Scuba divers carry tanks filled with compressed air or other breathing gas mixtures. Whenever a diver inhales, her regulator passes air from the tank into her mouthpiece. Depending on the size of a tank and the size of the diver, a full tank of air can allow a diver to remain underwater for as long as an hour.
Compressed Air
Scuba tanks carry compressed air at a pressure of about 3,000 or 3,500 pounds per square inch. The regulator’s first stage reduces the pressure of the tank’s air to an intermediate pressure of about 1,500 psi. The regulator’s second stage, or mouthpiece, further reduces the air pressure to a breathable level that matches the surrounding water.
Normal Air
Normal air contains about 78 percent nitrogen and 21 percent oxygen. Trace gases, such as argon and other inert gases, make up the last 1 percent of air. Most recreational, open-water scuba divers use tanks filled with normal air.
Risks of Normal Air
Nitrogen and oxygen both become toxic at greater depths and pressures. At depths of 90 feet or more, divers breathing normal air may experience “nitrogen narcosis,” a syndrome that can affect a diver’s judgment. Divers should not exceed depths of 130 feet while breathing normal air, because the risk of oxygen toxicity increases beyond this depth.
Alternative Gas Mixtures
Nitrox gas mixtures contain a higher percentage of oxygen and a lower percentage of nitrogen than normal air. Trimix and heliox mixtures contain helium. Trimix contains a mixture of oxygen, nitrogen and helium, and heliox contains only oxygen and helium.
Advantages of Alternative Gas Mixtures
Nitrox allows divers to have longer first dives, shorter decompression stops, shorter surface intervals and longer second dives. Because nitrox contains less nitrogen, the diver’s body absorbs less nitrogen into its tissues, meaning that the diver can spend more time underwater and less time letting the nitrogen escape from his body’s tissues after a dive.
Because trimix has less oxygen, it reduces the risk of oxygen toxicity for divers making deep dives. Trimix also reduces the risk of nitrogen narcosis at depth because it has less nitrogen than normal air. Unlike oxygen and nitrogen, helium does not become toxic at greater depths and pressures.
Risks of Alternative Gas Mixtures
Because nitrox contains higher levels of oxygen, divers breathing this gas mixture face a greater risk of oxygen toxicity. Divers must earn specialized nitrox certification before diving with this mixture. In a nitrox certification course, they learn how to manage the safety risks of diving with the mixture to avoid any injury or danger. Divers should not use nitrox for deep diving.
Only divers with the required training and experience should dive with trimix. That mixture diving requires additional decompression stops, during which the diver might breathe nitrox or pure oxygen.
Labels
All tanks filled with nitrox must bear a clear label that reads “Enriched Air Nitrox.” All Trimix-filled tanks must bear a clear label that reads “Trimix,” and show the percentages of oxygen and helium in the mixture.
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