Nitrox

What is nitrox?

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.

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

PPO2 ------------------------- FO2  x P

Where  PO2 = Partial Pressure Of oxygen on the Mixture             FO2  = Fraction of Oxygen (e.g. 0.32)             P = Absolute Pressure in Bar

 

Example 1. -   What is the MOD if I do not want to exceed 1.4 Bar and I have an EANx mix of 32% Oxygen. 1.4 ------------------------- .32 x  ? 1.4 ÷ 0.32 = 4.375 bar which is 33.75 metres Answer: MOD = 33.75 metres

Example 2. -   What is the MOD if I do not want to exceed 1.4 Bar and I have an EANx mix of 40% Oxygen. 1.4 ------------------------- .4 x  ? 1.4 ÷ 0.4 = 3.5 bar which is 25 metres Answer: MOD = 25 metres

Check out the MOD calculator

The Best Mix

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

PPO2 Best Mix =   -----------------------                      Absolute Pressure

Where  PPO2 = Partial Pressure of oxygen on the Mixture            Absolute pressure = depth (Bar) + 1

 

Example 1. -   I want to dive to 40m on Nitrox but don't want to exceed a PPO2 of 1.4. What is the best mix?                  1.4 Best Mix = -----------------------                  4 + 1 (bar) 1.4 ÷ 5 = .28 Answer: Best Mix = 28% Oxygen

Example 2. -   I want to dive to 30m on Nitrox but don't want to exceed a PPO2 of 1.4. What is the best mix?                  1.4  Best Mix = -----------------------                  3 + 1 (bar) 1.4 ÷ 4 = .35 Answer: Best Mix = 35% Oxygen

Check out the best mix calculator 

The Equivalent Air Depth

The formula for calculating the equivalent air depth is:

(FN2)    (D+10) EAD =  --------------------       -10                       0.79

Where:  D= Depth in Bar              FN2= Decimal equivalent of the nitrogen in the mix              0.79= Normal Nitrogen in air

 

Example 1. -   What is the EAD of EANx 32 at 30m (.68) x  (40) EAD = --------------------       -10 0.79 Answer: EAD = 24.43metres

Example 2. -   What is the EAD of EANx 40 at 30m (.6) x  (40) EAD = --------------------       -10 0.79 Answer: EAD = 20.37metres

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.