By John Hauxwell, Red Sea Tribe.

In the Oxford English Dictionary Enriched Air Nitrox (EANx – where x is the percentage of oxygen) is defined as “a mixture of oxygen and nitrogen used as a breathing gas by divers, especially a mixture containing a lower proportion of nitrogen than is normally present in air, to reduce the risk of decompression sickness.” As scuba divers we focus on the oxygen content, even though the benefit of Nitrox is derived by the lower partial pressure of nitrogen. 

The reason for this is that oxygen becomes toxic at depth. Since the Nitrox blends we use in SCUBA have an increased percentage of oxygen, the maximum operational depth – or MOD – of the gas is important. The higher the percentage of oxygen in the blend, the shallower the MOD. There is a balance between the percentage of oxygen (oxygen toxicity and desired depth) and extended dive time (reduced nitrogen exposure) which we exploit to determine the optimal gas mix to use on our planned dives. Knowing the maximum depth for a gas blend is important to prevent acute oxygen toxicity and the problems that this causes.

MOD (in atmospheres) is calculated by dividing our ppO2 limit (1.4 for recreational diving) by the percentage of O2 in the gas mixture, so for a 32% Nitrox blend our MOD would be 

MOD=1.4/0.32 = 4.375 atm = 33.75m 

I have heard a number of myths about diving on Nitrox, and in most cases they are exactly that; myths. Let’s take a look at the common ones.

Myth #1 – Nitrox is for deep diving.

Some divers think that EANx is for deep diving, when it’s not. The myth that Nitrox lets you dive deeper is a dangerous misconception. Nitrox has an increased oxygen percentage. As the amount of oxygen in the mix increases your diving depth (MOD) decreases. It does not increase. This is simple enough to calculate, but people still get it wrong. These misconceptions can be fatal so you must be sure to get the necessary training and gain the appropriate certification before you use Nitrox on a dive.

Myth #2 – Nitrox reduces your air consumption.

Whilst Nitrox will extend your bottom time and your no decompression limit, it’s not true that it will reduce your air consumption or somehow (through magical means) make your air last longer. Your air consumption when scuba diving depends on several factors. These include the conditions, depth of your dive, your buoyancy, streamlining, your technique, work rate, and your level of fitness. None of the above factors have anything to do with the percentage of oxygen contained in your tank. Your gas consumption will be the same regardless.

Myth #3 – Diving on Nitrox is safer than diving on air.

Many people say that diving on Nitrox is safer than diving on air. You could argue that there’s some truth to this, as you don’t build up the same levels of nitrogen in your body when diving on EANx.  This may reduce the risk of DCS. Safety is more about your approach to diving. Diving on Nitrox requires you to follow the same strict safety rules and procedures as you would if you were diving on air. If you’re diving on Nitrox, things can still go wrong, and if you don’t follow safe diving practices, you are just as likely to get into trouble.

Myth #4 – You’re less tired when you dive on Nitrox.

Diving on Nitrox and being less tired is a “half-truth”. As nitrogen builds up as we dive, we can end up with a lot of residual nitrogen in our system. This may be particularly true when diving to excess on a liveaboard. As nitrogen accumulates in your body tissues, it can cause you to feel weary. It follows that taking less nitrogen on board should reduce that feeling. However, as most information around this is anecdotal it is difficult to prove.

Myth #5 – You don’t get headaches when you use Nitrox.

This is again, a bit of a misnomer. Usually headaches are due to CO2 build-up caused by using an incorrect breathing technique such as breath holding or skip-breathing. This condition, also described as carbon dioxide retention, can cause other effects such as dizziness and fatigue, as well as serious complications such as seizures or loss of consciousness. Diving on Nitrox doesn’t change how you breathe, however, you may expunge the carbon dioxide from you system quicker using a higher percentage of O2, therefore reducing the chance of a headache. Again evidence for this is mostly anecdotal.

Now we have dispelled the myths around the “magic gas”

let’s look at the pros and cons of using Nitrox.

Pros

Cons

So how do you calculate your NDL using Nitrox if you haven’t got a Nitrox table?

The MOD for air is 56.6m based on a ppO2 of 1.4. The MOD for nitrox blends is much less. This is because of the increased amount of oxygen in the mix and thus a greater ppO2. So if you do not have a Nitrox table, you will need to calculate the Equivalent Air Depth (EAD) for the gas mix you are planning to use.

There’s actually quite a simple formula for calculating your EAD – and it’s based around the percentages of nitrogen in the gas we are using. We know that air is made up of a number of gases – but we will approximate it to be 79% nitrogen and 21 % oxygen  for our calculations.

To calculate the EAD in ATMOSPHERES we take the maximum diving depth and multiply it by the percentage of nitrogen in the mix we are using against the percentage of air. Both percentages are expressed in decimals. This gives us the EAD is atmospheres.

Depth(atm) x (MixN2/AirN2) = EAD (atm)

So for example if we are using 36 % Nitrox (nitrogen content is 64%) at a depth of 25m, our EAD is calculated thus

3.5atm x (0.64/0.79) = 2.8atm= 18m. 

Looking at our recreational dive planner this would give us an NDL of 56 minutes, as opposed to 29 minutes on air. A considerable difference! You would also calculate your surface interval and residual nitrogen based on this EAD.

For more information on using Enriched Air Nitrox or if you want to become an Enriched Air diver, please contact us and one of our team will help you.