Posted by northcoast diver on January 11, 2005 at 11:17:41:
In Reply to: I agree posted by seahunt on January 10, 2005 at 14:15:53:
If you run V-Planner repetitive dive plans with 1 hour surface intervals indefinitely, you can see that any dive with an Equivalent Air Depth of 65 ft or less and with a single 80 cu ft tank (for an average person with a Surface Consumption Rate of 0.75 cu ft per min) allows you to dive as many repetitive dives in one day as you could ever want, without going into decompression.
Ergo, if you are diving to 65 ft or less, you really do not need Nitrox, unless you just really want the extra safety margin of a lower EAD.
If you are diving deeper than 65 ft, then EAN35 is probably your best overall mix, in order to lower your EAD to 65 ft, and thus be able to dive as many repetitive dives in one day as you could ever want.
If you change the assumptions, such as a bigger than 80 cu ft tank, or twin tanks, or surface intervals shorter than 1 hour, then everything else changes as well, in which case Nitrox could become more useful even at shallower depths.
For 2-dive boat trips, the Nitrox is not always worthwhile, particularly if the second dive is at a really shallow depth, as many boat captains are likely to do. There could be benefits, but probably not major benefits, to Nitrox for this kind of diving. HOwever if anyone ever wants to stretch the diving to 3 or 4 or 5 dives per boat trip, then Nitrox becomes a practical necessity in order to safely do so. In this manner you would be reducing the EAD to 65 ft or less.
When I dive the north coast, most of the sites are fairly shallow, at less than 50 ft. On those dives I use my air tank. It is not worth wasting the money on the Nitrox, in my opinion.
When I dive the central coast, most of those dives are on pinnacles that go all the way to the recreational limits. I order EAN35 from those boats. And with EAN35 I can dive all day, 4 or 5 dives, with no decompression problems. I normally stick to the NACD cavern limit of 100 ft, therefore EAN35 is perfect for those dives.
In a nutshell, Seahunt's general rule of using Nitrox on all dives is fine, particularly if he is diving pinnacles or else is diving a lot in one day.