When you’ve spent hundreds of hours preparing for your dream dive, travelling to a new destination, or hoping to see that elusive albino frogfish you’ve been hunting, every minute of your dive counts and turning for the surface because you’ve run out of no-deco time can hurt. It’s moments like these that drive many divers to maximize their bottom time with enriched air nitrox (EANx, or nitrox), gases with increased oxygen content that can significantly increase your NDL or (if used on air tables within safe depth limits) increase your conservatism on deep or repetitive dives. Whether you dive nitrox already or are considering it for your next trip, learn how to safely maximize your bottom time before you go.
By Reilly Fogarty
Why use nitrox?
You know the role nitrogen plays in diving – as a diver you manage your safety by controlling the saturation and supersaturation of nitrogen and other inert gases during your dive. Reducing the amount of nitrogen you absorb is one way to increase your bottom time or increase conservatism, and you can do this by replacing some of the inert nitrogen in your tanks with oxygen that your body can metabolize. Because your body has less absorbed inert gas after a dive with nitrox, it has less to offgas and the risk of bubble production compared to the same dive on air is decreased significantly. This means that you can either reduce your DCS risk or extend your bottom time (but not both at once!).
Decreasing DCS risk with nitrox involves diving it instead of air and using air tables (or an appropriately configured computer) while respecting the maximum operating depth (MOD) of the gas. Comparing two identical dives, one on air and one on nitrox, the dive performed on nitrox will result in less nitrogen absorbed and a decreased risk of DCS. DCS risk is not entirely eliminated, but will be significantly decreased, which can be ideal for deep dives or trips to remote locations that involve repetitive dives.
Increasing bottom time with nitrox works because of the equivalent air depth (EAD) principle. Dive tables and computers calculate no-decompression limits using the partial pressure of nitrogen in breathing gas. When the percentage of nitrogen (and the corresponding partial pressure) of nitrogen is reduced, this provides the equivalent gas loading of a dive to a shallower depth on air. As long as you are within the MOD of your nitrox mix, this principle allows you to dive nitrox to a given depth for significantly longer than if you were on air.
Know Your MOD
In the early days of nitrox, the gas had a reputation for deep diving. The opposite is actually true – because exposure to elevated levels of oxygen beyond specific limits is dangerous, the greatest benefits from nitrox diving are seen within recreational depth limits. Typical recommendations call for a maximum partial pressure of oxygen of 1.4 ATA throughout the dive, and limits on exposure time both per dive and over 24 hours. You’ll learn more about these limits in your nitrox training course, but it is vital that you always know and respect the MOD of your nitrox mix at all times. For example, EANx 32, a nitrox mix with 32 percent oxygen, has an MOD of 112 feet, while EANx 36 has an MOD of 95 feet.
For more information on nitrox diving, visit www.DAN.org/Health.