Deco PPO2

[JohnD]

what PPO2 do people use, I am having an argument with my DO as he wants to stick to 1.4 and I like 1.6 on deco stops. gets me out of the water quicker and I am usually nowhere near my 80% CNS for an individual dive. comments? pros and cons, please.

[Juz]

Personally, and I honestly feel deco is a very personal thing, when I was on OC, I used 70% from 12m, giving me a PPO2 of 1.54, but then the PPO2 dropped to 1.33@9m and to 1.12@6m.

I found I preferred this than to going onto 80%@9m.

It's what worked for me.

Juz

[Narked@50]

Hi John

Use what you like if you feel safe with it, but your DO is correct in insisting you do 1.4 for club dives as that is what the BSAC recommend, and he is acting in your best interest, not out of spite.

If your doing this kind of diving I am sure you already know the pros and cons?, if you don't, you should definately be on 1.4!

James

[JohnD]

I know lots of pros and cons. just wanted to hear other peoples advice. for the 2 or 3 minutes at 1.6 (or slightly over) the CNS loading isnt too bad wouldnt want to do it for longer though, on the profile I planned it gets me out of the water 8 minutes earlier. PRO & CON!

[nigelH]

Originally Posted by JohnD

I know lots of pros and cons. just wanted to hear other peoples advice. for the 2 or 3 minutes at 1.6 (or slightly over) the CNS loading isnt too bad wouldnt want to do it for longer though, on the profile I planned it gets me out of the water 8 minutes earlier. PRO & CON!

Wooo. Can you quote a plan showing 2 or 3 mins at 1.6 saving 8 mins total deco over 1.4 please. I'm having problems envisaging one. Maybe I'm missing something.

I tend to plan with 1.3 as it's a YBOD and then boost toward 100% at 6m giving 1.5 and a bit and call it conservatism. But then I'm talking half an hour plus not 2 or 3 minutes.

nigelH

[Padowan]

Originally Posted by JohnD

what PPO2 do people use, I am having an argument with my DO as he wants to stick to 1.4 and I like 1.6 on deco stops. gets me out of the water quicker and I am usually nowhere near my 80% CNS for an individual dive. comments? pros and cons, please.

Getting out of the water fast is not necessarily what you are aiming for - getting out "clean" is. Depending on what you are planning as a bottom PPO2, you will probably get a better benefit, from a "speed" perspective from not ongassing it in the first place (ie. planning a bottom PPO2 of 1.3-1.4).

On the YBOD, running 1.3bar for the whole dive vs bumping it up on deco to 1.5-1.6 like Nigel, only decreases my average total deco obligation by about 5 minutes or so on a 30 minute hang, personally I'd rather do those 5 minutes and be more conservative, than get out faster.

Remember the earlier you get on the boat, the more people you have to help out of their kit!!

[Mark Chase]

OC I use 1.6 for deco starting at first gas switch. I prefer to use 80% for my rich mix to keep CNS below 100% and to overcome certen logistical problems of using 100% so my 6m stop is 1.3 but if the dive alowed I would be happy on 1.6.

This worked for us

On CCR I am using 1.3 for the whole dive at the moment but deco is NOT cleen so I am working on a few diferent aproaches to sort this problem out.

ATB

Mark Chase

[PrettyFishies]

"On CCR I am using 1.3 for the whole dive at the moment but deco is NOT cleen so I am working on a few diferent aproaches to sort this problem out. "


If your deco is not clean then it's time to try whacking the O2 up high on 6m deco, why not try doing the working ( or bottom time) part of the dive on 1.0 bar and then whack it up, for the 1.0 bar O2 you really won't rack up much extra deco on top of what you would at 1.3. This keeps CNS loading down then you can go on high O2 for long periods at 6m, it sounds like you need a good O2 flush round the bags at 6m then stay at 1.6 for entire deco. If you're really brave you could hammer it up higher but then I'd never recommend such a dangerous practice.

Don't bother with air breaks, disco dancing heats you up

Seriously, try this and I'll bet you'll get out cleaner and less fatigued. Also, keep He in deco mix, flushing with air and other stupid practices will get you bent/ sub clinical.

OR..you could try heliox.

[iainmsmith]

Originally Posted by JohnD

what PPO2 do people use, I am having an argument with my DO as he wants to stick to 1.4 and I like 1.6 on deco stops. gets me out of the water quicker and I am usually nowhere near my 80% CNS for an individual dive. comments? pros and cons, please.

If you're talking about shallow deco, I suspect using pure O2 (in preference to 80%) is probably more important in terms of a "clean" deco than 1.4 vs 1.6, in that you maximise the inert gas offgassing gradient. If using O2, then the extra 0.2 bar would seem primarily useful for keeping inert gas in solution. All else being equal, I'd rather deco on O2 at 4m than 80% at 6m.

Fortunately BSAC now lets you use O2 at 4m (see
http://www.bsac.org/techserv/o2use0304.htm) so if you're qualified to use it then you're presumably knowledgeable and experienced enough to make your own decisions as to what you do under the water.

If you're talking about 50% at 21m vs 18m, I'm not sure there's a great difference if diving on a nitrox backgas, though on trimix, I like to decrease the inert gas fraction as soon as possible. If you are doing extended gas switch stops from dives with a nitrox backgas, it's worth looking at the compartment loadings that result - some of them increase reasonably significantly, although you may reduce microbubble formation.

From your DO's point of view, he technically has to remain with BSAC Safe Diving Practices otherwise he is putting himself in a legal grey area in the event of an incident. Some DO's are happy to do so (especially if they themselves are qualified beyond BSAC limits and are on the same dive) others are not. If he tells you not to do violate BSAC guidelines and you choose to do so, it would make sense not to rub his nose in it - which means you and your buddy not talking about it - otherwise he would be quite entitled to restrict your diving for ignoring him. At the very least, it would be courteous not to publicise the fact that you are going against his wishes.

Iain

[NotDeadYet]

I choose gases to give 1.0 bar ppO2 during the dive for trimix or a max of 1.2bar on EAN and then hike them up on deco. I use O2 at 6m on most deco dives and to be honest I never bother calculating the %CNS thingie, even for long dives. I'm not even convinced the whole idea that the percentage concept even works, it has no experimental basis and was just thought up by IANTD when they were writing the courses. In my view, there are more factors at work. For example, you can tolerate 1.6 ppO2 on a nitrogen based gas a lot longer than you can tolerate 1.6bar on pure oxygen. I just don't think it is as simple as trading off the various limits as in the %CNS idea.