Diving cylinder
A diving cylinder or diving gas cylinder is a gas cylinder used to store and transport high-pressure gas used in diving operations. This may be breathing gas used with a scuba set, in which case the cylinder may also be referred to as a scuba cylinder, scuba tank or diving tank. When used for an emergency gas supply for surface-supplied diving or scuba, it may be referred to as a bailout cylinder or bailout bottle. It may also be used for surface-supplied diving or as decompression gas. A diving cylinder may also be used to supply inflation gas for a dry suit, 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 of between and a maximum working pressure rating from. Cylinders are also available in smaller sizes, such as 0.5, 1.5 and 2 litres; however these are usually used for purposes such as inflation of surface marker buoys, dry suits, and buoyancy compensators rather than breathing. Scuba divers may dive with a single cylinder, a pair of similar cylinders, or a main cylinder and a smaller "pony" cylinder, carried on the diver's back or clipped onto the harness at the side. Paired cylinders may be manifolded together or independent. In technical diving, more than two scuba cylinders may be needed 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 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 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 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 or dished 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. 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, 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.