The compressed air cylinder

Compressed air cylinders (popularly called high-pressure oxygen cylinders) are relatively small but very powerful cylinders that can hold large volumes of compressed gas. The fact that you can take your own gas/air supply with you during a dive is called SCUBA (Seleven Contained Underwater Breathing Apparatus). Hence the English term Scuba diving. The cylinder can be filled with compressed air or with enriched air nitrox (more oxygen and a little less nitrogen) or trimix (a mixture of three different gases) for technical diving.

The air cylinder used by recreational divers today was first developed by Jacques Cousteau. The air cylinder is in fact no more than a metal cylinder with an opening to which a tap has been attached. However simple it may look, the manufacturing of such a cylinder is a complex and well controlled process. The air cylinder is one of the most important pieces of diving equipment. Visityour IDD dive center or resort for advice on air cylinders.

Selecting the compressed air cylinder

Two things to consider when choosing a compressed air cylinder are, the capacity of the cylinder and the material the cylinder is made of. Compressed air cylinders are made of aluminum, steel or carbon. Each material has advantages and disadvantages. Steel cylinders are more solid, but more susceptible to oxidation than aluminium cylinders. Rust can form on or in the steel cylinder while an aluminium cylinder can form aluminium oxide. In the Netherlands, steel cylinders are usually used. Carbon cylinders are remarkably light because of their construction: a thin aluminum cylinder wrapped with carbon fiber. It's very light to take to the filling station. But for divers, this is not the only advantage: they have to take extra lead with them to compensate. Technical divers sometimes use them because they carry more than one cylinder. Firefighters who use cylinders above water also prefer carbon-wrapped cylinders. The most common cylinders have a water capacity of 7, 10, 12, 15 or 18 litres.

The requirements for a compressed air cylinder

A compressed air cylinder must be made of steel, aluminium or carbon fibre. In the Netherlands, the following information must be stamped on the air cylinder:

  • The name of the manufacturer or importer
  • The filling pressure (200/230/300 bar) and the test pressure (hydrostatic - 300/345/450) of the cylinder
  • The inspection date for the country approved hallmark (in the Netherlands, depiction of the Dutch lion) or European hallmark
  • The weight of the cylinder
  • The water content measured in litres
  • The type of gas the cylinder is used for

Using the compressed air cylinder

The most important thing when using a compressed air cylinder is to ensure that no moisture can enter it. The compressed air cylinder is filled with dry, clean air under high pressure. Since this air contains oxygen, any moisture that enters will cause the inside of the air cylinder to rust.

The best way to prevent moisture from entering is to never empty the compressed air cylinder completely. If for any reason an air cylinder does become empty, close the valve immediately to prevent moisture from entering the air cylinder. To ensure that there is no moisture in the air cylinder, have the air cylinder visually inspected once a year.

Diving shops have tools that can be used to remove the valve of the air cylinder and to inspect the inside of the cylinder with a special lamp. The diving shop will then use a special lamp to inspect the inside of the air cylinder to see whether there is any moisture or rust. In the mean time you can also check the cylinder yourself by looking, smelling or listening for water in the cylinder.

You can check this in the following way:

  • If the air leaving the compressed air cylinder forms a white vapour, this may indicate the presence of moisture in the compressed air cylinder, as dry air is invisible.
  • By smelling the air coming from the air cylinder you can determine whether there is oil or another substance in the air cylinder; pure, dry air is odourless.
  • By moving the air cylinder back and forth you can hear whether there is water or any other substance in the air cylinder (e.g. rust).
  • If you suspect moisture in the compressed air cylinder, have the cylinder visually inspected as soon as possible. 

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