Nitrox: Everything You want to know!

Nitrox is by far the most popular diving specialty there is. What was once considered a dangerous dark art over 30 years ago is now mainstream. Now Nitrox certification is as important as your mask and fins or underwater iPhone case. Moreover, current nitrox specialty courses are dry and can be taught virtually without getting wet. All you need to do is learn some theory, basic formulas, and how to analyze a nitrox tank, and you are set to go. So what exactly is the story behind nitrox, and why is it a scuba revolution?

Where Did Nitrox Come From

Similar to many diving innovations, the use of Nitrox was pioneered in the technical/cave diving world. Pursuing the limits of exploration, divers realized they could use gas with a higher percentage of oxygen (Nitrox) to achieve two things:

On the bottom stage of the dive, they had a longer no decompression limit. This was due to the lower percentage of nitrogen, which meant that less was absorbed into the divers' tissues compared to air. This then resulted in prolonged no decompression limits, which are always good news for explorers.

More interestingly, for divers making decompression dives with air or mixed gases, using Nitrox up to 100% oxygen during the depression phase of the dive is beneficial. The use of higher oxygen mixes "accelerates the deco" relative to using regular air. The reduction is substantial and can take multiple hours off decompression times compared to using normal air. These shorter times reduce fatigue and stress on the diver's body.

For the layman using higher nitrox mixes for decompression helps drive out the inert gases in your tissues quicker due to the elevated percentage/Partial Pressure of oxygen and lower percentage/partial pressure of nitrogen. Check out the gold standard presentation below (nitrox section around 40 mins)

The Pros & Cons of Nitrox

Over the years, a lot has been said about the benefits and issues of using nitrox in recreational diving.

Drawbacks of Nitrox

The main drawbacks of using nitrox are cost, additional training, and the risk of an oxygen toxicity hit.

When nitrox first came on the scene, it was expensive since blending it required a big setup, and tanks were sold at a premium (see the section on how it is made). Today, however, things have changed; costs have dropped substantially, and there is little difference in the price of nitrox vs. air cylinder fills. If you do some research, you will find various liveaboards around the world offering nitrox for free to certified divers. That is a far cry from several years ago when a fill would set you back around $10 a tank.

Historically training was expensive and took a couple of days, including dives. Again this has changed, and courses are now much cheaper and less intensive, making them much more convenient and affordable.

Finally, there were grave concerns about the risks of Oxygen toxicity hits from using nitrox by recreational divers. Over the past few decades, this risk has proven to be greatly exaggerated, and the number of incidents has been incredibly low. Those few that have occurred have resulted from a large number of errors compounding into one big problem!

The Pros Of using nitrox

There are various benefits of using nitrox, and we have discussed a few. Whether it is extended bottom times (see where nitrox came from section) or an increased safety margin when diving air limits (see if it is safer than using air section), by far, the biggest benefit of diving nitrox is anecdotal.

Many divers report a massive reduction in post-dive fatigue when using nitrox. Lots of divers using air on multi-dive days report feeling very tired at the end of the day and every dive. Huge numbers of divers have reported that they feel much less fatigued at the end of multi-dive days when using nitrox. Regardless of your fitness levels, using nitrox can be a significant benefit on vacation since it gives you time to enjoy other leisure activities as well as diving.

Keep in mind that the mechanism of why this happens is poorly understood. If you dive nitrox to its limits regularly, the effects of the reduction of this post-dive fatigue will slowly but surely fade away.

Is it Safer than Using Air?

One of the biggest questions and often poorly misunderstood statements about nitrox is that using it is “Safer than diving air.” While the statement has a grain of truth, it only applies in some very restricted situations.

Nitrox is indeed safer than air in terms of reduced probability of Decompression Sickness if you are diving within air limits. For instance, if you are to do the same dive to the same depth for the same amount of time. Using air, you will have more nitrogen absorbed in your tissues than using Nitrox 32. As a result, the probability of incurring a DCS hit is higher using air than nitrox. Bear in mind this only applies when the dives are identical in depth and time.

