Nitrox - Richness of Mix

Richness of Mix

The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which have 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). Divers may calculate an equivalent air depth to determine their decompression requirements or may use nitrox tables or a nitrox-capable dive computer.

Nitrox with more than 40% oxygen is uncommon within recreational diving. There are two main reasons for this: the first is that all pieces of diving equipment that come into contact with mixes containing higher proportions of oxygen, particularly at high pressure, need special cleaning and servicing to reduce the risk of fire. The second reason is that richer mixes extend the time the diver can stay underwater without needing decompression stops far further than the duration of typical diving cylinders. For example, based on the PADI nitrox recommendations, the maximum operating depth for EAN45 would be 21 metres (69 ft) and the maximum dive time available at this depth even with EAN36 is nearly 1 hour 15 minutes: a diver with a breathing rate of 20 litres per minute using twin 10 litre, 230 bar (about double 85 cu. ft.) cylinders would have completely emptied the cylinders after 1 hour 14 minutes at this depth.

Usage of nitrox mixtures containing 50% to 80% oxygen is common in technical diving as a decompression gas, which by virtue of its lower partial pressure of inert gases such as nitrogen and helium, allows for more efficient (faster) elimination of these gases from the tissues than leaner oxygen mixtures.

In deep open circuit technical diving, where hypoxic gases are breathed during the bottom portion of the dive, a Nitrox mix with 50% or less oxygen called a "travel mix" is sometimes breathed during the beginning of the descent in order to avoid hypoxia. Normally, however, the most oxygen-lean of the diver's decompression gases would be used for this purpose, since descent time spent reaching a depth where bottom mix is no longer hypoxic is normally small, and the distance between this depth and the MOD of any nitrox decompression gas is likely to be very short, if it occurs at all.