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Nitrox

Popularity seems to have declines in recent years but it is still the gas of choice for some.

Why Nitrox?

For a diver it is generally the element nitrogen that limits the duration of diving. Indeed as we descend, more and more of the nitrogen is absorbed and stored in the body, if there is insufficient time on the ascent to naturally breathe this nitrogen out of the body then it can form bubbles and become harmful. Thus ascents are controlled, slow with appropriate stops and time for 'degassing'.

Naturally divers looked at ways to decrease this quantity of nitrogen in the divers air supply by diluting it with other gasses, for example helium. This is commonly known as Trimix, and although it is used worldwide, the mixing and using of Trimix was too complex and costly for the average leisure diver. Moreover Helium has its own decompression requirements too.

An alternative solution would be to increase the quantity of Oxygen in the breathing mix and in doing so decrease the percentage of Nitrogen. Oxygen is "consumed" by the body as opposed to being stored so there are no decompression issues.  Furthermore it is much cheaper and easier to obtain. It's not without its risks though.

What is Nitrox?

Essentially Nitrox is any combination of nitrogen and oxygen. The Air we breath is Nitrox with 21% oxygen and 79% nitrogen). In diving the oxygen content is increased and consequently the Nitrogen content is decreased. For example, a diver may wish to dive on 32% oxygen and 68% Nitrogen, this particular mix would be called EANx32. Other mixes typically range from EANx28 (28% Oxygen and 82% Nitrogen) to EANx40 (40% Oxygen and 60% Nitrogen). All the mixes do two things first they raise the oxygen content and secondly they lower the Nitrogen content. Both have a significant impact on the diver

The reduced nitrogen levels result in a slower uptake of nitrogen for the diver at all depths.  This in turn causes a reduced level of nitrogen narcosis and decompression sickness (DCS). Ultimately this allows for longer and/or safer dives depending on how the diver chooses to utilise that advantage.

As most divers know, Oxygen becomes toxic at high partial pressures and breathing a higher level of oxygen  increases the partial pressure. as does depth, so great care must be taken in limiting your depth.

Nitrox - The Advantages

• Longer bottom times or larger safety margins.
• Less decompression stops.
• Less likely to experience nitrogen narcosis.
• Less tired after a nitrox dive than I would be on air.
• Some people say they are  warmer with nitrox and don't suffer from the "dry mouth" problem.

The Disadvantages

• It is more expensive than air, probably twice the price of an air fill.
• Divers must monitor their depth more rigorously so as not to exceed their MOD.
• Divers must also monitor the duration of their exposure to oxygen enriched air.
• Divers may have to get their cylinders and regulators "O2 cleaned".

Oxygen Toxicity

There are two types of oxygen toxicity; central nervous system (CNS) toxicity and pulmonary toxicity. CNS toxicity is caused by short term exposure to high oxygen partial pressures and can result in convulsions and death. Pulmonary toxicity is caused by longer term exposures to moderate oxygen partial pressures and leads to pulmonary problems. The latter is not usually harmful to sports divers.

To monitoring oxygen exposure over time an "oxygen clock" is used. This is a mechanism which calculates the accumulative effects of oxygen, based on the partial pressure and the duration of exposure

When you dive with Nitrox there are three initial things that you need to know:

1. What is the maximum operating depth (MOD) with a given mix.
2. What is the best mix to use for a given dive.
3. What is the equivalent air depth (EAD).

The Maximum Operating Depth

The IANTD suggest a safe Partial Pressure ceiling for Oxygen is 1.4 bar. With this and the fraction of Oxygen in our mix we can work out the MOD using this triangle

Check out the MOD calculator

The Best Mix

The best mix for a given dive can be calculated for the following:  

Check out the Best Mix calculator

The Equivalent Air Depth

The formula for calculating the equivalent air depth is:

Check out the EAD calculator 

Please note the above is merely an overview of Nitrox. If you plan to dive with Nitrox you must attend a recognised training course.