Definitions

Diving_cylinder

Diving cylinder

A diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing gas as a component of SCUBA (Self-Contained Underwater Breathing Apparatus). It provides gas to the SCUBA diver through the demand valve of a diving regulator.

Diving cylinders typically have an internal volume of between 3 and 18 litres and a maximum pressure rating of 200 bar to 300 bar, (about 3000 psi to 4500 psi). The internal cylinder volume is also expressed as "water capacity" - the volume of water which could be contained by the cylinder. When pressurised, a cylinder carries a volume of gas greater than its water capacity because gas is compressible. 696 (3 x 232) litres (25ft³) of gas at atmospheric pressure can be compressed into a 3-litre cylinder filled to 232 bar. Cylinders also come in smaller sizes, such as 0.2, 1.5 and 2 litres, however these are not generally used for breathing, instead being used for purposes such as Surface Marker Buoy, drysuit and buoyancy compensator inflation.

Divers use gas cylinders 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. They are also used for many purposes not connected to diving.

The term "diving cylinder" tends 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 when referring to diving cylinders. This is a misnomer. These cylinders typically contain (atmospheric) breathing air, or an oxygen-enriched air mix. They rarely contain pure oxygen, except when used for rebreather diving, decompression in technical diving or for oxygen therapy.

Parts of a cylinder

The diving cylinder consists of several parts:

  • the pressure vessel is normally made of cold-extruded aluminium or forged steel. An especially common cylinder available at tropical dive resorts is an "aluminium-80" which is an aluminium cylinder of 0.4 cubic feet rated to hold (about) 80 ft³ of 14.7 psi gas at its rated pressure of 3000 psi (in metric units, its internal capacity is approximately 11 litres, to be pressurized to about 200 bar). Aluminium cylinders are also used where divers carry many cylinders, such as in technical diving, because the greater buoyancy of aluminium cylinders reduces the extra buoyancy the diver would need to achieve neutral buoyancy. In cold water diving, where a diver 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. Kevlar wrapped composite cylinders are used in fire fighting breathing apparatus and oxygen first aid equipment, but are rarely used for diving, due to their high positive 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 countries require 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 pressurization.
  • a rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Halocarbon (i.e. "viton") 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 or "J-valve" (obsolete). 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 when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled a lever and finished the dive before the reserve (typically 500 psi) 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. The J-valve got its name from being item number 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.

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, G5/8) - 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, or cylinders which have DIN fittings, so a European diver with a DIN system abroad in many places will need to take an adaptor.
  • 300 bar DIN : (7-thread, G5/8) - these are similar to 5-thread DIN fitting but are rated to 300 bar working pressures. The 300 bar pressures are common in European diving and in US cave diving, but their acceptance in U.S. sport diving has been hampered by the fact that United States Department of Transportation rules presently prohibit the transport of metal scuba cylinders on public roads with pressures above about 230 bar, even if the cylinders and air delivery systems have been rated for these pressures by the American agencies which oversee cylinder testing and equipment compatibility for SCUBA (OSHA and CGA). Note that reference to M25 threads refers to the tank neck thread not the valve size.

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 to a cylinder, which 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 - which is exactly how the current system works.

Purposes of diving cylinders

Divers may carry one cylinder or multiples, depending on the requirements of the dive. In parts of the world where diving takes place in warm water and in good visibility, recreational divers usually carry only one cylinder. An example of this type is coral reef diving where it is possible to do an interesting dive without going deep or needing long decompression. Where diving risks are higher, for example in parts of the world where the water is cold and visibility is low or when recreational divers do deeper or decompression diving, divers routinely carry more than one gas source. An example of this type is north European diving where the temperature is often less than 15°C/60°F and visibility less than 10m/33ft and many interesting dive sites are shipwrecks in deeper water on the sea bed.

Each cylinder may have a different purpose:

  • 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 (bottle) - a small bail out.

Divers doing technical diving often carry different gases, each in a separate cylinder, for each phase of the dive:

  • travel gas - gas for use during the descent and ascent - typically air or nitrox with a medium oxygen content 21-40%.
  • bottom gas - gas for use at depth - typically a helium-based gas with a low oxygen content below 21%.
  • deco gas - gas for use at the decompression stop - nitrox with a high oxygen content - typically 80-100%.
  • stage - another way of referring to an additional cylinder holding travel or deco gas.

