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.^[13][14] 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.^[15][16]

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 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.[15][16]}

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.^[17]

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.^[18]

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),^[19] with a 12% taper right hand thread, standard Whitworth 55° form with a pitch of 14 threads per in

  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;^[20]
  • 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;^[20]
  • 3/4"x14 BSP parallel thread,

    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.^[25]

    • 

      Stamp markings on an American manufacture aluminum 40 cu ft 3000 psi cylinder

    • [25]

      • 

        Stamp markings on an American manufacture aluminum 80 cu ft 3000 psi cylinder

      • Stamp markings on a British-manufacture aluminium 12.2-litre 232 bar cylinder

      • 

        Stamp markings on an Italian manufacture steel 7-litre

        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.^[25]

      The cylinder valve

      Universally required markings include:

      • Identification of the

        A variety of other markings may be required by national regulations, or may be optional.^[25]

        The cylinder valve

        [29]

        Cylinder threads may be in two basic configurations: Taper thread and parallel thread.^[4] 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.^{[30][31][32][33]}

        Parallel threads are more tolerant of repeated removal and refitting of the valve for inspection and testing.^[34]:s9

        Connection to the regulator

        

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

        

        DIN-valve to regulator connection, showing the sealing surfaces, according to ISO 12209-2

        A rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. [34]^:s9

        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.^[35] 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.^[36] 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.^[37]
        • 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, h

          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 M26×2 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%).^[38] 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^[38] - 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.^[39] The regulator supplied with the rebreather had a compatible connection.

          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.^[40]

          • 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)^[40]
          • 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).^[40] The 300 bar pressures are common in European diving and in US cave diving.

          Adaptors

          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.^[40] There are two categories of adaptors: 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. These can be recognised by a dimple recess opposite to the outlet opening, used to locate the screw of an A-clamp. Block adaptors are generally rated for 200 bar, and can be used with almost any 200 ba

          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.^[40]

          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.^[40] There are two categories of adaptors: 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. These can be recognised by a dimple recess opposite to the outlet opening, used to locate the screw of an A-clamp. Block adaptors are generally rated for 200 bar, and can be used with almost any 200 bar DIN valve.

          

          <

          The most commonly used cylinder valve type is the single outlet plain valve, sometimes known as a "K" valve,^[16] 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.^[5] 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.^[41]

          Reserve valves

          

          A J-valve from c.1960

          

          Draeger 200 bar axial spindle cylinder valves with manifold and reserve lever

          

          Dräger taper thread axial spindle cylinder valve with reserve lever

          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.^[4][28] These valves became known as "J-valves" 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".^[16] 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,^[42] NOAA (the National Oceanographic and Atmospheric Administration) and OSHA (the national Occupational Health and Safety Administration) 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.^[42] 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 to 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 if it was necessary for the diver to descend during exit from a cave or wreck, breathing would become progressively more difficult as the diver went deeper, eventually becoming impossible until the diver could ascend to a low enough ambient pressure.^[16]

          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 valve knob on the diver's right side when back-mounted. Side-spindle valves used with manifolds must be a handed pair - one with the knob to the right and the other with the knob to the left, but in all cases the valve is opened by turning the knob anticlockwise, and closed by turning it clockwise. This is the convention with almost al

          Y and H cylinder valves have two outlets, each with its own valve, allowing two regulators to be connected to the cylinder.^[5] 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.^[41]

          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.^[4][28] These valves became known as "J-valves" 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".^[16] 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,^[42] NOAA (the National Oceanographic and Atmospheric Administration) and OSHA (the national Occupational Health and Safety Administration) 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.^[42] 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 to 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 if it was necessary for the diver to descend during exit from a cave or wreck, breathing would become progressively more difficult as the diver went deeper, eventually becoming impossible until the diver could ascend to a low enough ambient pressure.^[16]

          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 valve knob on the diver's right side when back-mounted. Side-spindle valves used with manifolds must be a handed pair - one with the knob to the right and the other with the knob to the left, but in all cases the valve is opened by turning the knob anticlockwise, and closed by turning it clockwise. This is the convention with almost all valves for all purposes. Left and right hand side-spindle valves are used by sidemount divers. These may be blanked off manifold valves or specially made for the purpose.^[15][43]

          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.^[4] 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. Burst disk over-pressure protection is specified in the CGA Standard S1.1. Standard for Pressure Relief Devices.^[29] Bursting disc rupture pressure is generally rated at 85% to 100% of test pressure.^[34]

          Accessories

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

          Manifolds

          Main article: Scuba manifold

          [16]

          Some cylinder valve models have axial spindles - in line with the cylinder axis, and are not handed. Standard side-spindle valves have the valve knob on the diver's right side when back-mounted. Side-spindle valves used with manifolds must be a handed pair - one with the knob to the right and the other with the knob to the left, but in all cases the valve is opened by turning the knob anticlockwise, and closed by turning it clockwise. This is the convention with almost all valves for all purposes. Left and right hand side-spindle valves are used by sidemount divers. These may be blanked off manifold valves or specially made for the purpose.^[15][43]

          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.^[4] 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. Burst disk over-pressure protection is specified in the CGA Standard S1.1. Standard for Pressure Relief Devices.^[29] Bursting disc rupture pressure is generally rated at 85% to 100% of test pressure.^[34]

