Same Day Switching Between Air and Nitrox

Some divers routinely switch between air and Nitrox, using air on the first dive to go deep, then Nitrox later in the day to extend bottom time. To monitor decompression, some switch back and forth on an air/Nitrox computer, while others follow the air tables all the way. Does it make sense?

At the 2000 Divers Alert Network (DAN) Industry Nitrox Conference (see January 2003 Undercurrent), consensus was that when breathing Enriched Air Nitrox (EAN or just Nitrox) mixes with 40 percent or less oxygen, it's not necessary to use dedicated, specially-cleaned tanks or regulators. Furthermore, many computers also offer air or Nitrox dive modes, allowing the user to preset the percentage of oxygen in the breathing mix.

The most tangible advantage of air is when a diver goes below the maximum operational depth for a given Nitrox mix. For instance, breathing Nitrox 32 below 130 fsw puts the diver at risk for developing oxygen toxicity, which can cause deadly convulsions. So some divers consider air the safer choice for deep dives. But Tim O'Leary, director of technical operations for NAUI, disagrees. He told Undercurrent that "nobody should use air on deep dives" because of the risk of nitrogen narcosis.

Bret Gilliam, who runs Technical Diving International, believes the risk of narcosis is acceptable for sport divers down to 145 fsw, whether on air or Nitrox. His agency allows instructors to train divers to reach 180 fsw on air. However, Gilliam told Undercurrent that susceptibility to narcosis below 130 fsw "is so individual, it should be individually determined, preferably under the supervision of an experienced technical instructor."

As an alternative to deep diving on air, Gilliam points out that most Nitrox stations can provide a custom mix, such as 30% oxygen, that will allow a diver to exceed 130 fsw. On subsequent, shallower dives the oxygen percentage can be increased to allow for less buildup of nitrogen in the system. "Why switch to air at all," he asks, "when you can have your Nitrox blended to match your maximum depth on each dive?"

When a dive computer is in the Nitrox mode, it uses a formula to calculate the potential for oxygen poisoning. "It's a hypothetical model, not a true measure of oxygen buildup in the diver's body," says Bill Hamilton, a physiologist with four decades of experience in decompression and breathing gases. Hamilton helped develop limits for NOAA that today are the basis for most recreational diving calculations.

During surface intervals, the body rids itself of excess oxygen, but there's debate among the experts about how rapidly this occurs. Hamilton points out that "Oxygen is very fickle and different people get rid of it at different rates." So it's best to err on the side of caution, as the computer manufacturers have done with their algorithms.

Computers that handle Nitrox display the partial pressure of oxygen (P02) because too much oxygen at depth is toxic to a diver. NOAA and most physiologists agree that a PO2 of 1.6 atm is as high as a scuba diver should ever breathe in the water. Many, including Hamilton, say 1.4 atm PO2 is a safer practical upper limit, and that's the limit at which most Nitrox computers begin displaying P02 warnings.

Gilliam and the other industry experts agree that "if you're diving a single 80 cu. ft. aluminum tank with Nitrox and following your computer, you'll hit your nodecompression limit on any given dive before you develop oxygen toxicity." Yet it's possible you may get a P02 warning on your computer. If such a warning occurs underwater, you should immediately begin a slow ascent in accordance with your Nitrox training, then take a surface interval long enough to allow the PO2 to drop down to safe levels (usually about two hours).

Some divers who get such warnings after doing repetitive dives on Nitrox switch to air to slow down oxygen accumulation. When doing this, it's best to keep one's computer in the Nitrox mode, so you can continue to track the P02 display. Some computers, such as Suuntos, won't allow you to switch to air mode for a day or two after diving in Nitrox mode. If you decide to breathe air after Nitrox, Jim Clymer, of Suunto's Technical Department, recommends setting the Nitrox mode for 21 percent, the equivalent of air.

"Just remember," warns Hamilton, "that you are dealing with toxicity, not accumulation of oxygen, and the recovery period is not really known."