Decompression Planning

[DRE]

"I have an advanced open water certification, with basic NITROX to 40%. I am in no manner an intructor or a guru. I post this info based upon what I have learned, and would like to have an open dialog.

My first point in planning this dive would be to figure the "best mix" for the dive, assuming that NITROX is the only gas used, and that only one dive is planned.

To mimimize nitrogen exposure, which will reduced decompression obligation and also the effects of nitrogen narcosis, I will want to maximize the oxygen content of my breathing gas, as long as I stay withing the oxygen exposure limits."

Thinking that a higher O2 % in your mix will actually reduce the narcosis is old school thinking. Although the reasoning behind it seems correct, the reality is that it doesn't (just try it: mix up some trox and also have a tank of air and see how you feel). Instead of talking about Equivalent Narcotic Depth for a nitrox mix (which I'd reserve for helium based gas mixtures), I'd rather talk about Equivalent Nitrogen Depth as the only benefit is in terms of deco.

"For this dive, I have chosen 28% O2 for my best mix. At a depth of 130 ft., my PO2 should be about 1.4. This should give me a single dive limit of about 150 minutes, which should be plenty to complete the dive.

Running this dive plan on my PC shows a 2 min stop at 30ft, 3 min stop at 20. and a final stop at 15 ft fot 25 minutes, for a total dive time of 64 minutes."

What software are you using. I don't like this profile very much, as it pulls you to close to the surface too soon. I'd prefer to have my first actual stop at around sixty feet, or about halfway from max depth, with 10 second stops starting at 80% of max depth. Your total run time for the dive would be very similar, but the distribution of your stops would be different. The deeper you start your stops the cleaner your deco will be.


"With a working SAC of 0.70, and a DECO SAC of 0.50, this equates to a gas consumption of 127cuft.... If using the "Rule of Thirds", you couldn't find cylinders large enough to complete this dive. What have I figured wrong ????"

You haven't, that's the amount of gas you need, which means you have to carry at least a set of doubles (which is necessary for any real deco dive). Bear in mind that the rule of thirds demands that you keep 1/3 of your gas supply for emergencies, which in this case means the 127cf of gas consists of 2/3 of your total volume.
That's why for dives like this an Advanced Nitrox/Deco Procedures course is very beneficial.