Table of Contents
The Air We Breathe
The air we breathe is a mixture of gases, composed of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases and vapor. For our purposes, however, let’s leave out everything except oxygen and nitrogen. For our purpose let’s assume we have the air containing 21% oxygen and 79% nitrogen.
When you dive with normal air, you breathe the mix described above. Your dive computer calculates your nitrogen intake—how much nitrogen enters your body tissues—based on the 79% nitrogen content in the air you breathe. It uses this information to determine your no-decompression limits, surface intervals, and no-fly times, among other factors.
Enriched Air - Nitrox
Nitrox, or Enriched Air (EANx), as the name suggests, has been enriched with something—and that something is oxygen. A Nitrox mix contains more oxygen than normal air, hence the name ‘Enriched Air.’ The percentage of oxygen in enriched air typically ranges from more than 21% to 40%.
As mentioned above, during a dive, nitrogen dissolves into your body tissues. By increasing the percentage of oxygen in the air you breathe, you reduce the percentage of nitrogen, which means less nitrogen enters your body during the dive. For example, increasing the oxygen content from 21% to 30% (an increase of 9%) reduces the nitrogen content by the same amount, from 79% to 70%. More oxygen equals less nitrogen.
Nitrox Pros and Cons
What You Get
Like everything in the world, Nitrox has its pros and cons. By reducing your nitrogen intake, Nitrox allows you to perform longer dives compared to using normal air. With less nitrogen being absorbed by your body, it takes longer to reach your no-decompression limits. It also helps reduce fatigue after repetitive dives, which is especially useful on liveaboard trips where you might be doing 3 to 4 dives a day. From this perspective, Nitrox seems like an ideal solution to maximize your diving experience. However, it also comes with risks that you need to fully understand.
Nitrogen narcosis and Nitrox
Nitrogen toxicity can be an issue when diving with air, typically occurring at depths of around 30 meters. While it might seem logical that breathing less nitrogen would reduce the likelihood of narcosis, this has not been scientifically proven. Therefore, the same precautions should apply as when diving with normal air. Susceptibility to narcosis varies between individuals, as we are all different. Additionally, factors such as fatigue, stress, and exertion can also contribute to its onset.
Oxygen Toxicity
Oxygen toxicity is not a concern when diving with normal air, as the oxygen exposure remains low. However, due to its higher oxygen content, it can become an issue when diving with Nitrox.
Simply put, oxygen under pressure can become toxic, leading to serious conditions such as central nervous system (CNS) toxicity or pulmonary oxygen toxicity. These conditions can result in fatal accidents, including convulsions and drowning. The primary cause is exceeding safe oxygen exposure limits.
The Culprit: Oxygen Partial Pressure
Each gas in a gas mixture exerts pressure proportional to its percentage in the mix, a concept known as partial pressure. Oxygen is no exception, and its partial pressure is referred to as PO₂. At normal atmospheric pressure (1 atmosphere, the pressure of the air we breathe), air contains 21% oxygen, resulting in a partial pressure of 0.21 atmosphere or bar/ata.
As you descend underwater, the pressure increases by 1 atmosphere for every 10 meters of depth. At 10 meters, the pressure doubles to 2 atmospheres, and the oxygen partial pressure doubles as well, rising from 0.21 to 0.42 bar/ata. At 20 meters, the pressure increases to 3 atmospheres, and the oxygen partial pressure rises to 0.63 bar/ata. This progression continues with each additional 10 meters of depth.
How pressure and oxygen partial pressure (PO₂) increase with depth
As mentioned earlier, high oxygen concentrations can be toxic. Oxygen toxicity is almost certain at a partial pressure of 1.6 bar/ata. For air with 21% oxygen, this critical threshold is reached at approximately 65 meters—a depth well beyond the 40-meter maximum limit for recreational diving.
