Nitrox in a nutshell

Diving with oxygen-enriched air – known as Nitrox or EAN (Enriched Air Nitrox) for short – has become established in recreational diving because it offers a clever way to reduce nitrogen absorption. Nitrox contains more oxygen than normal compressed air. This leaves less space for nitrogen in the breathing gas, so the proportion of nitrogen automatically decreases. Less nitrogen means longer bottom times, less nitrogen narcosis, shorter surface intervals, and a lower risk of decompression sickness (DCS).

Foto: Patrick Oswald

Nitrox diving is not rocket science. Once you understand the basics, you can benefit from it safely and effectively. You don't need years of experience – but there are three simple golden rules that you must follow.

1. Maximum oxygen partial pressure 1.4 bar

Unlike air diving, nitrogen is no longer the only limiting factor in nitrox diving. As the oxygen content in the breathing gas increases, so does the risk of CNS toxicity (central nervous system poisoning).

What is CNS toxicity?

This is an acute form of oxygen poisoning in which the central nervous system (brain and spinal cord) becomes overstimulated. This can result in epileptic seizures without warning – which can lead to immediate death by drowning underwater. This must be prevented at all costs.

The golden rule for recreational divers is: the maximum partial pressure of oxygen (PpO₂) must not exceed 1.4 bar.

The following factors increase the risk of CNS toxicity:

• Lack of sleep

• Cold

• Physical exertion

• Elevated CO₂

• Stress or anxiety

• Various medications

Lack of sleep in particular is often overlooked – it lowers the brain's stimulus threshold and can greatly reduce tolerance to oxygen. Diving when overtired puts you at unnecessary risk. Going on a nitrox dive after a night of partying while on diving vacation is not a good idea.

2. MOD – how deep can I dive with my mixture?

MOD stands for Maximum Operating Depth, i.e. the maximum depth to which a nitrox mixture can be used safely. It is calculated from the oxygen content of the mixture and the maximum permissible oxygen partial pressure (PpO₂ 1.4 bar).

The calculation is quite simple in principle: take the desired maximum oxygen partial pressure, i.e. 1.4 bar, and divide it by the oxygen content (FIO₂) of the mixture in decimal form. Subtract 1 from the result (because 1 bar corresponds to the surface pressure) and multiply the remainder by 10 to obtain the depth in meters.

An example:

You are diving with Nitrox 30, i.e., with an oxygen content of 30%. The maximum PpO₂ of 1.4 bar corresponds to 30% of the ambient pressure = 4.66 bar. Subtract 1 bar from the result and multiply the remainder by 10 (m/bar) to obtain the depth in meters, in this case 36.6 m.

The formula looks like this:

Maximum Operating Depth (MOD) m = ((PpO₂ / FIO2) - 1) x 10

--> ((1.4 bar / 0.3) - 1) x 10 m/bar = 36.6 m.

This means that with Nitrox 30, you should not dive deeper than 36 meters if you do not want to exceed the safe limit of 1.4 bar PpO₂.

The MOD is the depth limit for each gas mixture. If you know and respect it, you will dive safely.

3. „Always analyse your gas!“

Nitrox offers clear advantages – but only if you know exactly what you are breathing. Many diving accidents involving Nitrox can be traced back to a simple mistake: the diver assumed the wrong mixture or the wrong MOD.

That is why the third golden rule in Nitrox diving is: “Always analyze your gas!”

Each tank must be checked with an oxygen analyzer before the dive. Don't rely on stickers, labels, or your memory. Determine the oxygen content yourself and calculate the MOD.

Conclusion

Nitrox makes many things better – but not everything easier. Those who know and follow the rules get:

• more bottom time

• less nitrogen narcosis

• shorter surface intervals

• and a lower risk of DCS.

Nitrox unfolds its full potential wherever diving is frequent and extensive – for example, on diving trips with several dives per day. Due to the lower nitrogen saturation of the tissue, you can use longer no-decompression times, reduce your risk of DCS, and feel fitter and more rested overall – especially when diving several times in a row.

But: oxygen partial pressure, MOD, and gas analysis are non-negotiable.

Nitrox is not rocket science. If you follow these three points, you will dive safely.