Diving cylinder

A diving cylinder or SCUBA tank is used to store and transport high pressure breathing gas as a component of an Aqua-Lung. It provides gas to the SCUBA diver through a diving regulator. Diving cylinders are typically filled in the range of 186 to 300 bar (2700 to 4300 psi, or 18.6 to 30.0 MPa) and have a volume of 1.5 to 18 litres or a gas carrying capacity of 850 to 3600 litres (30 to 120 ft&sup3;).

Gas cylinders are used above water for many purposes including storage of gases for oxygen first aid treatment of diving disorders and as part of storage "banks" for diving air compressor stations; and also for many purposes not connected to diving.

Parts of a cylinder
The diving cylinder consists of several parts:


 * the pressure vessel is normally made of forged aluminium or steel. Kevlar composite cylinders are used in fire fighting breathing apparatus, but are rarely used for diving due to their high positive buoyancy. Aluminium cylinders have a lower density than steel cylinders. This can be an advantage in technical diving because it reduces the extra buoyancy the diver needs to carry many cylinders. It can be a disadvantage to divers who carry few cylinders due to the extra weight needed on the diving weighting system to counteract this buoyancy.


 * the pillar valve is the point at which the pressure vessel connects to the  diving regulator. The purpose of the pillar valve is to control gas flow to and from the pressure vessel and to form a seal with the regulator. Some counties insist that the pillar valve includes a burst disk, a type of pressure 'fuse', that will fail before the pressure vessel fails in the event of over pressurisation.


 * a rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Halocarbon o-rings are used with cylinders storing oxygen-rich gas mixtures to reduce the risk of fire.


 * Y pillar valves. Most pillar valves only have one output and one valve. A Y valve has two outputs and two valves allowing two regulators to be connected to the cylinder. If one regulator “freeflows”, which is a common failure mode, its valve can be closed and the cylinder breathed from the regulator connected to the other valve.


 * Reserve lever. Diving cylinders until the 1970s, before pressure gauges on regulators came into common use, often used a mechanical reserve mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled a lever and finished the dive before the reserve was consumed.



Types of pillar valve
There are three types of pillar valve:


 * A clamp or Yoke - 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. This type 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 over-pressurisation.


 * 232 bar DIN (5 thread, metric M 25×2) - the regulator screws into the pillar valve trapping the o-ring securely. 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, so a diver abroad will need to take an adaptor.




 * 300 bar DIN : (7 thread, metric M 25×2) - these are similar to 5 thread DIN fittings but are rated to 300 bar working pressures.

The new European Norm EN 144-3:2003 introduced a new type of valve, similar to existing 232 bar or 300 bar DIN valves, however, with a metric M 26×2 fitting on both the cylinder and the regulator. These are to be used for breathing gas with oxygen content above that normally found in natural air in the Earth's atmosphere (i.e., 22% –100%). From August 2008, these shall be required for all diving equipment used with Nitrox or pure oxygen. The idea behind this new standard is to prevent a rich mixture being filled in a cylinder which is not oxygen clean. However there is nothing except human procedures to ensure that a cylinder with a new valve remains oxygen clean - which exactly what the current system works. It is an unfortunate sympton of madness from the EU.

Purposes of diving cylinders
Divers often carry more than one cylinder. Each cylinder has a different purpose.

Divers doing recreational diving often have a:
 * primary breathing source - the cylinder intended for most of the dive
 * bail out or bale out - a cylinder used purely as an independent safety reserve
 * pony - a small bale out

Divers doing technical diving often carry different gases, each in a separate cylinder, for each phase of the dive:
 * travel gas - a cylinder holding gas for use during the descent - typically a nitrox with a medium oxygen partial pressure
 * bottom gas - a cylinder holding gas for use at depth - typically a helium-based gas with a low oxygen partial pressure
 * stage - a cylinder holding gas for use at the decompression stop - typically nitrox with a high oxygen partial pressure

Rebreathers also use internal cylinders:
 * oxygen rebreathers have an oxygen cylinder
 * semi-closed circuit rebreathers have a "diluent" cylinder, which often contains air, nitrox or a helium based gas
 * closed circuit rebreathers have an oxygen cylinder and a "diluent" cylinder, which often contains air, nitrox or a helium based gas

Breathing capacity
A commonly asked question is 'what is the underwater duration of a particular cylinder?'

