I thought about calling this post “The Pros and Cons of Diving with Nitrox”, but there aren’t really a lot of “cons” other than the ones you’re already familiar with, cost and convenience: The time and cost associated with getting certified for Nitrox diving, the extra cost of Nitrox, and the inconvenience of Nitrox being unavailable in some locations. On the “pro” side, you’re already aware of the increased bottom times Nitrox allows, compared to air.
What You Don’t Know About Diving With Nitrox
In getting certified for Nitrox diving, you learned that the Nitrox dive tables were developed by extending air dive tables using the “Equivalent Air Depth” (EAD) rule, the intent being to develop NDL’s for Nitrox that were equivalent in decompression risk to the NDL’s for air. In that respect, the EAD rule earns a big “FAIL”.
The EAD rule results in Nitrox tables whose NDL’s are vastly different in decompression risk from the NDL’s for air. (More about that below)
The EAD Rule – What it Does and Doesn’t Do
It’s tempting to say “There’s nothing equivalent about the Equivalent Depth Rule”, but that would be incorrect. In your certification classes, you learned to calculate equivalent air depths for Nitrox (in case you happen to have your air tables, but not your Nitrox tables handy). Some of you may even still remember how to do those calculations. What the calculations actually do is produce equivalence for the nitrogen partial pressure being breathed in from air and Nitrox at their “equivalent” depths. Or, to put it in less technical terms, you’re breathing in the same amount of nitrogen from Nitrox as you would be from air at their equivalent depths. So, in that respect alone – the amount of nitrogen being inhaled – the depths are in fact equivalent.
The problem with the application of the EAD Rule comes from the idea that, since DCS risk stems from excess nitrogen in the blood and tissues, if the nitrogen partial pressure being breathed is the same for two gases (e.g. air and Nitrox) at two different depths, then the risk is the same for these gases at these two depths, provided the same time is spent at these depths. Unfortunately, this idea is overly simplistic. While the rule equalizes the nitrogen entering the system through the lungs at the equivalent depths, the amount of excess nitrogen that builds up in tissues depends additionally on all parts of the dive profile, not just the deepest portion. As just one example, if both dives include (as they should) a safety stop at 15′ for 3 minutes, less nitrogen is entering the system from breathing Nitrox than from breathing air during the safety stop.
The great advantage of probabilistic decompression theory is that it elevates the discussion of decompression risk to a quantitative level. By using it, “risk” becomes calculable, with actual numerical values assigned to it for specific profiles. But, even thinking in terms of whatever model of decompression you are currently most familiar with, you can see that, by focusing solely on the pressure of gas breathed in at depth, and ignoring the full on- and off-gassing of the tissues, the EAD Rule is unlikely to actually result in equivalent DCS risks for profiles at the “equivalent” depths.
Comparing DCS Risks for Air and Nitrox at “Equivalent” Depths
“Equivalent” depths for Air and Nitrox will, of course have identical NDL’s (i.e., allowing the same bottom times at their respective actual depths). So, for example, a 32 Nitrox dive to 75 fsw is treated the same as an air dive to 60 fsw. They share the same maximum bottom time of 55 minutes. What they don’t share is the same risk of DCS. As shown in the table below, the air dive for 55 minutes will have a risk of .32 percent (or, roughly, almost a 1 in 300 chance) of DCS. The Nitrox dive for 55 minutes will have a risk of .089 percent (less than a 1 in 1000 chance) of DCS. On these two “equivalent” dives, the DCS risk on air is more than 3 times the DCS risk on Nitrox.
The table below illustrates the differences in DCS risk between supposedly equivalent dives on air and 32 Nitrox. All the dives include a stop at 15′ for 3 minutes. The probability calculations are based on SAUL. (A link to the SAUL Dive Planner is in the header of this page.) The Nitrox dives shown in red print (the three deepest) are included solely for the DCS risk comparison. NEVER DIVE THOSE DEPTHS ON NITROX. The risk there is not of DCS – which, as you can see, is minimal – but of oxygen toxicity. Not much is known about the degree of risk for oxygen toxicity or what other factors may affect it. But two things are clear. First, in the recreational dive range, the risk is very much greater with Nitrox than with air, where it is almost negligible. And, second – but more important – the result of oxygen toxicity at depth is almost invariably fatal (by way of convulsions and loss of consciousness, leading to drowning). For this reason, I would recommend an even more cautious approach of using 32 Nitrox only under 120 fsw.
COMPARISON OF DCS RISK FOR AIR AND 32 NITROX AT THEIR EAD RULE EQUIVALENT DEPTHS
(DCS risks calculated using SAUL and all dives assume 60 fsw/min ascent and descent rates and include a stop at 15′ for 3 min.)
“equivalent” Depths(fsw) PADI (NDL) Risk of DCS (%)
Air 32 Nitrox BT (min) Air 32 Nitrox
50 63 80 .21 .028
60 75 55 .32 .089
70 86 40 .40 .124
80 98 30 .41 .133
90 110 25 .47 .173
100 121 20 .41 .118
110 133 16 .30 .060
120 144 13 .20 .016
130 156 10 .080 0.0
As you can see from the above table, the “equivalent” depths for air and Nitrox are far from equivalent in terms of DCS risk. The good news is that Nitrox is a lot safer in terms of DCS risk than current practices assume.