Accidental DECO and mild panic in a non tech certified diver.

[Gareth J]

Thanks so much for finding and posting that article. That much being said, it's clear from the article -that shows only 1 relevant example- that the convulsions only occurred at a P02 of 1.6 after a dive of 40 minutes duration during which time there was heavy exercise during the entire dive. It's also not stated if the diver in the given example was using a single tank and a blend of at or less than 40% O2 (as per my stated parameters).

So I "might" need to clarify that there has "never been a documented case of an 02 hit on a single tank recreational (non tech) single tank diver breathing <40 percent 02 other than a test case where heavy exercise was done for 40 minutes and maintaining a 1.6 the entire time.

I was lazy and clicked on the first DAN document I found. Which is about OXTOX, the history etc. I skipped all the Navy studies and went for the first 'recreational' diving example. Its not a list of incident, just the data around Oxtox, and why the limits are where they are and the fact they are not as conservative as some believe.

I would really love to get my hands on the US Navy document I saw back in the 90's when I did my original Advanced Nitrox qualification. But that was pre internet. So I doubt its online. That was really scary.
When I started using Nitrox it was a bit wild west, a lot of brew your own. Most cylinders un marked because we weren't supposed to use Nitrox (according to our agencies), and the skippers wouldn't let us onboard in case the Nitrox cylinders blew the boat up.

When a diver such as myself sets their dive computer for 1.6, it is highly unlikely that they would be diving diving 1.6 the entire dive- if they even get that high at all. It's just the upper limit. I don't think I EVER hit 1.6 during a dive.



The article does not say that- what it does say is P02 levels ABOVE 1.6 are a credible risk. And there is no other research data in that article other than the 1 example that addresses the parameters I stated (and again we don't know the 02 blend or if it was a single tank) which is what we are addressing here.

You can work the blend out, they give a depth, and a PO2.
The problem with the incident, is it is a just a comment about a reported incident, not the details of the incident itself. It is there as an example of how boarder line the 1.6PO2 recommendation is.
I would love the full details, was the diver dry (in a chamber), on a bike, or a ffm supply controlled by the operator. Or in a pool on OC equipment. That's not clear.

The decompressing diver is real world. Warm water, drysuit, at rest, 100% O2 on a 6m stop. He will have been in the water a long time. Its not clear if this was OC or CCR. If OC he/she would have a variable PO2 during the bottom and ascent phase, probably between 1.2 and 1.4 on the bottom. If CCR, he/she would have had a fixed PO2, around 1.2 (possibly lower) probably throughout the dive until the stops.

It is really easy to have a high work load on a dive. Coastal waters have tides, caves have water flow. I've certainly overbreathed both my CCR and OC regulators in the past in tidal waters. I breathed a cylinder of gas in 2m of water trying to cut the propeller free on a dive boat once. They had to pull me out, I couldn't physically climb the ladder.

 

[caruso]

I skipped all the Navy studies and went for the first 'recreational' diving example.

But you don't know that it was a 'recreational' diving example. To clarify- I use the term 'recreational' to mean single tank, no decompression diving, at or below 130' and I also add "breathing at or below P02 1.6". There is nothing in the study that states whether the one example fits this criteria.

You can work the blend out, they give a depth, and a PO2.

Whatever those numbers work out to be, I'll bet that either the 02 level is higher than 40% and or the depth (real or simulated) is higher than my stated acceptable diving parameters.

It is there as an example of how boarder line the 1.6PO2 recommendation is.

You and I both read the same study, and I'm thinking the study does more to support my side of the argument (P02 levels at or below 1.6 in recreational diving are safe) and you of course feel the opposite. It's rather amusing if you think about it.

It is really easy to have a high work load on a dive..

The article makes it clear that exertion increases risk of an 02 hit. I'm not debating that in the least. My issue is with working hard AND maintaining P02 of 1.6 for 40 minutes. Most if not literally ALL single tank recreational divers breathing less than 40% 02 will never experience that. It's almost beyond the realm of possibility.

 

[Gareth J]

Whatever those numbers work out to be, I'll bet that either the 02 level is higher than 40% and or the depth (real or simulated) is higher than my stated acceptable diving parameters.

For clarity, the diver must have been breathing a 40% mix at 30m.
po2 (1.6) / absolute pressure at depth (4) = 40% mix

If you choose to dive the bottom phase with a planed PO2 of 1.6, that is your choice. There are no dive police.
But please don't say it is a safe option, or encourage others. Even in the early 90's when I did my Advanced Nitrox we where told not to do it, and we have learnt a lot of lessons since then, all of them say high PO2's during the bottom phase are not a good idea. Remember 1.6 - 0.1 for any adverse factor (depth, temperature, work, fatigue, stress, etc).

I will and have planned decompression schedules with a PO2 of 1.6 - but that is at rest, on the shallow stops. I have had off board bailout schedules with a PO2 of 1.6, but that is an immediately falling PO2, the bailout directly precedes an immediate ascent.

