A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator.
Diving cylinders typically have an internal volume of between 3 and 18 litres (0.11 and 0.64 cu ft) and a maximum pressure rating from 200 to 300 bars (2,900 to 4,400 psi). 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. 600 litres (21 cu ft) of gas at atmospheric pressure is compressed into a 3-litre cylinder when it is filled to 200 bar. 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.
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. They are also used for many purposes not connected to diving. For these applications they are not diving cylinders.
The term "diving cylinder" tends to be used by gas equipment engineers, manufacturers, support professionals, and divers speaking British English. "Scuba tank" or "diving tank" is more often used colloquially by non-professionals and native speakers of American English. The term "oxygen tank" is commonly used by non-divers when referring to diving cylinders; however, this is a misnomer. These cylinders typically contain (atmospheric) breathing air, or an oxygen-enriched air mix. They rarely contain pure oxygen, except when used for rebreather diving, shallow decompression stops in technical diving or for in-water oxygen recompression therapy. Breathing oxygen at depths greater than 20 feet (6.1 m)(equivalent to a partial pressure of oxygen of 1.6 ATA) can result in oxygen toxicity, a highly dangerous condition that can trigger seizures and thus lead to drowning.
Nitrox refers to any gas mixture composed (excluding trace gases) of nitrogen and oxygen; this includes normal air which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon.
However, in scuba diving, nitrox is normally differentiated and handled differently from air. The most common use of nitrox mixtures containing higher than normal levels of oxygen is in scuba, where the reduced percentage of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues and so extending the possible dive time, and/or reducing the risk of decompression sickness (also known as the bends).
Enriched Air Nitrox, nitrox with an oxygen content above 21%, is mainly used in scuba diving to reduce the proportion of nitrogen in the breathing gas mixture. Reducing the proportion of nitrogen by increasing the proportion of oxygen reduces the risk of decompression sickness for the same dive profile, or allows extended dive times without increasing the need for decompression stops for the same risk. Nitrox is not a safer gas than compressed air in all respects; although its use can reduce the risk of decompression sickness, it increases the risk of oxygen toxicity and fire, which are further discussed below.
Breathing nitrox is not thought to reduce the effects of narcosis, as oxygen seems to have equally narcotic properties under pressure as nitrogen; thus one should not expect a reduction in narcotic effects due only to the use of nitrox. Nonetheless, there are people in the diving community who insist that they feel reduced narcotic effects at depths breathing nitrox. This may be due to a dissociation of the subjective and behavioral effects of narcosis. However, it should be noted that because of risks associated with oxygen toxicity, divers tend not to utilize nitrox at greater depths where more pronounced narcosis symptoms are more likely to occur. For a reduction in narcotic effects trimix or heliox, gases which also contain helium, are generally used by divers.
There is anecdotal evidence that the use of nitrox reduces post-dive fatigue, particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue. There was, however, some suggestion that post dive fatigue is due to sub-clinical decompression sickness (DCS) (i.e. micro bubbles in the blood insufficient to cause symptoms of DCS); the fact that the study mentioned was conducted in a dry chamber with an ideal decompression profile may have been sufficient to reduce sub-clinical DCS and prevent fatigue in both nitrox and air divers. In 2008, a study was published using wet divers at the same depth and confirmed that no statistically significant reduction in reported fatigue is seen.
Further studies with a number of different dive profiles, and also different levels of exertion, would be necessary to fully investigate this issue. For example, there is much better scientific evidence that breathing high-oxygen gases increase exercise tolerance, during aerobic exertion. Though even moderate exertion while breathing from the regulator is a relatively uncommon occurrence in scuba, as divers usually try to minimize it in order to conserve gas, episodes of exertion while regulator-breathing do occasionally occur in sport diving. Examples are surface-swimming a distance to a boat or beach after surfacing, where residual "safety" cylinder gas is often used freely, since the remainder will be wasted anyway when the dive is completed. It is possible that these so-far un-studied situations have contributed to some of the positive reputation of nitrox.