion, small areas of local damage from abrasion and normal wear, and limited depths of damage from pitting, line corrosion, or other physical impacts. 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 – doubling the diameter will also double the basic wall thickness. Wall thickness is also proportional to working pressure and test pressure for a given diameter and material specification. The cylindrical section has the lowest wall thickness, and it is consistent within manufacturing tolerances for the entire cylindrical section. End thickness of the base allows for considerably greater wear, impact, and corrosion on the bottom of the cylinder, while 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. 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.
Mass Faber steel cylinders carrying the CE mark have slightly decreased in mass for a given cylinder size from 2023. A 200-bar 15-litre cylinder with 203 mm-outside-diameter (8.0 in) with domed bottom, has reduced from 16.2 kg to 14.5 kg. The equivalent 232-bar cylinder reduced in mass from 18.2 to 16.7 kg.
Buoyancy Buoyancy of a scuba 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). This affects the cylinder orientation for sling and side mount. The mass of a seamless metal diving 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. 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 (BC), also called buoyancy control device (BCD), 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 consistently 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 a 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, 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