 

[caruso]

For clarity, the diver must have been breathing a 40% mix at 30m

So to summarize. The only example of an 02 hit that comes even close to my stated parameters, is a diver at a P02 of 1.6 under heavy exertion for the entire 40 minutes at EAN40 under what is most likely an artificial, controlled laboratory setting intentionally inducing a heavy workload + an unusually long P02 1.6 level at the highest possible concentration of recreational Nitrox, for the entire dive.

I'm not convinced that is sufficient proof to dispute my contention that diving a single tank within recreational limits on Nitrox at or below 40% with a computer set to P02 1.6 is anything other than within reasonable safe limits.

 

[Gareth J]

So to summarize. The only example of an 02 hit that comes even close to my stated parameters, is a diver at a P02 of 1.6 under heavy exertion for the entire 40 minutes at EAN40 under what is most likely an artificial, controlled laboratory setting intentionally inducing a heavy workload + an unusually long P02 1.6 level at the highest possible concentration of recreational Nitrox, for the entire dive.

I'm not convinced that is sufficient proof to dispute my contention that diving a single tank within recreational limits on Nitrox at or below 40% with a computer set to P02 1.6 is anything other than within reasonable safe limits.

No, to summarise.

a/ Typing into google gave me an instant paper by DAN that included this particular incident.
b/ US Navy found CNS hits where very unpredictable in combat swimmers when breathing high PO2's, both as a group and as individuals.
c/ UK Navy found CNS hits where very unpredictable in clearance and chariot swimmers when breathing high PO2's, both as a group and as individuals.
d/ NOAA also has concerns on high PO2 mixes. Giving an absolute maximum PO2 of 1.6, which should be modified by 0.1 for adverse factors. WRONG - See (g)
d/ All the diving agencies, including the technical ones, teach caution when using high PO2's. All only allow PO2's of 1.6 during the decompression phase of the dive when divers are at rest and under minimal stress.
e/ some diving agencies will not sanction PO2's above 1.4 for no-stop single cylinder dives.
f/ technical diving agencies teach keep the bottom phase of the dive below a PO2 of 1.4 when using OC, and under some conditions significantly lower.
g/ CCR diving is all done with a PO2 set point of 1.3 or lower, other than when at 6m where the PO2 may be raised to 1.6.
f/ DAN's guidance on OXTOX, states that 1.6 PO2's are actually quite aggressive and should be treated with extreme caution.
g/ The NOAA Diving Program's maximum oxygen partial pressure (PO2) level is 1.4 ATA for no-decompression dives. (from Google Search)

It appears to me that progressively, the maximum PO2 that was assumed safe has been reducing over time (Royal Navy Studies during the 2nd world war, and subsequent Navel studies since). A PO2 of 1.6 should be treated with extreme caution.


Post edited after google check. Statement (d) is wrong. NOAA does not allow no-stops dives with a PO2 above 1.4. See link and correction (g)

 

[caruso]

@Gareth J

Thanks for supplying the additional information about agency concerns and unpredictability, as well as the first article you instantly found when doing a Google search. That much being said, there's nothing new there.

 

[Bob DBF]

@GarthJ

Interesting read, it does seem that all the research was done during WWII, and shortly thereafter, with 100% O2 at shallow depth, and all the limits used by government and agencies are based on their interpretation if that research.

From the article:
"Dr. Lanphier's work is certainly compelling enough that divers should be very cautious before extrapolating oxygen exposure limits based on 100 percent oxygen rebreathing directly to nitrox diving at higher gas densities. Ideally, nitrox limits should be tested at the maximum gas density anticipated for their use."

That research on Nitrox at depth has not been done, or overlooked by the article.


Bob

 

[chillyinCanada]

Superlike!

More seriously though, is there *any* associated pain or tenderness and does the rash look like mottled bruising?

 

[Gareth J]

@GarthJ

Interesting read, it does seem that all the research was done during WWII, and shortly thereafter, with 100% O2 at shallow depth, and all the limits used by government and agencies are based on their interpretation if that research.

From the article:
"Dr. Lanphier's work is certainly compelling enough that divers should be very cautious before extrapolating oxygen exposure limits based on 100 percent oxygen rebreathing directly to nitrox diving at higher gas densities. Ideally, nitrox limits should be tested at the maximum gas density anticipated for their use."

That research on Nitrox at depth has not been done, or overlooked by the article.


Bob

I like the fact that they actual urge caution, because the initial Royal Navy figures where developed whilst they where involved in combat operations, in Alexandria in 1941

A typically British understatement - The initial limits "produced enough convulsions that the British Admiralty decided some sort of studies were needed to define the scope of the problem and, hopefully, find a solution."

The solution - whilst still involved in combat operations issued in 1943, was to have an absolute limited maximum exposure to a PO2 of 1.76.

A lot of those charioteers never made it home, both the Italians and British, and they new that before setting out.

 

[caruso]

More seriously though, is there *any* associated pain or tenderness and does the rash look like mottled bruising?

She says no pain, just itching and I had her look at pictures of the more severe form of skin bends with the mottling and she says it doesn't look at all like that. I spoke to her briefly this morning she was busy at work and said she's fine.