The Challenge
The challenge with Nitrox is its higher oxygen content. This means the critical 1.6 bar/ata partial pressure is reached at much shallower depths. For example, if a Nitrox mix contains 32% oxygen, the critical depth would be closer to 34 meters. The higher the oxygen percentage, the sooner you reach the critical depth limit for your gas mix and maximum partial pressure (PO₂ – 1,4 bar/ata).
For safety reasons, the maximum oxygen partial pressure limit for enriched air diving is set at 1.4 bar/ata. The difference between 1.4 and 1.6 bar/ata provides a margin for error.
Let’s consider a mix containing 30% oxygen. At a depth of 10 meters, the partial pressure would not be 0.42 bar/ata (0.21 + 0.21) as with normal air, but 0.6 bar/ata (0.3 + 0.3). This means the critical pressure of 1.4 bar/ata would be reached at a much shallower depth with nitrox mix than with the normal air.
It might seem like a lot of math, but don’t worry…
All Hail the Computer
As you’ve seen, depth is a critical factor when diving with Nitrox. This is where your dive computer becomes invaluable. With the appropriate settings, it can guide you safely through your dive.
Your computer needs to be set to Nitrox mode and configured with key values: the partial pressure limit (1.4 bar/ata) and the oxygen percentage in your gas mix (typically 21–40%). Using this data, the computer calculates your maximum depth limit, which must not be exceeded.
Maximum depths for different oxygen content in EANx. Calculated for PO₂ = 1,4 bar/ata.
|
% of O2
|
Maximum depth
|
|---|---|
|
21%
|
56 m
|
|
22%
|
53 m
|
|
23%
|
50 m
|
|
24%
|
48 m
|
|
25%
|
46 m
|
|
26%
|
43 m
|
|
27%
|
41 m
|
|
28%
|
40 m
|
|
29%
|
38 m
|
|
30%
|
36 m
|
|
% of O2
|
Maximum depth
|
|---|---|
|
31%
|
35 m
|
|
32%
|
33 m
|
|
33%
|
32 m
|
|
34%
|
31 m
|
|
35%
|
30 m
|
|
36%
|
28 m
|
|
37%
|
27 m
|
|
38%
|
26 m
|
|
39%
|
25 m
|
|
40%
|
25 m
|
How Do You Know the Oxygen Percentage?
You determine the oxygen percentage by analyzing the air from your tank using an oxygen analyzer. It’s crucial to perform this analysis yourself—don’t rely on the person who filled your tank or your dive guide. By measuring the oxygen content yourself, you ensure that the value you input into your computer is accurate, allowing it to calculate the correct depth limit.
Handling Nitrox Safely
Due to its increased oxygen content and the flammable and explosive nature of oxygen, special care must be taken when handling Nitrox. For blends up to 40%, no special equipment is required for regulators or hoses; however, cylinders must meet some requirements.
- Use EANx-dedicated cylinders only (based on blending method, manufacturer recommendations, and local regulations).
- Ensure clear cylinder markings for identification – green and yellow colours are used, depending on the country and local regulations.
Why are regulators and hoses acceptable, but cylinders require special EANx types?
When a Nitrox blend is made using the partial blending method, oxygen is first added to the cylinder, followed by the introduction of air. This means the cylinder and its valve come into direct contact with pure 100% oxygen. As a result, the cylinder must meet specific oxygen service standards, which standard scuba cylinders used for air do not. The reason for this is the potential danger of explosion, as the cylinder must be designed to handle pure oxygen safely.
Becoming a Nitrox Diver
If you want to dive with Nitrox, you must complete a special course to become a certified EANx diver. The course will cover diving theory, the rules for handling enriched air, and everything you need to know to dive safely with Nitrox.
Final Thoughts
Nitrox is an excellent alternative to normal air and a great way to enhance your diving capabilities. When used correctly, its benefits are well worth the extra effort. Always remember to set your dive computer properly and monitor your depth regularly. It’s also a good idea to set the depth alarm slightly above your maximum depth to ensure you stay within safe limits, especially if you get distracted during your dive.
Dive safe, dive well…