There are two parts to this answer:

1. What is the cylinder's capacity to store gas?

Two features of the cylinder determine its gas carrying capacity:


 * working gas pressure : this normally ranges between 200 bar/3000 psi and 300 bar/4500 psi


 * internal volume : this normally ranges between 3 litres and 18 litres

So, a 3 litre, 300 bar cylinder can carry up to 900 litres (33 ft&sup3;) of gas.

2. How much gas does the diver consume?

There are two factors at work here:


 * The breathing rate or respiratory minute volume (RMV), in liters per minute (lpm), of the diver. In normal conditions this will be between 10 and 25 lpm. At times of high work rate or panic, breathing rates can rise to 100 lpm.


 * The ambient pressure: the depth of the dive determines this. The ambient pressure at the surface is 1 bar / 14 psi. For every 10 metres/33 feet the diver descends the pressure increases by 1 bar / 14 psi.

So, a diver with a breathing rate of 20 lpm at 30 meters (4 bar) consumes 80 lpm. If this diver only had the 3 litre 300 bar cylinder to breathe from, the gas in the cylinder would be exhausted after a little over 11 minutes.

Reserves
It is strongly recommended that a portion of the usable gas of the cylinder be held aside as a safety reserve. The reserve is designed to provide gas for longer than planned decompression stops or to provide time to resolve underwater emergencies.

The size of the reserve depends upon the risks involved during the dive. A deep or decompression dive warrants a greater reserve than a shallow or a no stop dive. In recreational diving for example, it is recommended that the diver plans to surface with a reserve remaining in the cylinder of 500 psi, 50 bar or 25% of the initial capacity, depending of the teaching of the diver training organisation. On technical dives, such as penetration diving or deep diving, divers plan larger margins of safety using the Rule of Thirds: one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.

Some training agencies teach the concept of minimum gas and provide a simple calculation that allows a diver to work out an acceptable reserve to get two divers in an emergency to the surface. See DIR diving for more information.

Configuring cylinders
For safety, divers often carry an additional redundant aqualung (a second scuba tank and scuba valve) to mitigate out-of-air emergencies should the primary breathing source fail.

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


 * Single cylinder or single aqualung : consists of a single large cylinder with one regulator. This configuration is simple and cheap but it is only a single system: it has no redundancy in case of failure. If the cylinder or scuba regulator fails, the diver is totally out of air and faces an emergency. All training agencies train divers to rely on a buddy to assist them in this situation. The skill of gas sharing is required at the most basic scuba course. This equipment configuration, although common with entry-level divers, is not recommended for any dive that is deeper than 30m or where decompression stops may be needed or where there is an overhead environment (wreck diving, cave diving, ice diving) preventing the diver from performing an emergency ascent.


 * Main cylinder plus a small independent cylinder: this configuration uses a larger, main cylinder along with an independent smaller cylinder often called a "pony". The diver has two independent systems, but the total 'breathing system' is now heavier, more expensive to buy and maintain.
 * The pony is tyically a 2 to 5 litre cylinder. Its capacity determines the depth of dive and decompression duration for which it provides protection. Ponies are generally fixed to the diver's buoyancy compensator (BC) or main cylinder behind the diver's back. They can also be clipped to the BC at the diver's side or chest. Ponies provided an acceptable emergency supply but are only useful if the diver trains to bail out i.e. to use one.
 * Another type of separate small air source is a micro-aqualung: a hand-held 0.5 litre cylinder with a diving regulator directly attached. This source provides a few breaths of gas and is suitable as a shallow water bailout, say from a maximum of 10 metres / 33 feet.




 * Independent twin set/doubles: this consists of two independent cylinders and two regulators. This system is heavier, more expensive to buy and maintain and more expensive to fill. Also the diver must swap demand valves during dive to preserve a safety reserve of air in each cylinder. If this is not done then should a cylinder fail the diver may end up having no reserve. Independent twin sets do not work well with air-integrated computers - as they usually only monitor one tank. Many divers feel the complexity of switching regulators periodically to ensure both cylinders are evenly used is offset by the redundancy of two entirely separate breathing supplies.


 * Manifolded twin set/doubles with a single regulator: two cylinders are joined at their pillar valves with a manifold but only one regulator is attached to the system. This makes it simple and cheap but means there is no redundant breathing system, only a double gas supply.


