On today's dive boats, you'd be hard-pressed not to see one or two of the telltale green and yellow tanks that signal someone on board is diving with nitrox, also called enriched air, oxygenated air, EAN, EANx, or simply "nitrox " too short. It is doneused for recreational diving for decades, and every major training agency has an enriched skydiving training program.
Like everything else in a diving bagmechanismand knowledge — nitrox is a tool that can be effective when used correctly, but dangerous if not. Its purpose is to increase non-stop dive time so you can have longer dives and/or shorter surface intervals, all other things being equal and the gas supply permitting. But it has less depth limits and other concerns that you don't have with air within the limits of recreational diving.
What is Nitrox?
In terms of recreational diving, enriched air nitrox (EANx) refers to any nitrogen/oxygen gas mixture with more than the 21 percent oxygen found in normal air. For example, 32 percent and 36 percent oxygen are common. you will hear them called nitrox 32 (or 36) or EAN32 (or 36). To clearly identify them from air cylinders, EAN filled tanks are marked with a yellow and green tank band on top just below the crown. The percentage of oxygen mixture is indicated on a mark or label, which is important because the percentage affects your depth limits and dive time.
How does EAN Nitrox work?
Enriched air has more oxygen than air, which lowers the nitrogen content. As a result, you breathe less nitrogen during the dive than if you were using normal air.
We all learned on our ownOpen Water Diver kursusthat when you dive, the water pressure causes nitrogen from the air you breathe to dissolve in your bloodstream and tissues. The higher the pressure (the deeper the dive) and the longer the time under pressure (time down), the more nitrogen dissolves. If you surface with too much dissolved nitrogen in your tissue, bubbles can form, causingdecompression sickness(DCS). Staying within the no-stop limits limits the theoretical amount of dissolved nitrogen so you can go straight up to the surface (but stop your safety). If you exceed this limit (like tec divers and commercial divers), you must perform mandatory decompression stops gradually as you ascend to keep the risk of DCS low. Based on the Recreational Dive Planner, for example, when a diver reaches 100 feet of no-decompression air after 20 minutes. At 60 feet, the diver's maximum time would be 55 minutes. But nitrox changes those numbers.
Because you inhale less nitrogen, less dissolves in your body, all else being equal, allowing for a greater no-decompression limit. This can be calculated using an equivalent air depth (EAD), which is a smaller depth where nitrogen would enter the solution at the same rate if you were using air. For example, if at 105 feet with EANx32 it has an EAD of 90 feet, so you absorb nitrogen as if you were 15 feet lower. Therefore, the diver's usual no-decompression limit of 15 minutes is extended to 25 minutes - a 66 percent increase in bottom time.
Advantages of Nitrox Diving
The best use of nitrox is in the 50 to 100 foot range. Stop times for dives below 50 feet are often so long that you want to empty your tank before the dive time is up. When using EANx36 at 60 feet, the non-stop limit is 80-90 minutes or more (depending on your computer) for your first dive, so again you will be limited by the gas supply, not the decompression limit.
You are taught to dive well within the limits and by pushing the limits you can dive further and still be conservative. Many enjoy nitrox diving to or even beyond the air limits without being close to the true non-stop max. As with air, this does not guarantee 100 percent safety, but staying within the limits reduces the practical risk of DCS for most divers.
Interested in nitrox diving? Start your adventurehere with the PADI Enriched Air Diver course.
Nitrox Diving Hazards
This benefit does not come without a cost. Oxygen, although useful and necessary for life, can become dangerous in high concentrations. Too much oxygen under pressure affects the central nervous system - oxygen toxicity - causing convulsions or convulsions, visual disturbances, ringing in the ears, nausea, convulsions, irritability and dizziness, with convulsions or convulsions being most severe during diving. Oxygen toxicity shocks are very rare in scuba diving, but if they do happen underwater, they happen without warning and lead to drowning if you lose your regulator while breaking. Any percentage of oxygen can cause toxicity if you go deep enough, but with air you have to go way beyond the stated depth limits for that to be possible. With nitrox the depth limit is within recreational diving depths and the more oxygen the lower the limit - the limit being the depth where the PPO2 would be 1.4 ata. Prolonged exposure to elevated oxygen can also cause another type of oxygen exposure. Nitrox certification is required because, among other things, EANx divers must now learn to manage their oxygen exposure.
Calculation of your PPO2
When enriched air first entered recreational diving, there were no EANx dive computers and few tables. Well, you figured it all out with a pencil and calculator.
If you have math phobia, wait! NOBODY does that in recreational nitrox diving anymore. You look up the maximum depth on a table (easy), set your computer with the EANx mix you're using (easy), and it calculates your oxygen exposure and adjusts the time without interruption. In fact, when doing EANx diving with a computer in multi-level profiles, you will almost always end the dive due to low gas, cold, hunger or urination. You need to learn how to analyze your mix (not difficult) and some equipment issues, but all of this is covered in the PADI Enriched Air Diver course.
However, you need to understand what your computer is doing for you so that you can use this information accurately and safely.
For example, the pressure component of the oxygen boundary is calculated using partial pressure. You can find it by multiplying the EANx oxygen percentage by the absolute pressure (in atmospheres or bars). For example, a diver breathes air (21 percent O2) at 132 feet (5 ata) with a partial pressure of oxygen (PPO2) of 1.05. This would be equivalent to breathing 105 percent oxygen at the surface. In other words, you cannot breathe this PPO2 at the surface, even using pure oxygen.