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/4400 psi
  • internal volume : this normally ranges between 3 litres and 18 litres

To calculate the quantity of gas:

 Volume of gas at atmospheric pressure = (cylinder volume) x (cylinder pressure) / (atmospheric pressure)

So a 12 litre cylinder at 232 bar would hold almost 2784 litres (98 ft³) of air at atmospheric pressure. In the US you might find a cylinder with an internal capacity of 0.4 ft³ filled to 3000 psi; Taking air pressure as 15 psi, this gives 0.4 x 3000 / 15 = 80 ft³ (although it would be described as an "80 cubic foot cylinder", as the US normally refers to cylinder capacity as free-air equivalent at its working pressure, rather than the internal volume of the cylinder commonly used in metric countries).

Up to 200 bar the ideal gas law remains valid and the relationship between the pressure, size of the cylinder and gas contained in the cylinder is linear; at higher pressures there is proportionally less gas in the cylinder. A 3 litre, 300 bar cylinder can only carry up to 810 litres (28.6 ft³) of atmospheric pressure gas and not the 900 litres expected from the ideal gas law.

2. How much gas does the diver consume?

There are three factors at work here:

  • breathing rate or respiratory minute volume (RMV), in litres 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.
  • time
  • ambient pressure: the depth of the dive determines this. The ambient pressure at the surface is 1 bar / 14.7 psi. For every 10 metres/33 feet in salt water the diver descends, the pressure increases by 1 bar / 14.7 psi. As a diver goes deeper, the diver's lungs are compressed and this must be offset by breathing gas at a pressure equal to ambient water pressure. Thus, it requires twice as much mass of gas to fill the same volume (the diver's lungs) at 10 metres/33 feet as it does at the surface. This equation repeats itself with each bar of pressure. If a given cylinder consumed at a constant rate would last a diver one hour at the surface, it would last thirty minutes at 10 metres/33 feet, 20 minutes at 20 metres/66 feet and just 15 minutes at 30 metres/99 feet.

To calculate the quantity of gas consumed:

 gas consumed = breathing rate x time x ambient pressure

Thus, a diver with a breathing rate of 20 lpm will consume at 30 meters (4 bar) the equivalent of 80 lpm at 1 bar (80 lpm at the surface). If this diver only had a 10 litre 200 bar cylinder to breathe from, the gas in the cylinder would be exhausted after a little over 2000/80 = about 25 minutes.

Keeping this in mind, it is not hard to see why technical divers who do long deep dives require multiple cylinders or rebreathers.

Breathing Time

For Metric users:

Absolute maximum breathing time (BT) can be calculated as

BT = available air / rate of consumption

which, using the ideal gas law, is

BT = (available cylinder pressure * cylinder volume) / (rate of air consumption at surface) * (ambient pressure)

This may be written as

(1) BT = frac {(CP-AP)*CS} {BR*AP}

with

BT = Breathing Time (in minutes)
CP = Cylinder Pressure (in bars)
CS = Cylinder Size (in liters)
AP = Ambient Pressure (in bars)
BR = Breathing Rate (in liters per minute)

AP is deducted from CP, as the quantity of air represented by AP can in practice not be used for breathing by the diver as she needs it to overcome the pressure of the water (AP) when inhaling.

However, in normal diving usage, a reserve is always factored in. The reserve is a proportion of the cylinder pressure which a diver will not expect to use other than in case of emergency. The reserve may be a quarter or a third of the cylinder pressure or it may be a fixed pressure, common examples are 50 bar and 500 psi. The formula above is then modified to give the usable breathing time as

(2) BT = frac {(CP-RP)*CS} {BR*AP}

where RP is the reserve pressure.

Ambient pressure (AP) is the surrounding water pressure at a given depth and is made up of the sum of the water pressure and the air pressure at the surface. It is calculated as

(3) AP = frac {D*g*rho} {100000} + atmospheric pressure

with

D = Depth (in meters)
g = Standard gravity (in meters per second squared)
ρ = Water Density (in kg per cube meter)

In practical terms, this formula can be approximated by

(4) AP = frac {D} {10} + 1

For example (using the first formula (1) for absolute maximum breathing time), a diver at a depth of 15 meters in water with an average density of 1020 kg / m³ (typical salt water), who breathes at a rate of 20 liters per minute, using a dive cylinder of 18 liters pressurized at 200 bars, can breathe for a period of 72 minutes before the cylinder and supply line pressure has fallen so low as to prevent her from inhaling. In most open circuit scuba systems this happens quite suddenly, from a normal breath to the next abnormal breath, a breath which typically cannot be fully drawn. (There is never any difficulty exhaling). In such circumstances there remains air under pressure in the cylinder, but the diver is unable to breathe it. Some of it can be breathed if the diver ascends, and even without ascent, in some systems a bit of air from the cylinder is available to inflate BCD devices even after it no longer has pressure enough to actuate the mouthpiece valve.

