Compressed Air for Scuba Diving

Air, not oxygen

People often say divers carry "oxygen tanks". They don't. A standard scuba tank holds compressed air, the same mix you are breathing now: about 21 percent oxygen and 78 percent nitrogen, with around 1 percent argon and small traces of carbon dioxide and other gases. Breathing pure oxygen at depth would quickly become dangerous, as explained in oxygen toxicity.

How a tank is filled

Dive centres fill tanks with a high-pressure compressor. The compressor draws in outside air, squeezes it in several stages and cools it between each one. Filters then remove moisture, oil and other contaminants. The finished air is pumped into the scuba tank, usually to a working pressure of about 200 to 232 bar (3,000 to 3,400 psi), and some steel tanks to 300 bar.

Tanks warm up as they are filled and lose a little pressure as they cool. A tank filled hot may read slightly below its working pressure an hour later.

Air quality

Because divers breathe air at high pressure, small amounts of contamination matter far more than on land. Breathing-air standards, such as EN 12021 in Europe, set limits for carbon monoxide, carbon dioxide, oil and water vapour. Reputable fill stations test their air regularly.

Contaminated air is rare but serious. Carbon monoxide from a compressor intake placed near an engine exhaust is the classic cause. Warning signs include an oily taste or smell, headache, nausea and dizziness. If your air smells or tastes wrong, don't dive with that tank, and tell the dive centre.

The limits of air

Air is cheap and available almost everywhere divers go, but it has two limits at depth. The nitrogen in it causes nitrogen narcosis from around 30 m (100 ft), and it dissolves into your body, which limits how long you can stay without decompression stops. That's why most training agencies set a recreational limit of 40 m (130 ft) on air. Oxygen toxicity on air only becomes a concern far beyond that depth.

How long a tank lasts

How long a tank of air lasts depends on its size, your breathing rate and your depth. Because of Boyle's law, each breath at 20 m (66 ft) takes three times as much gas as a breath at the surface. A relaxed diver at 10 m might get an hour from a standard tank, while the same tank might last half that or less at 30 m. Divers plan to surface with a reserve, often 50 bar (500 psi), and agree a turn-around pressure before the dive.

Air versus nitrox

The main alternative to air for recreational diving is enriched air nitrox, which has more oxygen and less nitrogen. It allows longer no-decompression times at moderate depths but has a shallower maximum depth. Many divers carry air for deeper dives and nitrox for repeated dives in the 18 to 30 m range.

Frequently asked questions