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12 litre and 3 litre steel diving cylinders
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. 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 litres to 18 litres or a gas carrying capacity of 850 to 3600 litres (30 to 120 cubic feet).
Gas cylinders are used elsewhere in diving, above water, for oxygen first aid treatment of diving disorders and as part of storage "banks" for diving cylinder diving air compressor stations.
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 a regulator freeflows, which is a common failure mode, its valve can be shut and the cylinder breathed from the regulator connected to the other valve.
- diving cylinders until the 1970s, before contents gauges on regulators came into common use, often used a mechanical resave mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off when the gas pressure reached a low level. To release the reserve, the diver pulled a lever and finished the dive before the reserve was consumed.
An A clamp type pillar valve
Types of pillar valve
There are three types of pillar valve:
- A clamp - an A clamp (or Yoke) connection on the regulator surrounds the pillar valve and pulls the output O-ring of the pillar valve onto 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) - 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.
A 232 DIN type pillar valve
- 300 bar DIN : (7 thread) - these are similar to 5 thread DIN fittings but are rated to 300 bar working pressures.
Purposes of diving cylinders
Divers, especially those doing technical diving, often carry more than one cylinder. Each cylinder has a different purpose:
- primary breathing source - the cylinder intended for most of the dive
- bail out or bale out - a cylinder used purely as a safety reserve
- pony - a small bale out
- stage - a cylinder holding gas for use at the decompression stop
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 and 300 bar
- volume : this normally ranges between 3 litres and 18 litres
So, a 3 litre, 300 bar cylinder can carry up to 900 litres (33 cubic feet) of gas.
2. How much gas does the diver consume?
There are two factors at work here:
- The breathing rate, RMV or respiratory minute volume, in liters per minute, of the diver: in normal conditions this will be between 10 litres and 25 litres per minute. At times of high work rate or panic breathing rates can rise to 100 litres per minute (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 the diver descends the pressure increases by 1 bar.
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 breath from, the gas in the cylinder would last for just 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. 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.
The reserve is designed to set aside a significant volume gas to provide for longer than planned decompression stops or to provide time to resolve underwater emergencies.
On high risk dives, such as when cave diving or technical diving, divers frequently plan with larger margins of safety using the Rule of Thirds: one third is for the outward journey, one third is for the return journey and one third is a safety reserve.
Configuring cylinders
For safety, divers often carry an additional redundant aqualung to mitigate out-of-air emergencies. There are several options for the combined cylinder and regulator system:
- Single aqualung (no redundancy): consists of a single large cylinder with one regulator. This configuration is simple and cheap but it is only a single system. If the single aqualung fails, the diver is totally out of air and faces a desperate emergency. This configuration, although common with entry-level divers, is not recommended for any dive where decompression stops may be needed.
- Main aqualung plus pony aqualung: this configuration uses a larger, main aqualung with smaller pony aqualung. The diver has two independent systems, but the total 'breathing system' is now heavier, more expensive to buy and maintain. The pony cylinder is of low capacity and so is only provides a reserve for shallow dives.
7 litre, 232 bar, DIN independent twin set. The left cylinder shows manufacturer markings. The right cylinder shows test stamps
- Independent twin set: this consists of two independent aqualungs. Again 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 safety reserve of air in each cylinder with possibility of accidentally breathing a cylinder empty resulting in having no reserve. Independent twin sets do not work well with air-integrated computers.
- Manifolded twin set each with a regulator: consist of two aqualungs with their pillar valves joined with a manifold that has a valve that can isolate the two pillar valves. The diver keeps the valve open, by one turn only, until an air loss occurs. Closing the valve preserves a safety reserve or gas. The pros and cons of this configuration are similar to independent twin sets except, there is no need to change regulators underwater, there is a danger of losing all air if the manifold valve cannot be closed when an air loss occurs, the manifold is another potential point of failure and the manifold is expensive.
- Single regulator manifolded twin set: 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 system.
- Spare air type micro-aqualung: a hand held 0.5 litre cylinder with a diving regulator directly attached provides a few breaths of gas and is suitable as a shallow water bailout, say from a maximum of 10 metres / 33 feet.
Filling tanks
Tanks should only be filled with air or other gas mixes from reliable sources 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 salt 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.
A hydrostatic test involves pressurising the cylinder to its test pressure and measuring its volume before and after the test. An 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.
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.
de:Pressluftflasche
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