On the other hand, diving nitrox, a nitrox table, and air using an air table, you will get to spend longer at depth before reaching the same pressure group of saturation. Ending the dive at the same nitrogen pressure group means the probability of a DCS incident is the same in both cases, and nitrox is not safer than air. You can only argue that nitrox is somehow safer when you use it for diving air limits.

Understanding Partial Pressure Limits

Understanding partial pressure limits and how they evolved can clarify much of the mystery surrounding them. While lots of divers take the 1.4 Partial Pressure of Oxygen (PPO2) as a gospel treating it like a speed limit, if you exceed it, terrible things will happen, but that is simply not the case.

Historically partial pressure limits for oxygen were much higher, and while there were more accidents, they were by no means happening to everybody across the board. Technical divers regularly switch gases at 1.6 bar PPO2 to accelerate their decompression. Furthermore, when recompression chambers are used, one of the basic treatments involves taking the individual to 18m/60ft depth and breathing 100% oxygen. This generates a colossal PPO2 of 2.8 bar, and yet thousands of individuals are treated in chambers every year without ever succumbing to oxygen toxicity.

If you are wondering about human tolerances of high PPO2 in the water, check out the video below of what was and still is the world record for the deepest air dive (not to be recommended under any circumstances). Nearly 25 years ago (1999), a British diver set the record at 156m/511ft using air. That works out at an arguably insane PPO2 of 3.48 bar, which is nearly two and half times the recommended limit of 1.4! Not only did the diver survive the eventful dive setting the record, but he also did not suffer any long-term consequences.

What divers should keep in mind is that these limits are a result of a learning curve. Human Physiology is highly variable, and limits need to be respected and understood. However, if you slightly exceed a limit( never recommended) in an unlikely scenario), the key is not to panic, go back within the limit, and stay safe.

How is Nitrox Made

There are two ways nitrox is made. The first is called partial pressure blending and is the oldest way to make nitrox. The second is called membrane separation and is a more modern way to make nitrox, although it has limitations.

Partial Pressure Blending

Partial Pressure Blending is pretty straightforward. You take a big cylinder full of pure oxygen, then make a few simple calculations before decanting some Oxygen into your target cylinder. At this point, you then top up your cylinder using an air compressor to your tank's fill pressure. Making nitrox this way is straightforward and allows you to make any nitrox percentage from 21% to 100%. However, partial pressure blending techniques have a couple of main drawbacks.

First, because you are using pure oxygen, special handling and procedures must be used to ensure everything is done safely and minimize the risks of an oxygen fire. All these additional costs make the cost per cylinder of nitrox even more expensive.

Secondly, the technique is inefficient for liveaboard operations. Making nitrox this way for a group of divers over a week without restocking on large O2 cylinders is problematic. The sheer volume of cylinders needed would occupy a lot of space and create a serious hazard by having that much high-pressure oxygen around. Partial Pressure blending works best from shore-based operations or when mixes higher than 40% are needed.

Membrane Separation

Membrane separation, or a nitrox membrane, is newer nitrox-making technology and is the perfect method for making nitrox at recreational dive facilities as well as liveaboard operations. Nitrox made using this technology takes normal air and passes it through a membrane which removes some of the nitrogen in the air, leaving the air with a higher percentage of oxygen or nitrox! This is then fed into a compressor and used to fill Nitrox tanks.

The main advantage of this system is size and simplicity. There is no need for large tanks of O2, and the membrane systems are relatively compact. When it comes to operation, things are also much more straightforward, with no complex calculations required. There is little difference between filing air tanks or Nitrox 32% tanks using a membrane system.

The main drawback of using this method is that you can only make nitrox up to 40%, although for safety, many facilities only offer up to 36%. So, If you are looking for richer mixes to use, technical diving partial pressure blending is the way to go.

Should You Get Certified

The simple answer is yes, without a shadow of a doubt! If you only ever do one more diving course in your life beyond open water and advanced open water then nitrox it must be! Beyond the two basic certifications, it is arguably the most important certification you will do. Nitrox will make your diving journey easier and more fun, especially if you indulge in big multi-day multi-dive adventures like on a liveaboard.