Posted by Karl S. on July 06, 2003 at 15:28:09:
seahunt (small-s) mentioned xeno-gas for diving after purple striped sea urchins in an earlier post.
Having re-though nitrox (see earlier post), I have also re-thought trimix as well.
From discussions by Florida IANTD tech divers, I have come to view the perceived need for trimix as akin to the view of the perceived need for nitrox. In other words, do you really need this enriched gas?
It is strictly a perception. Repeated often enough, it becomes a doctrine and then a dogma and finally a presumed self-evident truth, according to the law of the fallacy of affirming the consequent.
First of all, what gas should you put in your scuba tanks?
NAUI speaks to this issue fairly directly by stating (or rather, NAUI's tech officials state) that air is the preferred gas for scuba for depths down to 50 fsw, and that enriched air nitrox is appropriate for depths deeper than 50 fsw, down to whatever depth corresponds with the partial pressure of oxygen to a maximum of 1.4 atmospheres absolute, and no deeper. Or at least so I have been told by NAUI.
TDI teaches the same concept, or at least so I have been told by two independent TDI instructors.
The issue viewed in the strictest terms is: to what depth is a given nitrox mix safe? The presumption being that more oxygen in the mix equals less inert (or quasi-inert) gas in the mix and therefore less risk of inert gas decompression sickness.
For myself, I have come to view air as the primary mix for all diving, with the only variable to the air mix being the adjustment of the partial pressure of the nitrogen and the partial pressure of the oxygen for safe survival at whatever depth the mission of the dive takes you, and the use of enriched air nitrox for the return trip.
And I do not view a maximized partial pressure of oxygen as necessarily a good thing, just because it lowers the amount of inert (helium) or quasi-inert (nitrogen) gas in the mix.
Here is my list of preferred dive mixes, then:
100% air for dives to 150 fsw;
40% helium & 60% air for dives to 210 fsw;
50-50 helium & air for dives to 260 fsw;
60% helium & 40% air for dives to 330 fsw;
70% helium & 30% air for dives to 460 fsw.
For the return trip, some combination of nitrox is in fact highly desirable if not absolutely required.
NOAA somehow came up with EAN36. By some coincidence, this is a perfect nitrox mix for switching to and from at 100 fsw, where the PO2 is 1.45 for this mix. It works as a travel mix going down, when your Heli-Air is too hypoxic to breath from the surface, and it works as an initial deco mix starting at 100 fsw up to whatever depth you next deco mix safe to breathe at.
Since it is perfectly feasible to sling as many as 4 deco tanks, that makes 3 different deco mixes besides EAN36 possible. You can choose whatever you think is best.
So, to answer seahunt's (small-s) question about the xeno gas, I myself would plan the dive as follows:
1) what is the mission of the dive? an external survey of a shipwreck or oil platform or reef etc? the recovery of items from depth? a penitration dive for internal exploration? a penitration dive for internal recovery of an item or items?
2) what is the MOD of the dive? convert this to ATAs and plan the Heli-Air mix to minimize the ppN2 to 3.5 atas or some other comfortable level.
3) what is the buddy plan for the dive? solo individual independent self-sufficient divers, or buddy pairs, or buddy teams of 3 or 4 divers?
4) what is the gas management plan for the dive? rule of thirds for back gas? rule of halves for deco gas? where will the backup tanks be located? will there be teams of safety divers and at what depths?
5) how good is the equipment? has it been serviced recently?
6) what is the maximum feasible time we can each spend at maximum depth?
7) what is the P(max) divided by 1.6? this will be the first deep stop depth. (note: 20 fsw has been found by the USN and by NOAA to be an indefinite NDL, which is where we get the 1.6 factor: 20/33 + 1 = 1.6)
8) what does the deco software (Abyss or DecoPlanner) say about the next several deep stops?
9) when will we switch to the first deco mix? how much gas will we need for the phase of the egression on this deco tank? how much extra gas do we need besides deco in order to use this tank as a travel mix as well?
10) what other deco mixes will we need, and at what depths will we switch to them? how much gas will be needed, and what size deco tanks should we use? 40s 63s or 80s?
11) what is the total time underwater including all deco time, and is it too cold to dive for this long given the temperature of the water? do we have to modify the bottom time and deco plan?
Back to my original point:
No nitrox is ever needed on any dive shallower than 50 fsw.
EAN36 is a great mix for returning from dives deeper than 50 fsw, and some people assert it is a good mix for diving to and from 100 fsw as well. That also makes it an excellent travel mix and lean deco mix, although leaner deco mixes would work too.
Other EAN mixes may be required for returning from dives deeper than 150 fsw as part of the decompression profile.
You really dont need nitrox for diving shallower than 150 fsw, except for the egression phase, in which case it helps you to off-gas inert gas better than air. Therefore slinging a stage tank with EANx in it works better than putting nitrox into your main back tank(s).
You really dont need trimix. Heli-Air works just fine, if not better, than trimix.
OK that should open up Pandora's box, nice and wide, and invite all sorts of flames! Flame on.
seahunt, did you know what you were getting yourself into? What do purple striped sea urchins taste like? Are they any better than the black sea urchin uni we had at The Sea Ranch that we freedived for together?
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