1.4 ....1.6...or ?????

[battles2a5]

I am fairly certain the rebreather incident is a completely separate accident, not the toxing one that has been mentioend.

 

[sphillips3]

In fairness, that guy was switching from a rebreather to a open circuit at the end of a 4000ft dive solo, then back. When he got back his rebreather was flooded. Other craziness ensued. OxTox was not the problem here.

Nope - not that one either. The name of the diver in the event I am refering to was Jeff Thompson - it was a standard open circuit nitrox cave dive with buddies. Nothing special and everything was within accepted standards - I think that's why it got so much attention.

 

[Rec Diver]

Most technical divers use 1.6 for extended periods of time without any issue at all as a matter of routine.

I use 1.3 for bottom gas and 1.6 for deco.

 

[mkutyna]

The incident with the female cave diver is written up here:
http://www.divers4life.com/TheCrack.html

Here's a ScubaBoard thread about Jeff Thompson:
http://www.scubaboard.com/forums/accidents-incidents/209504-fatality-ginnie-yesterday.html

The specific incident I was referring to in my original post was the death at the Crack. After re-reading the account, it seems I was wrong about the ppO2 which was 1.4.

I still wouldn't want to be testing out my personal ox tox threshold at 130'.

 

[Blackwood]

I use 1.3 for bottom gas and 1.6 for deco.

String clearly meant "for deco." Many long+deep dives will necessitate decoing (constant 1.6, less backgas breaks) for longer than I imagine these two dives lasted in total.

And the line between working and not-working can easily be blurred by heavy currents.

 

[wedivebc]

As with so many things in diving (like the NDLs, for example) there is no hard and fast line where you can say that 1.4 is "safe" and 1.5 is "not safe". The studies that were done to look at oxygen toxicity showed that there is very wide variability between divers, and more importantly, with an individual diver on repeated exposures. The variations were really huge -- it's quite daunting to look at the figures.
.

One theory I have heard is if helium is a CNS stimulant as HPNS seems to suggest and pseudoephinephrine is also a CNS stimulant is it reasonable to be concerned about high PO2 when diving high He as opposed to when diving with a CNS depressant such as nitrogen?
Do CNS stimulants compound the effects of each other?

 

[Charlie99]

If you sucked in a 2.5 chances are youd have an issue near immediately. Below that and the clock is more of an issue.

While it's not a very good idea, a ppO2 of 2.5ata isn't likely to cause any immediate problems. While oxtox events sometimes happen in chambers at their normal ppO2 of 2.8ata, going into convulsion isn't by any means a universal experience of divers being treated.

Oxtox underwater often leads to drowning so it is wise to be very conservative, but a momentary spiking of ppO2 isn't as certain to cause a problem as most divers seem to think. Too many conversations on CNS oxtox gloss over the fact that it is a time-dose sort of problem, where both ppO2 AND length of exposure are important.

You only have to go back 40 years or so to find limits of 2.0+ ata that were used with occasional incidents. For example, the Belgian Navy had a 2.3ata ppO2 limit, based upon experiments and testing. The USN "excursion limits" for their O2 rebreathers also allowed brief periods at depths well beyond 2.0ata. I've not advocating routine use of these limits --- just trying to point out that short spikes of high ppO2 are NOT the instant death sentence that some divers believe they are.

==================================

My contingency limit is 2.2ata ---- that's how deep I'll go for a brief period to assist a diver in a life threatening situation. With EAN32 and the single tank I use, for all practical purposes my gas volume sets my emergency depth limit, not CNS considerations.

A good question for us each to ask ourselves is at what ppO2 do we abandon our buddy.

Charlie Allen

p.s. someone (Dr Deco??) posted a graph of ppO2 vs time to convulsions of 10% of test subjects. At 2.0ata that was about 75 minutes. At 4.0ata ppO2, about 10% have convulsed at 15 minutes, at 5ata about 8 minutes. If I can find the source, I'll post/link it.

 

[String]

Agree with that. I just plucked a number out of the air and was too low. Was trying to point out that for normal ranges its more of a time than acute risk.

 

[TSandM]

I have seen discussions of deep air where it has been suggested that part of the reason divers seem to tolerate high ppO2s on deep air is the depressant effect of nitrogen. It makes some intuitive sense. I'm not sure which is safer, though -- being deep and stupid, or being deep and likely to seize. Low ppO2s, He, and doing the required deco seems like a better answer to me. (Or my strategy, which is to stay shallow enough that it's not an issue )

 

[emttim]

I recently did two dives with two dive buddies. Both dives were to 130 ft. I was diving air so I was not concerned with oxygen toxicity...however the other divers were diving eanx32. I mentioned to them at the start of the first dive that I thought they were pushing the depth limits for eanx32 as 110ft is the max depth at 1.4 ata and 130 is right at 1.6 ata.
They both expressed that those numbers are considered too conservative and that the Navy has indeed abandoned those numbers. Personally , I'm sticking to the 1.4. Any opinions???

I would never go above 1.4 except in a contingency as 1.6 was designed for. Sure, the convulsions themselves are relatively harmless, but at 130'....yeah screw that. Seems like some of the responses on this thread indicate that ppO2s in excess of 2.0 are possible without convulsions, as well as some thinking 1.6 is too conservative, which may or may not be true. However, until I see literature that does indeed prove 1.4 ppO2 is too conservative, 1.4's the magic number I'm sticking to. For a dive to 130', I'm using either a blend of EANx28 or EANx26.