 * Manifolded twin set/doubles with two regulators: consist of two cylinders with their pillar valves joined with a manifold with a valve that can isolate the two pillar valves. In the event of a problem with one cylinder the diver may close the isolator valve to preserve gas in the cylinder which has not failed. The pros of this configuration are you have a large gas supply, there is no need to change regulators underwater, management of gas supply is automatic and if most failure situations the diver may close a failed valve or isolate to leave himself with an emergency supply. The cons of this solution is that there is a danger of losing all air if the manifold valve cannot be closed when a problem occurs and the manifold is another potential point of failure. This type of cylinder is often used in Technical diving.


 * Stage bottles/cylinders: are a type of independent cylinder used for technical diving. They are independent cylinders with their own regulators.  Their primary purpose is not to provide redundant gas supply, but rather to carry either "stage", "travel" or "decompression" breathing gas while the main cylinder carries "bottom" gas.

Closed-circuit
Diving cylinders are used in closed-circuit diving in two roles:


 * As part of the rebreather. 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. See the main article: rebreather.


 * Bail out: rebreather divers often carry one or more redundant gas sources should the rebreather fail:
 * Open-circuit: The number of open-circuit bail outs, their capacity and the breathing gases they contain depend on the depth and decompression needs of the dive. So on a deep, technical rebreather dive, the diver will need a bail out "bottom" gas and a bail out "decompression" gas for use. On such a dive, it is the capacity and duration of the bail out that limits the depth and duration of the dive - not the capacity of the rebreather.


 * Closed-circuit: using a rebreather are a bail out is possible but uncommon. Although the long duration of rebreathers seems compelling for a 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.

Filling tanks
Tanks should only be filled with air from diving air compressors or with other breathing gases using gas blending techniques. Both these services should be provided by reliable suppliers such as dive shops. Breathing industrial compressed gases can be lethal because the high pressure increases the effect of any impurities in them.

Special precautions need to be taken with gases other than air:


 * oxygen in high concentrations is a major cause of fire and rust.
 * oxygen should be very carefully transferred from one tank to another and only ever stored in tanks that are certified and labeled for oxygen use.
 * gas mixtures containing proportions of oxygen other than 21% could be extremely dangerous to divers who are unaware of the proportion of oxygen in them. All cylinders should be labeled with their composition.

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

The blast caused by a sudden release of the gas pressure inside a diving cylinder makes them very dangerous if mismanaged. The greatest risk of explosion exists at filling time and comes from thinning of the walls of the pressure vessel due to corrosion. Another cause of failure is damage or corrosion of the threads and neck of the cylinder where the pillar valve is screwed in.

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

Manufacture and testing
Most countries require tanks to be checked on a regular basis. This usually consists of an internal visual inspection and a hydrostatic test. In the United States, a visual inspection is required every year, and a hydrostatic every five years. In European Union countries a visual inspection is required every 2.5 years, and a hydrostatic every five years. In Norway a hydrostatic (including an visual inspection) is required 3 years after production date, then every 2 years.

Legislation in Australia requires that cylinders are hydrostatically tested every twelve months regardless.

A hydrostatic test involves pressurising the cylinder to its test pressure and measuring its volume before and after the test. A permanent increase in volume above the tolerated level means the cylinder fails the test and should be destroyed.

When a cylinder in manufactured, its specification, including Working Pressure, Test Pressure, Data of Manufacture, Capacity and Weight are stamped on the cylinder.

On testing, the test date, or the test expiry date in some countries such as Germany, is punched into the neck of the tank for easy verification at fill time.

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

Gas cylinder colour coding
In the European Union gas cylinders are beginning to be colour coded according to EN 1098-3. The "shoulder" is the top of the cylinder close to the pillar valve.


 * Air has a white and black quartered shoulder.
 * Heliox has a white and brown quartered shoulder.
 * Nitrox has a white and black quartered shoulder.
 * Pure Oxygen has a white shoulder.
 * Pure Helium has a brown shoulder.
 * Trimix has a white, black and brown segmented shoulder.

Worldwide, in many recreational diving settings where air and nitrox are the widely used gases, nitrox cylinders are colour-coded with a green stripe on yellow bottom. Air scuba cylinders are often all yellow, so they can be seen easily underwater, although in some industrial cylinder identification colour tables yellow means chlorine.

Cylinder labelling


In the European Union breathing gas cylinders must be labelled with their contents. The label should state the type of breathing gas contained by the cylinder.

Cylinders that are subject to gas blending with pure oxygen also need an "oxygen service certificate" label indicating they have been prepared for use in an oxygen-rich environment.

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