The “Mystery” of Nitrox (EAN) DivingSimon Vetterli
While Nitrox diving has been used in recreational diving for over 25 years, there is still a degree of mystery about it. Even some misunderstanding what it can do for your diving. Because of this, many divers never undertake the short additional training that is required to qualify to dive Nitrox. Once you are training, diving with Nitrox only involves verifying the percentage before each dive. Nitrox is short for Nitrogen-oxygen, and is any blend of air where those two elements are the only ones other than trace elements. The air we breath is 21% oxygen and 79% nitrogen with small amount of trace elements. In recreational scuba diving, we can use what is called Enriched Air Nitrox (EAN). Blending an EAN mixture, we add 100% oxygen to normal air to increase the percentage of oxygen in the blend. We will generally see it as EAN-XX, where the XX is the percent oxygen in the blend. EAN-32 and EAN-36 are the most common and are used in the EAN dive tables. These tw
o blends are commercially available, however, most dive centers that dive Nitrox can blend the mixture themselves so can have other percentages based on their needs. Divers qualified as Enriched Air Nitrox divers can dive using up to a 40% blend.
Does EAN allow me to dive deeper?
It is a widely held believe that EAN-XX will allow you to dive deeper. Depending on how you look at it, that is only partially true. The belief revolves around the concept of equivalent air depth (EAD). Since, the air in the EAN-XX cylinder has a higher percentage of oxygen, that means it has a lower percentage of nitrogen. So less nitrogen is being absorbed into your tissues. If we have a standard fill at 21% oxygen (air) and dived to 80 feet /24 meters, we would have a NDL of 29 minutes (figures rounded).Using an EAN-32, we can dive to 100 feet/30 meters with the same bottom time of 29 minutes. At 32% oxygen, nitrogen is being absorbed at the same rate as 21% oxygen at 24 meters. So the EAD of EAN-32 at 100 feet/30 meters is 80 feet /24 meters. So for the same bottom time we can dive deeper.
There is another factor that needs to consider, and that is Dalton’s Law of Partial Pressure. While some explanation of it is in your open water training and an expansion is in your advance open water and Nitrox / EAN training, we do not need a full understanding for our purpose here. When we are on land breathing, the partial pressure of oxygen, is .21ata. That value is made up of the percentage of oxygen times atmosphere pressure. When we dive to 33 feet/10 meters, we reach 2ATM and the partial pressure of oxygen (po2) becomes .42. Around this point the characteristics of how oxygen affect our bodies starts to change and oxygen becomes a toxin. However, the time it would take where the concentration would cause concerns are much longer than any diver would be still in the water. When the po2 increase to 1.6ata, the brain can start to become affected. This is called central nervous system (CNS) oxygen toxicity. At this point individuals will start showing signs of oxygen toxicity. These can rapidly escalate to convulsions and even death. For safety reasons, 1.4ata is considered the maximum safe partial pressure. When we are diving 21%, 1.4ata is reached at around 185 feet/56 meters. This is much deeper than we can dive due to the effects of nitrogen. However, if we switch to EAN-36 we will hit the 1.4ata safety limit at only 94 feet/ 28 meters. This is referred to as Maximum Operating Depth (MOD)and is well within range of a deep diver and before hitting the NDL based on nitrogen absorption. If we use EAN-38, the MOD is 87 feet/ 26 meters. So clearly, you cn not say EAN-XX can allow you to dive deeper. A final point, the oxygen off loading is the same as off loading nitrogen. The surface interval will reduce the amount in your system. Diving before the total is off loaded will not affect the ata point, however it will influence the time it takes to build up other toxic properties. While 100% oxygen is used in different medial situations, 100% oxygen will reach a po2 of 1.6ata in about 19 feet/6 meters of water.
Longer or safer?
The other benefits you will hear supporting EAN diving is that you can dive longer and safer. It is more accurate to say, EAN will allow you to dive longer or safer. Let’s use the same example as we used above, using air and diving to 80 feet /24 meters. We would have a NDL of 29 minutes for that dive. A diver using EAN-32 can stay at that depth giving him a NDL of 45 minutes almost 50% longer dive, before reaching the NDL. The air diver at 29 minutes and the EAN diver at 45 minutes have the same theoretical risk threshold. The EAN diver has had a longer dive but has not increased his safety. If both divers dive to the same depth and to the 29 minute mark, the air diver is at the NDL while the EAN diver has a large buffer, being still a time distance from his NDL. If they surface together, the EAN diver has a much safer profile.
Many dive professionals will use EAN to reduce their intake of nitrogen, even when diving with students on air. This gives them a greater safety margin.
When should I dive Nitrox / EAN and How to I become trained?
How long you can dive is really based on three factors, how fast you use your breathing gas (air or EAN), the NDL due to nitrogen and the oxygen loading. Few divers will benefit from the reduced nitrogen of an EAN blend if they are diving in less than 60 feet/ 18 meters. The deciding factor is the gas in the dive cylinder and not the other two factors.
Dives deeper than 60 feet/ 18 meters will benefit from an EAN mix, as long as it remains within the limits of the MOD. Diver preparing to dive EAN-XX must verify the oxygen content themselves, calculate the MOD, record the MOD on the tank and certified they did those steps. Those are the only task different for dive preparation. If using dive tables they will use EAN tables, it a dive computer they will enter the percentage.
Enriched Air Nitrox is a very simple certification class and can be combined with most other dive training. It consists of the knowledge portion and two open water dives. The two dives can be integrated into other training. Open water dive training will only integrate one dive, the second dive is additional. If you are not currently Nitrox / EAN qualified, you should explore a certification and join those with extended or safer diving.