A cylinder, scuba tank, or dive cylinder as they are often called is a pressure vessel containing air under extreme pressure and in most recreational diving cylinders that’s all there is in it.
The air we breathe is made up of 79% nitrogen and 21% oxygen. An enriched air (nitrox) cylinder will contain just that: air, enriched with more oxygen, normally somewhere between 32% and 40% depending on the dive plan. This extra oxygen decreases the nitrogen content of the gas in the cylinder.
A recreation dive cylinder is not an “oxygen tank”. Pure oxygen, if breathed below 10 metres, can result in oxygen toxicity: this can kill you.
The most commonly used dive cylinder is a 12 litre and it has a working pressure of 232 bar (you have around two bar in your car tyres). Pumped to 200 bar, a 12 litre cylinder will have 2400 litres of air inside it (12 litres multipled by 200). In this part of the world (South Africa) a scuba cylinder is required by law to be painted yellow with a grey neck. An enriched air or Nitrox cylinder is required to be marked with an additional green band around the top, preferably so labeled.
Reference to an “oxygen tank” in recreational diving is just that, a cylinder filled with pure oxygen, painted black and white These are primarily for medicinal use in oxygen first aid, in treating decompression sickness or other emergencies. Their other use is or for very shallow decompression stops (6 metres or less) usually done by technical or deep divers. Oxygen tanks are also used (as in the picture below) for blending of enriched air mixes for Nitrox diving.
If you want to talk about the cylinder of air that a scuba diver wears on his back while diving, the correct terminology (or the term that would be understood by most divers) is dive tank, cylinder or tin of sky (OK, that last one is not entirely serious).
