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12 litre and 3 litre steel diving cylinders
12 litre and 3 litre steel diving cylinders

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). Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Air is the most common and only natural breathing gas. Other artificial gases either pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such It provides gas to the SCUBA diver through the demand valve of a diving regulator. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. A diving regulator is a Pressure regulator used in a Scuba set that supplies the diver with breathing gas

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 bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. 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. A Diving Air Compressor is a Gas compressor that can fill Diving cylinders with high- Pressure Air pure enough to be used as a Breathing gas 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. Surface Marker Buoy (standard A Surface Marker Buoy, SMB or simply a blob is an inflatable Buoy used by SCUBA divers, A dry suit or drysuit provides Thermal insulation or passive thermal protection to the wearer while immersed in water and is worn by divers,

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. A gas cylinder or tank is a Pressure vessel used to store Gases at high Pressure. Oxygen therapy is the administration of Oxygen as a therapeutic modality Divers face specific physical and Health Risks when they go Underwater (e A Diving Air Compressor is a Gas compressor that can fill Diving cylinders with high- Pressure Air pure enough to be used as a Breathing gas 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. British English or UK English ( BrE, BE, en-GB) is the broad term used to distinguish the forms of the English language used in the "Scuba tank" or "diving tank" is more often used colloquially by non-professionals and native speakers of American English. Phonology North American English regional phonology In many ways compared to English English, North American English is conservative in its Phonology. The term "oxygen tank" is commonly used by non-divers when referring to diving cylinders. An oxygen tank is a storage vessel for Oxygen, which is either held under Pressure in Gas cylinders or as Liquid oxygen in a Cryogenic This is a misnomer. These cylinders typically contain (atmospheric) breathing air, or an oxygen-enriched air mix. Nitrox refers to any gas mixture composed (excluding trace gases of nitrogen and oxygen this includes normal Air which is approximately 78% Nitrogen They rarely contain pure oxygen, except when used for rebreather diving, decompression in technical diving or for oxygen therapy. A rebreather is a type of Breathing set that provides a Breathing gas containing Oxygen and recycled exhaled gas Technical diving (sometimes referred to a Tec diving) is a form of Scuba diving that exceeds the scope of Recreational diving (although the vast majority Oxygen therapy is the administration of Oxygen as a therapeutic modality

Contents

Parts of a cylinder

A 15 litre, 232 bar cylinder with "A clamp" type pillar valve
A 15 litre, 232 bar cylinder with "A clamp" type pillar valve
A 12 litre, 232 bar cylinder with DIN type pillar valve
A 12 litre, 232 bar cylinder with DIN type pillar valve

The diving cylinder consists of several parts:

Types of pillar valve

A 232 DIN type pillar valve
A 232 DIN type pillar valve

There are three types of pillar valve:

A yoke (A-clamp) to DIN adaptor
A yoke (A-clamp) to DIN adaptor

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. European Committee for Standardization or Comité Européen de Normalisation ( CEN) is a private non-profit organisation whose mission is to foster the European economy These are to be used for breathing gas with oxygen content above that normally found in natural air in the Earth's atmosphere (i. Air is the most common and only natural breathing gas. Other artificial gases either pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five e. , 22% –100%). From August 2008, these shall be required for all diving equipment used with Nitrox or pure oxygen. Nitrox refers to any gas mixture composed (excluding trace gases of nitrogen and oxygen this includes normal Air which is approximately 78% Nitrogen The idea behind this new standard is to prevent a rich mixture being filled to a cylinder, which is not oxygen clean. An oxygen tank is a storage vessel for Oxygen, which is either held under Pressure in Gas cylinders or as Liquid oxygen in a Cryogenic 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. Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment A decompression stop is a period of time a diver must spend at a constant depth in shallow water at the end of a dive to safely eliminate absorbed Inert gases from 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. Wreck diving is a type of Recreational diving where Shipwrecks are explored

Each cylinder may have a different purpose:

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

Rebreathers also use internal cylinders:

Breathing capacity

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

There are two parts to this answer:

1. A rebreather is a type of Breathing set that provides a Breathing gas containing Oxygen and recycled exhaled gas What is the cylinder's capacity to store gas?

