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

[Sas]

Ok well I have never any kind of course on Nitrox or dived with it, or had to do the calculations but I have heard the 1.4 limit, with 1.6 as a contingency. I was confused by this when I read a bit about chamber treatment as one story I read had the chamber set to 18m deep whilst breathing 100% oxygen (Table 62 treatment) for 60mins, which would put the ppO2 at 2.8? I do know that people under going recompression have to be watched carefully to make sure they don't seize so I guess this could be because of the oxygen content of what they are breathing?

I heard that oxygen becomes lethal at ppO2 of 2, which I guess can't be correct then given the chamber treatment. As emttim asked, some links to proper studies would be cool if anybody has some as I am interested now. The google results are many and contradictory.

 

[sphillips3]

Chamber rides are a completely seperate issue. During a chamber ride you are:

1) Resting
2) Under direct medical supervision (usually an attendant in the chamber with you)
3) At no risk of drowning if you do Ox-Tox
And
4) Are being treated for a potentially fatal medical condition (embolisim and/or DCS)

In addition, I believe that the chamber protocol involves air breaks but I'm no expert in this area.

What is done in the chamber has essentially no relevance to what is an acceptable limit for a recreational activity where the consequences of a mistake involve death.

 

[battles2a5]

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.

Sure the convulstions are seem rather harmless.... if you weren't underwater. If you seize underwater the reg will likely fall from your mouth and your buddy's best efforts probably won't be able to get the reg back in your mouth. And after the seizure stops you should be able to breathe through a reg just fine while unconscious. You will be hard-pressed to find cases of a successful tox rescue. If you seize underwater with standard OC gear, it probably will not end well.

High PO2 exposures in the chamber are different story. It's a medical treatment for one thing. And you are not underwater, you are being tended to by a technician, and the exposures are carefully times. And if you seize, well, you just seize you don't drown.

 

[Sas]

Chamber rides are a completely seperate issue. During a chamber ride you are:

1) Resting
2) Under direct medical supervision (usually an attendant in the chamber with you)
3) At no risk of drowning if you do Ox-Tox
And
4) Are being treated for a potentially fatal medical condition (embolisim and/or DCS)

In addition, I believe that the chamber protocol involves air breaks but I'm no expert in this area.

What is done in the chamber has essentially no relevance to what is an acceptable limit for a recreational activity where the consequences of a mistake involve death.

Ok cool thanks. What I thought was, that ppO2 of 2 was lethal under any circumstances so now I know it is not. I was not meaning to suggest that a chamber ride would be equivalent to a dive with regards to ppO2 Still interested to hear what kinds of pp02s are possible under different circumstances though.

 

[Sas]

For a dive to 130', I'm using either a blend of EANx28 or EANx26.

Why do you use those mixes when diving to 130'? Do they give you that much extra bottom time to be worth it? I am not sure so just asking how long you've spent at depth with those mixes.

 

[sphillips3]

Here is a link to a document that shows the results of a study on the rate of Ox Tox (Both Paul Bert and Lorraine Smith) in chambers. They studied exposures up to 2.9 ATA and the study says that exposures up to 3.3 ATA are permitted under the protocol being studied. The patients in the study were all being treated for diving-related injuries.

One interesting result is that females appear to be at a significantly greater risk of a toxic event than males, at least for the parameters of this study. I make no assertion that this makes any difference in the diving environment.

http://archive.rubicon-foundation.org/dspace/bitstream/123456789/4010/1/15485081.pdf

 

[Blackwood]

1) Resting
2) Under direct medical supervision (usually an attendant in the chamber with you)
3) At no risk of drowning if you do Ox-Tox
And
4) Are being treated for a potentially fatal medical condition (embolisim and/or DCS)

5) You are dry

There seems to be some physiological response to being underwater that makes you more susceptible to toxing. I'm not sure it's been explained, but it's certainly been observed.

 

[Rick Murchison]

Whew!
Lots of good, bad, and indifferent info in this thread...
Not surprising considering what we "know" about oxtox hits.
I'll qualify this list with "AFAIK" - that is, from what I've been able to glean from the available literature as "fact" - and what's floating around out there as "anecdotal evidence."
(1) As TS&M has already said, the spread on the data is positively daunting; one might say even baffling. Within the general population there is a positively tremendous variation on susceptability, and experimental data shows that even the same individual may have a very wide variance from day to day... In the one intentional high PO₂ chamber experiment I know of, subjects were exposed to pure O₂ at 3.1 ATA and held there (performing various "normal activity" tasks) until symptoms were felt or observed. Times from initial switch to oxygen to symptoms varied from 5 minutes to over 2½ hours! But even 5 minutes at 3.1 isn't "a few seconds if you exceed 1.6"...
This vast variance in susceptability is also supported from the decompression treatment oxtox incidents report already cited.
(2) The Navy's limit for surface-supplied mixed gas dives has been pulled back to 1.3 ATA.
(3) As for confirmed cases of oxtox in the absence of drugs, AFAIK, within the NOAA time limits, there has been one case of a hit in an individual at 1.6 ATA who had previously exceeded 1.6 ATA and then ascended back to 1.6; there have been zero cases of hits where an individual has exceeded 1.6 but safely returned to 1.4 or shallower, and there have been zero cases of hits when 1.6 has not been exceeded.
(4) There is no study (yet) validating a correlation between the use of drugs and oxtox susceptability, but there is a mounting pile of anecdotal evidence that drugs that "heighten nervousness" (pseudoephedrine is the main contender here) may lower oxygen tolerance and increase susceptability to oxtox.
(5) CO₂ levels can get high enough to cause convulsions at normal (.21ATA) oxygen levels - which one causes the convulsion is really irrelevant, eh?.
---
(6) It is my opinion that there are individuals who are "borderline epileptic" who may be susceptable to oxtox at any elevated PO₂.
---
So... what do I do? And what am I willing to do?
(a) My planned MOD is based on 1.4 for bottom gasses; 1.6 for deco gasses
(b) About the only drug I'll dive with is an aspirin.
(c) If I accidentally exceed my MOD for a few seconds I don't get excited about it, just swim back up to where I'm supposed to be, and if in deco, add a backgas break.
(d) It'd have to be a mighty expensive piece of equipment to get me to go below 1.6 to retrieve it, but I ain't saying I wouldn't.
(e) If it's a lifesaving situation, I wouldn't hesitate to go to 2.0 or even more for a few seconds to retrieve someone.
(f) I try to avoid heavy work at all times, especially underwater. If I plan to do any heavy work underwater, it ain't on nitrox.

Rick

 

[Blackwood]

Another thing to note about chamber rides. While you may be able to go for a few hours at at PO₂ of 3ATM without having a CNS hit, your lungs certainly aren't going to feel too good when you're done

(2) The Navy's limit for surface-supplied mixed gas dives has been pulled back to 1.3 ATA.

Do you happen to know why they did that?

Was it for "instant CNS hits"? Or because surface supplied divers are generally exposed for a long time (i.e. CNS clock)? Or was it for pulmonary issues? Or a cost savings?

 

[Ken abucs]

it's difficult when a lot of places just pump 32, but it really makes sense to just pick a different mix in this case.

Even if your shop banks 32%, they can easily mix lower percentages. For example, if you fill 2400PSI of 32%, and 600PSI of air, you get 30%.

My shop does this all the time, usually because they don't have enough EAN32 left to fill my HP tanks. So I often end up with 29-30% EANx.

There's no excuse for having the wrong gas for the planned dive. As others have mentioned, the penalty can be pretty steep.