Using the same conditions and a reserve of 50 bar, the formula (2) for usable breathing time is worked thus:

Ambient pressure = water pressure + atmospheric pressure = 15/10 + 1 = 2.5 bar
Usable air = usable pressure * cylinder capacity = (200-50) * 18 = 2700 liters
Rate of consumption = surface air consumption * ambient pressure = 20 * 2.5 = 50 liters/min
Usable breathing time = 2700 liters / 50 liters/min = 54 min

This would give a dive time of 54 min at 15 m before reaching the reserve of 50 bar.

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. This is because recreational divers practicing within "no-decompression" limits can normally make a direct ascent in an emergency. On technical dives where a direct ascent is either impossible (due to overhead obstructions) or dangerous (due to the requirement to make decompression stops), 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 sometimes carry an additional redundant aqualung (a second scuba tank and scuba valve) to mitigate out-of-air emergencies should the primary breathing source fail. For most common recreational diving (for example dives of 20 m to examine typical coral reefs) such extra equipment is usually not needed or used.

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 first-stage regulator, and usually two secondary regulator/mouthpieces. 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 first-stage 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 and for most sport diving, is not recommended for any dive that is deeper than 30 m or where decompression stops are needed, or where there is an overhead environment (wreck diving, cave diving, or ice diving). Generally, these conditions, because they prevent immediate emergency ascent, define technical diving.
  • 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 typically 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 in most failure situations, the diver may close a failed valve or isolate a cylinder, 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 configuration of cylinders 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 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. See the main article: rebreather.
  • In a bail out system: rebreather divers often carry one or more redundant gas sources should the rebreather fail:
    • Open-circuit: a simple diving cylinder and regulator. 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: a rebreather containing a diving cylinder and regulator. Using another rebreather as 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 can 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. Aluminium cylinders have been observed occasionally to fail explosively, fragmenting the cylinder wall. Steel cylinders usually remain mostly intact, and tend to fail at the neck.

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, see gas cylinder. This usually consists of an internal visual inspection and a hydrostatic test. In the United States, a visual inspection is NOT required every year (This is an industrial standard that is not DOT required), 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 a 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 is 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. Note: this is a European requirement.

Most compressor operators check these details before filling the cylinder and may refuse to fill non-standard or out-of-test cylinders. Note: this is a European requirement and a requirement of the USA DOT.

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. For mixed gases, the colours can be either bands or "quarters".

  • Air has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) "quartered" shoulders.
  • Heliox has either a white (RAL 9010) top and brown (RAL 8008) band on the shoulder, or white (RAL 9010) and brown (RAL 8008) "quartered" shoulders.
  • Nitrox, like Air, has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) "quartered" shoulders.
  • Pure oxygen has a white shoulder (RAL 9010).
  • Pure helium has a brown shoulder (RAL 9008).
  • Trimix has a white, black and brown segmented shoulder.

Note: As of the end of 2006, the quartered parts is obsolete, and new cylinders are now with the band, and the old system is repainted.

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. The normal colour of aluminium diving cylinders is their natural silver. Steel diving cylinders are often painted, to reduce corrosion, mainly yellow or white to increase visibility. In some industrial cylinder identification colour tables, yellow shoulders means chlorine and more generally within Europe it refers to cylinders with Toxic and/or Corrosive contents; but this is of no significance in SCUBA since gas fittings would not be compatible.

Cylinder labeling

In the European Union breathing gas cylinders must be labeled 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.

References

  • CEN. EN 1089-2:2002 Transportable gas Cylinders, Part 2 - Precautionary labels Superseded by EN ISO 7225:2007.
  • CEN. EN 1089-3:2004 Transportable gas Cylinders, Part 3 - Colour coding Current standard.

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