          Accessories

          Add

          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.^{[42][44]:164,165} 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.^[44]

          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.^[44] 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.^[44] 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.^[16]

          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^[44]:166 and from rolling the valve closed by friction of the handwheel against an overhead. A valve cage is often made of stainless steel,^[44] 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.^[45] Some boots have flats moulded into the plastic to reduce the tende

          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.^[44] 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.^[44] 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.^[16]

          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^[44]:166 and from rolling the valve closed by friction of the handwheel against an overhead. A valve cage is often made of stainless steel,^[44] 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

          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.^[45] Some boots have flats moulded into the plastic to reduce the tendency of the cylinder to roll on a flat surface.^[46] 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.^[45][47] 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.^[46]

          Cylinder handle

          [46]

          Cylinder handleA 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.^[48]

          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 th

          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.^[48]

          Pressure rating

          The thi

          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;^[38]

          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 a

          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.^[27]

          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

          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.

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

          Developed pressureThe 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 US 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

          

          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 volumepounds 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.

          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 one decimal place accuracy.^[25]

          Standard sizes by internal volume

          • 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).^[55]
          • 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 cu ft) and working pressure of 3,000 pounds per square inch (207 bar).^[55] 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).^[55]
          • 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 cu ft) and working pressure 3,000 pounds per square inch (207 bar).^[55]
          • Aluminum S63 (9.0 l) 3,000 pounds per square inch (207 bar),^[55] 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 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.^[38]
          • 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.^[56]

          Applications and configurations

          [27]

          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 approximately 11 litres (0.39 cu ft).

          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).^[57] 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:^[58]

          • "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.
          • "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.^[59]

          Open-circuit scuba

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

          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).^[57] 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:^[58]

          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.^[59]

          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 th

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

            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.^[64]
          • closed circuit rebreathers have an oxygen cylinder and a "diluent" cylinder, which contains air, nitrox or a helium based gas.^[64]

          • 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.^[65] 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.^[65] 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.^[65] 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.^[66][67][68] 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.^[62][69]

          Emergency gas supply on diving bells

          

          A closed bell used for saturation diving showing emergency gas supply cylinders

          Diving bells are required to carry an onboard supply of breathing gas for use in emergencies.^[70][71] 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

          

          Submersible argon cylinder for dry suit inflation. The blue colour is a legal requirement in South Africa

          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 diving disorders and as part of storage "banks" for diving air compressor stations, gas blending, surface supplied breathing gas and gas supplies for decompression chambers and saturation systems. Similar cylinders are also used for many purposes not connected to diving. For these applications they are not diving cylinders and may not be subject to the same regulatory requirements as cylinders used underwater.

          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.^[72]

          There are two parts to this problem: The capacity of the cylinder and the consumption by the diver.

          The cylinder's capacity to store gas

          Two features of the cylinder determine its gas carrying capacity:

          • internal volume : this normally ranges between 3 litres and 18 litres for single cylinders.
          • 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.

          At the pressures which apply to most diving cylinders, the ideal gas equation is sufficiently accurate in almost all cases, as the variables that apply to gas consumption generally overwhelm the error in the ideal gas assumption.

          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 example, 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 useful and the relationship between the pressure, size of the cylinder and gas contained in the cylinder is approximately linear; at higher pressures this linearity no longer applies, and 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 air and not the 900 litres (32 cu ft) expected from the ideal gas law. Equations have been proposed which give more accurate solutions at high pressure, including the Van der Waals equation. Compressibility at higher pressures also varies between gases and mixtures of gases.

          Diver gas consumption

          

          A closed bell used for saturation diving showing emergency gas supply cylinders

          Diving bells are required to carry an onboard supply of breathing gas for use in emergencies.^[70][71] 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

          

          Submersible argon cylinder for dry suit inflation. The blue colour is a legal requirement in South Africa

          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 [70]^[71] 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

          

          Submersible argon cylinder for dry suit inflation. The blue colour is a legal requirement

          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 diving disorders and as part of storage "banks" for diving air compressor stations, gas blending, surface supplied breathing gas and gas supplies for decompression chambers and saturation systems. Similar cylinders are also used for many purposes not connected to diving. For these applications they are not diving cylinders and may not be subject to the same regulatory requirements as cylinders used underwater.

          Gas calcu

          Divers also 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, surface supplied breathing gas and gas supplies for decompression chambers and saturation systems. Similar cylinders are also used for many purposes not connected to diving. For these applications they are not diving cylinders and may not be subject to the same regulatory requirements as cylinders used underwater.

          Gas calculations

          Further information: [72]

          There are two parts to this problem: The capacity of the cylinder and the consumption by the diver.

          The cylinder's capacity to store gas

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

          Maximum breathing duration (T) for a given depth can be calculated as

          T = available air / rate of consumption^[75]

          which, using the ideal gas law, is

          T = (available cylinder pressure × cylinder volume) / (rate of air consumption at surface) × (ambient pressure)^{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 fs

          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.

          Maximum breathing duration

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

          Maximum breathing duration (T) for a given depth can be calculated as

          T = available air / rate of consumptionideal gas law, is

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