The air we breathe is a mixture various gases. It is composed mostly of nitrogen and oxygen with small amounts of carbon dioxide, argon, and other gases. SCUBA tanks can also contain mixtures of different gases that will allow divers to reach further depths below the ocean’s surface. The most common diving air mix is simply atmospheric air. It allows for dives less than 40 meters. While some divers have gone deeper than 40 meters on standard air, 40 meters is considered the safest acceptable depth by most training agencies. For deeper or longer dives, mixtures such as nitrox, heliox, and hydrox are a safer alternative to simple compressed air. These mixes all allow a diver more freedom and require their own certifications in order to use.
Regular air is over three quarters nitrogen, less than one quarter oxygen, and trace amounts of other gases. Nitrox typically increases the amount of oxygen to between 32% and 40% of the total gas volume. This mixture allows for longer bottom time at shallower depths and a significantly lower chance of suffering from the bends. However, there are risks involved with using nitrox. Exceeding the limits for a dive with nitrox can lead to inhaling oxygen at an unsafe partial pressure which can eventually lead to oxygen toxicity.
Trimix & Heliox
With Trimix, oxygen and nitrogen are partially replaced with helium. This allows divers to go deeper than nitrox allows with less risk of oxygen toxicity. Heliox is a diving gas mix that may have the same amount of oxygen as regular air (although typically less) but completely replaces the nitrogen with helium. Removing the nitrogen from the mix extends the depth and time limits. Since it has a lower percentage of oxygen than nitrox, there is no risk of oxygen toxicity. Heliox use does require a secondary tank of air or nitrox to be used for the shallower sections of the dive. The most immediate disadvantage of heliox use is loss of body heat. Helium conducts temperature poorly. This allows the body to lose more of its natural heat more quickly, leaving a diver vulnerable to hypothermia. Another disadvantage is the rarity of helium and the expenses related to gathering it.
Hydrogen is cheaper and much more abundant than helium. This makes hydrox a cheaper alternative to heliox. Hydrogen has little to no toxicity issues and can be used at all depths. The main problem with hydrogen is risks due to its extreme flammability. Hydrox is relatively new and is used predominantly in extreme commercial diving beyond the depths of trimix or heliox.
Don’t Try This At Home
Before attempting to dive with gases beyond standard air, be sure to enroll in the proper training. A common starting point is the PADI Enriched Air Diver specialty course which will teach you how to safely use nitrox up to 40% oxygen.