Two features of the cylinder determine its gas carrying capacity:

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. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically 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. The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834 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:

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. Technical diving (sometimes referred to a Tec diving) is a form of Scuba diving that exceeds the scope of Recreational diving (although the vast majority A rebreather is a type of Breathing set that provides a Breathing gas containing Oxygen and recycled exhaled gas

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. These four properties that constitute an ideal gas can be easily remembered by the acronym RIPE which stands for - R andom Motion (molecules are in constant random motion

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. Standard gravity, usually denoted by g 0 or g n is the nominal acceleration due to gravity at the Earth's surface at sea level 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. A decompression stop is a period of time a diver must spend at a constant depth in shallow water at the end of a dive to safely eliminate absorbed Inert gases from

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. Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment This page lists SCUBA Diver training organizations Commercial diving: Surface supplied air training organizations International 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. In Scuba diving, the rule of thirds is a Rule of thumb that divers use to plan dives so they do not consume all the Breathing gas from the Diving

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

15 litre, 232 bar, A clamp single cylinder open circuit breathing set
15 litre, 232 bar, A clamp single cylinder open circuit breathing set
7 litre, 232 bar, DIN pillar valve independent twin set. The left cylinder shows manufacturer markings. The right cylinder shows test stamps
7 litre, 232 bar, DIN pillar valve independent twin set. The left cylinder shows manufacturer markings. The right cylinder shows test stamps
Manifolded twin 12 litre, 232 bar breathing set with two A-clamp pillar valves and two regulators
Manifolded twin 12 litre, 232 bar breathing set with two A-clamp pillar valves and two regulators
Two 3 litre, 232 bar, DIN cylinders inside a Inspiration Diving Rebreather closed circuit breathing set
Two 3 litre, 232 bar, DIN cylinders inside a Inspiration Diving Rebreather closed circuit breathing set

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. Aqualung was the original name for the first open-circuit scuba diving equipment, developed by Emile Gagnan and Jacques Cousteau in 1943 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:

Closed-circuit

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

Filling tanks

Tanks should only be filled with air from diving air compressors or with other breathing gases using gas blending techniques. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five A Diving Air Compressor is a Gas compressor that can fill Diving cylinders with high- Pressure Air pure enough to be used as a Breathing gas Air is the most common and only natural breathing gas. Other artificial gases either pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such Gas blending or gas mixing is the filling of Diving cylinders with non- Air Breathing gases such as Nitrox, Trimix and 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:

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. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single

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. A gas cylinder or tank is a Pressure vessel used to store Gases at high Pressure. This usually consists of an internal visual inspection and a hydrostatic test. A hydrostatic test is the common way in which leaks can be found in pressure vessels such as pipelines and Plumbing. 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. The United States of America —commonly referred to as the In European Union countries a visual inspection is required every 2. The European Union ( EU) is a political and economic union of twenty-seven member states, located primarily in 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. Norway ( Norwegian: Norge ( Bokmål) or Noreg ( Nynorsk) officially the Kingdom of Norway, is a Constitutional

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. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. 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 European Union ( EU) is a political and economic union of twenty-seven member states, located primarily in The "shoulder" is the top of the cylinder close to the pillar valve. For mixed gases, the colours can be either bands or "quarters".

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. Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment 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. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings 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. Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and Toxicity is the degree to which a substance is able to damage an exposed organism

Cylinder labeling

A contents label for oxygen usage
A contents label for oxygen usage

In the European Union breathing gas cylinders must be labeled with their contents. The European Union ( EU) is a political and economic union of twenty-seven member states, located primarily in The label should state the type of breathing gas contained by the cylinder. Air is the most common and only natural breathing gas. Other artificial gases either pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such

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. Gas blending or gas mixing is the filling of Diving cylinders with non- Air Breathing gases such as Nitrox, Trimix and Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

References

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