Table of contents

1 Parts of a cylinder 2 Types of pillar valve 3 Purposes of cylinders 4 Breathing capacity 5 Reserves 6 Configuring cylinders 7 Filling tanks 8 Manufacture and Testing

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.

• 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 guages on regulators came into common use, often used a mechanical reseve 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.

Divers, especially those doing technical diving, often carry more than one cylinder. Each cyclinder 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 decompression gas

• working gas pressure : this normally ranges between 200 bar and 300 bar

• volume : this normally ranges between 3 litres and 18 litres

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

Reserves

It is strongly recommended that a portion of the usable gas of the cylinder is held aside as a safety reserve. In recreational SCUBA diving for example, it is recommended the the diver plans to surface with a reserve remaining in the cylinder. This should be at least 500 psi, 50 bar or 25% of the initial capacity.

On high risk dives, such as when cave diving or technical diving, divers frequently maintain 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. The reserve provides additional gas, and therefore time to resolve underwater emergencies.

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.

• 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 twinsets 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 twinsets 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.

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 requires 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 cyclinder 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.