Why not 30%?

[DA Aquamaster]

I ran several nitrox blends from 21 to 32% through decoplanner for a 3 hour dive at 100 feet with a single deco gas at 100% O2. The O2 toxity clock stayed under 100% during the dive for all mixes (of course dropping for lower PO2 mixes), but in all cases exceeded 100% during deco with the final O2 clock upon surfacing being virtually the same for all bottom mixes. The reason for this was that dives on lower PO2 ran longer deco at the higher PO2.

Bob

We played with basically the same thing today and it is true that at best you only save a few percent on the CNS clock if you back off on the PO2 - at least with the more conservative models in general use today.

On the other hand, there are advantages to having a lower PO2 on the higher work portions of the dive, and on deco, you can at least add in some air breaks.

It does however also creates a potential argument for pumping 80% as a deco gas rather than 100%.

[atedeschi]

I agree that it does not save your CNS clock much to grant the change in banked gases, and i heard about the working part of a dive. Where are you diving that you are at 110' and working hard? No to 80%!!! Just use 100%, I never saw any benifit to it or why people use it.

[amphipod06]

There was at least one, 3/9/02. It is on the IUCRR website.

There was a more recent one, can't recall the exact date but it was after Halloween back in 2007... witnessed by the buddies.... two diff gas analyzers at the scene... 32% +/- 0.5%...

Dive safe,

Celia

[amphipod06]

I use 1.3 which gives me @ 1.25 in the loop. How long are your dives running? In my old age I don't do dives much over 3 hours.
Bill

I try to keep it at 1.0-1.1 during the dive and then up it while on deco...
Avg dives 120 mins long...

Dive safe,

Celia

[adam0321]

There is a HUGE difference in the propensity for 02 tox seizures during exertion, and resultant increased metabolic demand, as opposed to the resting state where the baseline metabolism is low. Patients at rest in a hyperbaric chamber can tolerate 2-3 ATA of pure 02 without seizing; this level of 02 would invariably induce seizures with even minimal exertion during a dive.

There are many unknown variables resulting in ones' individual susceptibility to 02-induced seizures, but one of the most recognized risks is the increased metabolic demand of exercise.

FWIW, I dive 31% in Ginnie, just not comfortable with 32% given the exertion levels necessary at times.

not to mention there is a huge difference not just in a chamber but in the difference between a wet diver and a dry diver. It has to do with the body being suspended in fluid. for instance we teach in the Marine Corps that cns o2 tox becomes a risk at 1.3 ata in a wet diver and 1.6 ata in a dry diver. Now I have done treatments in a chamber that divers have been subjected to ppo2 levels reaching 2.0 ata. On a tt6(treatment table) for instance a diver is slammed to 60fsw as quick as possible then put on o2 right away. the length of the tt with out extensions is 4hr48 minutes which they are on o2 for the entire table with brief air breaks. Now this table is used to treat serious dcs, Arterial gas embolism, or symptoms reacurance. So you take a diver that has been on a 95 minute dive @20fsw with a mk-25 which is a 100% o2 rebreather then throw him in a chamber on a tt6 for age and his ppo2 is going to be well above 2.0 ata and we have seen this happen with no signs of cns o2 tox and then again we have seen it in divers with much less exposure. the moral being everyone is different

[divindoc]

not to mention there is a huge difference not just in a chamber but in the difference between a wet diver and a dry diver. It has to do with the body being suspended in fluid. for instance we teach in the Marine Corps that cns o2 tox becomes a risk at 1.3 ata in a wet diver and 1.6 ata in a dry diver. Now I have done treatments in a chamber that divers have been subjected to ppo2 levels reaching 2.0 ata. On a tt6(treatment table) for instance a diver is slammed to 60fsw as quick as possible then put on o2 right away. the length of the tt with out extensions is 4hr48 minutes which they are on o2 for the entire table with brief air breaks. Now this table is used to treat serious dcs, Arterial gas embolism, or symptoms reacurance. So you take a diver that has been on a 95 minute dive @20fsw with a mk-25 which is a 100% o2 rebreather then throw him in a chamber on a tt6 for age and his ppo2 is going to be well above 2.0 ata and we have seen this happen with no signs of cns o2 tox and then again we have seen it in divers with much less exposure. the moral being everyone is different

Agreed! Your last line says it all - "everyone is different" - we know what the tolerable levels of 02 exposure are for the general diving population, but there is a wide range of individual variation in that equation, so what's right for me may not be right for you or the next guy. Being conservative with pp02 just makes common sense.

[Bob Cree]

Agreed! Your last line says it all - "everyone is different" - we know what the tolerable levels of 02 exposure are for the general diving population, but there is a wide range of individual variation in that equation, so what's right for me may not be right for you or the next guy. Being conservative with pp02 just makes common sense.

And further to that - based on other published Navy testing I have read - not only is there high variability of onset of CNS O2 toxicity symptoms from person to person, but also for the same person day to day...what is right for you today may not be right for you tomorrow....so maybe, "everyone is different every time" better describes it.

Makes coming up with hard rules that everybody wants, kind of tough to do. There is not much warning, and not much you can do except switch to a significantly lower O2 content gas if you are functionably able to and if you even have it or blow yourself to the surface and take the associated DCS issues if you can.

Particularly insidious.

As times and depths are pushed, be very, very careful folks...and my opinion, expect the problem to happen during deco, not on the dive.

Bob

[adam0321]

on deco?? hmm not sure about that. I guess its as good a place as ever. i would think you would be more likely to discover onset during an exertion enduced dive. For instance I know going into jb against the flow you are working harder. It is true that ones o2 toleration does vary day to day. And dive to dive. exertion,cold water,diet,exericse,dehydration etc etc. where I think the divier gets in trouble is they assume since every one is so different that there is such a "fudge Factor" built into the tables that since they have been diving xx amount of years or have xx amount of dives under there belt that they "can take it" . Now in my humble opinion the navy (and Marine Corps) tables are built using young healthy well trained divers. At nedu(naval experimental diving unit) they put the tables to the limit every day to see what the human body can take. Bit the bottom line is every one on any day is different. And the most important thing that I tell our students all the time is "there is no gutting out a diving injury"