Accelerated deco: low PO2 vs high PO2?

[DepartureDiver]

50% is a nice mix in that if I end up with a serious CF at 130-140 I can PROBABLY make a CESA to 70' where the 50% is breathable.

A point not to be missed. I was limiting my comment (which I didn't state) to deco optimization using one gas and the thread started off discussing 70% or 100%. Hopefully a CESA never occurs due to either redundancy or having a properly trained buddy.

 

[Rage]

As for Question #2, there is none. You didn't really mean CNS toxicity as another user showed using Vplanner? If so, the risk for CNS tox is greater with pp02 1.6 versus 1.12, but in deco, there is no known DCI protection using the lower 02.

Thanks Saturation, for the sticking with the question, straight answer and the links.

S.C.

 

[DA Aquamaster]

In open water, always carry all your gas with you. Also, while not a standard mix or "DIR", if carrying only one gas during a nitrox dive, consider switching to a 60-63% blend at 50'.

I agree with your reasoning as 63% does make some sense as it basically splits the difference between Nitrox 50 and Nitrox 70 in terms of depth it can first be used versus the benefit of max PPO2 at shallower stops. Much of it really depends on your profile and where the stops are occurring.

But overall for the deco diving I do, I agree with Genesis. 50% can be used on ascent from 70' up but in a things-went-seriously-wrong-situation, a short exposure during a relatively rapid ascent from 100' or even deeper is not going to present serious issues. It's extreme, but a valid consideration in regard to the fact that you are going to breathe what you got if there is nothing else and a breath or two during an esa from 150 to 70 ft could conceivably come in handy, and would be a lot safer than a significantly hotter deco mix.

From a practical real world standpoint, 50% also offers a large margin of saftey when used with a computer that does not accomodate gas switching and assumes the deco is on the bottom air or nitrox mix. Yet at the same time 50% does not hit you so hard on the CNS clock that it normally becomes an issue.

In my case if I am using a deco bottle it is most likely Nitrox 50 and there will be either Nitrox 80 or 100% in a safety tank hanging at 20' on the ascent line.

 

[padiscubapro]

hum: Switch to 50% @100fsw.....

It has to be a real good emergency to do that.... At a PO2 above 2, better get to 70 ASAP...


I think that both to 50% and 100% will help shorten the deco time with the added safety of having an intermediate mix. Also, if you are coming from a high He trimix (ex 18/40), having an intermediate trimix (30/20) could be a good solution.

Swithing to 50% @ 100 fsw would be for a OOA situation.. a 2.0 isn't that big a deal in itself.. If you are ascending (and it doesn't have to be like a rocket) your PO2 is dropping and your exposure is negligable.. I remember the days when I routinely used a 1.6 as a BOTTOM mix and deco'd on a higher po2.

The key is not to over exert yourself to keep co2 loading to a minimum...

High PO2 by itself will not trigger a seizure or other CNS problems, other factors such as exposure TIME and CO2 loading have to be considered.

I will be doing a series of dives in the near future that will require me to go onto oxygen starting at 30fsw (1.9 po2) to keep the runtimes in the 6 hour or so range.. (the o2 will be delievered by a FFM)

 

[Genesis]

Swithing to 50% @ 100 fsw would be for a OOA situation.. a 2.0 isn't that big a deal in itself.. If you are ascending (and it doesn't have to be like a rocket) your PO2 is dropping and your exposure is negligable.. I remember the days when I routinely used a 1.6 as a BOTTOM mix and deco'd on a higher po2.

The key is not to over exert yourself to keep co2 loading to a minimum...

Which is extremely unlikely (no CO2 issues) immediately following an OOA!

I would not want to switch to 50% at 100fsw. Would I, if I had no other choice? Yeah. Would I do it knowing that it might kill me? Yes. Of course if the alternative is CERTAIN death, any shot at living looks good.

 

[padiscubapro]

Which is extremely unlikely (no CO2 issues) immediately following an OOA!

I would not want to switch to 50% at 100fsw. Would I, if I had no other choice? Yeah. Would I do it knowing that it might kill me? Yes. Of course if the alternative is CERTAIN death, any shot at living looks good.

Its not at likely as you might expect (co2 loading), you might have a slightly elevated CO2 in the lungs(until you take a few breaths of the 50%) but if you aren't working its not going to be that high in your blood stream.. The falling PO2 should be enough to curtail any problems and if you aren't working you shouldn't be elevating you CO2. Now if you are busting your butt against a current, now thats a different story...
Too many people think a 1.6 is a magic number, remember in chambers they do a 2.5, and many divers have done dives with po2s that high.. also the narcotic effect at depth is likely to reduce the likelihood of fast CNS onset.. remember even on the extreme air reorcd dives noone has ever toxed like that, the liklihood of a tox is actually higher when you are shallower..
The record AIR dives have po2s in the range of 3+.

 

[Genesis]

True, but we're not talking about a depth where the PN2 is significantly protective, and the exact mechanisms by which PN2 protects and PO2 excites a CNS event are not all that well understood.

I'd do it if I had to, but I certainly wouldn't make a common practice of it....

 

[Saturation]

Thanks Saturation for your links...

This brings a reflexion to my mind:

If I can only bring one deco bottle, should I opt for:

1- a 72/28 Heliox and start my deco at 40 fsw and keep that mix at 30, 20 and 10 fsw

2- a 100 O2 mix from 20 fsw.

Since each inert gaz off-gaz independantly, having the He at 33 fsw should not slower the N2 off-gas.

V-Planner confirms that the 2nd option will save about 6 minutes (7%) of deco on one test I did with a 220f 20 min with 18/40 trimix.

$$$ wise, it's about the same, since I save on my bottom mix, the real difference is less than dollars (CAD) total.

Have fun!

Hi,

Right now, this is a personal choice and I still carry the 100% 02, if I had to choose but one gas. I think the jury's still out on how to use heliox best in technical diving.

Edmonds in-water recompression protocol, long term USN use with various 100% schedules, and ordinary chamber tables [TT6 and TT5] use 100% to good effect.

I have received email from very experienced technical divers who've added a bit of helium to their deco mixes, no real fixed amount, and they claim they feel much better after dives although they deco well without it. Its a qualitative sensation. Since they dive so often, they are most likely to feel the slightest change in its effect ... I have not heard of divers using heliox to improve on existing schedules [shorten time] or because the schedules are not working right [bends, niggles, off-ness] in shallower than 300' dives.

What it comes down to is if its really not a deco issue on regular dives, helium is 2-6x more costly per cuft than 02. However, I hear of inconsistent results of deco in dives in the 300' range [niggles, joint bends] enough that I would consider all deep deco gases as trimix for dives past 250'.