- If you are interested, calculate PPO2 to look like this: 5 ata x 0.21 percent O2 = 1.05 PPO2
- It's easy to find the corresponding area percentage - drop the decimal
- The depth limit is based on the depth at which a mix would have a PPO2 of 1.4, so this is calculated: Maximum depth = *(1.4 ÷ mix%) - 1) X 33
- The mixing percentage is entered as a decimal (eg 32% uses 0.32)
Note:Technical diversthey are often immersed in a partial pressure of oxygen of up to 1.6 PPO2, but are trained in additional risk management.
What Nitrox doesn't want
Enriched air is sometimes talked about as if it is a cure for every disadvantage of diving. This is not the case, so let's take a moment to dispel some myths.
Myth 1 - Using it means you're less likely to get stuck.
That sounds reasonable. If nitrogen is sedating and you breathe less of it, you should have less sedation. The problem is that the depth range where you start to worry about narcosis (100 to 130 feet) is also where you should stop using nitrox because of the risk of oxygen toxicity. And even if it wasn't, oxygen is thought to be roughly the same substance as nitrogen (not always, but usually - it gets complicated), so you're one step up and one step back anyway. The preventive value of nitrox is "very small, so marginal that I think it should be ruled out," writes Dr. Peter Bennett in the chapter on inert gas anesthesia in The Physiology and Medicine of Diving.
Myth 2 - Reduces gas consumption.
It also sounds reasonable. Naturally, if there is more oxygen in each breath, you don't need to take another as quickly, and gas consumption is lower. Unfortunately, it doesn't work that way, because most of the oxygen you inhale is exhaled unused anyway. What drives the next breath is the need to exhale carbon dioxide, and this is not affected by what's in your tank.
Myth 3 - Nitrox allows you to dive with less fatigue.
The theory here is that the work of outgassing nitrogen is a major cause of fatigue during diving, so less of it should make you less tired. Many nitrox divers swear it's true, but Bennett cites a blind study that proved otherwise. Using unmarked tanks, one group of divers was given nitrox, another was given air, and later both were asked how they were doing. "There was no difference," says Bennett. "It's a placebo effect."
Myth 4 - You can dive deeper with enriched air.
Some divers think that these green-yellow cylinders look serious and "technical" and associate it with going deep. As you just learned, EANx introduces a shallower floor, and the penalty for going below it - potentially dying - is also severe. Many of us would risk chasing that eagle radius out to 150 feet in a normal air mix, but we wouldn't think about it on nitrox. However, tech divers use EANx to decompress in the shallows after a deep dive, which may be one reason why some think it is a deep diving gas.
Get the right Nitrox mix
The premixes offered in most dive shops - usually 32 or 36 percent - make life simple. Simply lift the tank, analyze the mixture, confirm the maximum depth and enter it into your computer.
But you can use other mixes. Ask and your dive shop will probably match a tank to the percentage you need for your dive, as long as it's below the 40% recreation limit. Custom mixes like this really give you the best possible result. For example, using EANx32 at 70 feet has an EAD of 56 feet (a 55 minute NDL table), but using EANx40 it is 45 feet (100 minute table limit), giving you more bang for your buck. In places where a given mix is particularly useful due to depth, dive centers usually stock the EANx mix.
Nitrox and recreational equipment safety
The US Navy and the National Oceanic and Atmospheric Administration (NOAA) say that standard scuba gear can be used for oxygen concentrations as low as 40 percent. Since this is also the maximum recreational limit for nitrox diving, no recreational diving equipment has ever been used with equipment-related incidents within nitrox recreational limits. (Note: Some manufacturers use a more conservative rule. Always follow the manufacturer's recommendations for their equipment.)
Contrary to popular belief, oxygen does not burn. However, it increases the flammability of everything it comes into contact with – the higher the oxygen concentration, the easier most substances ignite and burn hotter and faster. For this reason, there is one exception for equipment, and that is the scuba tank, for two main reasons.
One of the reasons is that a common method of making nitrox (custom mixes) requires putting pure oxygen in the tank first and then specially filtered air compatible with oxygen. Because the tank contains pure oxygen during mixing, they must meet oxygen service standards, which means materials (o-rings, valve seats) and any lubricants must be rated for use with pure oxygen. The tank must also be specially cleaned, which is done annually.
The other reason, as you have learned, is that the tank must be clearly marked so that it is not accidentally submerged as air. The distinctive EANx band is located at the top of the cylinder so that it is visible even when surrounded by a cluster of cylinders.
Earn Nitrox Diving Certification
PADI's Enriched Air Diver kursusincludes eLearning that can be completed in a few hours. The personalized learning, done in a day or two, allows you to analyze a tank, set up yourdive computerfor nitrox and fill an enriched air log with a PADI pro there to guide you.
Nitrox types and definitions
- Normoxic nitrox: Normal air or clean air with 21 percent oxygen
- EAN, EANx, enriched air, enriched nitrox air: Any mixture with more than 21 percent oxygen
- EAD: Equivalent air depth. The ratio of the nitrogen absorbed at a given nitrox breathing depth to the depth at which an equivalent rate of absorption would occur by breathing air.
- PPO2 (or PO2): Partial pressure of oxygen, atmospheric pressure x percent of O2 = PPO2
- MOD or maximum depth: Maximum depth = *(1.4 ÷ mix%) - 1) X 33
- Best mix: 1.4 ÷ ((Depth + 33) ÷ 33) = best mix