From birth, man carries the weight of gravity on his shoulders. He is bolted to earth. But man has only to sink beneath the surface and he is free. - Jacque Cousteau
SCUBA diving has changed dramatically since the early days when the breathing air was pumped to divers from the surface. The freedom to move without being tethered to an air line was made possible with the use of compressed air tanks. Now, divers often choosing to use gas blends instead of atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases). These blends are obtained from dive shops, which can either buy premixed fill tanks or can utilize gas mixing systems to create custom blends at a large cost savings.
A variety of blends are common, and as with any SCUBA diving, each carries its own risks. It is essential that each diver is properly trained and certified for the conditions in which they plan to dive.
The most commonly used blends are:
Nitrox Mixes: The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which allow maximum operating depths (MODs) of 34 metres (112 ft) and 29 metres (95 ft) respectively when limited to a maximum partial pressure of oxygen of 1.4 bar (140 kPa).
TriMixes: A trimix is a gas blend which adds increased amounts of Helium to the mix. Helium is added to help minimize the potential narcotic effect of high levels of nitrogen. Also, due to its size, it enters and leaves body tissue more easily. While this allows for less inflammation, rapid off-gassing has to be controlled using slow decompression stops when ascending to the surface.
A number of "standard" mixes have evolved (referred to as 21/35, 18/45 and 15/55). These mixes evolved because they optimize the benefits and are the simplest to create in a dive shop using the most commonly stored fill tanks.
Heloix: Due to its expense, heliox tanks (blends with higher Helium content) are used almost exclusively in deep commercial diving. It is also sometimes used by divers using rebreathers, which conserve the breathing gas. The proportion of oxygen can vary depending on dive conditions, but it is often hypoxic and typically around 10%. Because sound travels faster in heliox than in air (think breathing in helium from a party balloon), those on the surface often use a "helium de-scrambler," which electronically lowers the diver's voice to make it easier to understand.
Interested in learning more? Search online to check out some great resources or contact a local dive shop!