Deep Diving on Nitrox question

[boulderjohn]

My understanding is that it is a long descending swim through that begins around 80 feet and exits anywhere from 125 to 138 (I have seen different numbers given). So the vast majority of the dive, before and after, is well below the 1.6 limit and even the 1.4 PPO2 limit commonly used.

You were misinformed. The entrance is at about 90-95 feet IIRC, then you do a straight down plunge, then level off for the exit. The exit is very large, so you there can be a wide range of depths depending upon the path you take. Stay high and you might be at about 125. You will only be below 1.4 for less than a minute--probably well less than a minute.

To me, the reason for the nitrox is what follows. The first times I did it, we did it on air. We went down, did the passage, and then ascended. Frankly, it was not that much of a dive. When I later did it with EANx 32 and steel 120s, it was a totally different dive. After we were done with the Devil's throat, we did a long, slow, leisurely ascent through a maze of tunnels--a truly great dive.

 

[uncfnp]

You were misinformed. The entrance is at about 90-95 feet IIRC, then you do a straight down plunge, then level off for the exit. The exit is very large, so you there can be a wide range of depths depending upon the path you take. Stay high and you might be at about 125. You will only be below 1.4 for less than a minute--probably well less than a minute.

To me, the reason for the nitrox is what follows. The first times I did it, we did it on air. We went down, did the passage, and then ascended. Frankly, it was not that much of a dive. When I later did it with EANx 32 and steel 120s, it was a totally different dive. After we were done with the Devil's throat, we did a long, slow, leisurely ascent through a maze of tunnels--a truly great dive.

Thanks for the corrected depth. I had seen that number referenced as well. Either 80 or 95 the rationale for my post stands. The added benefit for the dive profile after the swimthrough is a consideration but would probably not play a significant role in my decision whether 32 is "safe" for the DT section.

 

[Lorenzoid]

I'm returning to this thread because I had exactly this discussion with an Aldora divemaster last week. We were heading out for the first dive of the day--all divers' tanks filled with approximately 32%--and discussing where we would like to go. Devil's Throat was probably never a candidate due to the mixed experience levels of the divers, but I made some off-handed comment (with this thread in mind) to the effect of "not Devil's Throat" and then asked the DM "by the way, what mix would you use for Devil's Throat?"

He said that as a matter of fact he had recently had a discussion with someone about this, and he had argued his preferred mix would be 36%. To be clear, he did not imply Aldora would have their divers use 36%--as I understood his response, he was relating his personal opinion only. I replied that I thought even 32% was too rich, as that at least briefly gives a ppO2 of 1.6 as the diver nears the exit (Wikipedia says the depth there is about 135 fsw, and other sources say about 130), and what if for some unforeseen reason a diver is delayed there and ends up spending several minutes at 1.6? Now, I know there's really nothing at the exit that would prevent a diver from immediately ascending, but my reply was based on the idea that unforeseen things happen, and a desire to plan conservatively for the unforeseen to whatever extent possible.

The divemaster then started asking me if I understood the concept of the Oxygen Clock. I said it was my understanding that the concept of the Oxygen Clock (sometimes called CNS Clock) relates to cumulative exposures, and so it wouldn't be relevant to the question of what mix to use for a single dive at Devil's Throat. I said maybe the NOAA Single Exposure table might be relevant--though for 1.6 I think the table lists a generous maximum exposure of 45 minutes--but that the Oxygen Clock should have nothing to do with this. He kept harping on "Oxygen Clock." I don't get it.

Also, in response to my thought that a diver might conceivably be delayed and therefore spend more than a few minutes at a ppO2 of 1.6, the diver sitting across from me, who was apparently a recent beneficiary of a TDI AN/DP course, chimed in and said, "So what?" He opined that a ppO2 of 1.6 for the working portion of a dive is normal for tech divers (like him). I said that although I have admittedly not taken any deco course, it was my understanding that 1.6 is believed to be tolerable for deco stops, when the diver is not exerting himself, but that the consensus seems to be that a diver should keep his ppO2 lower than that, such as a maximum of 1.4, for the working portion of the dive when the diver is exerting himself. My suggestion that exertion is believed to have some relationship with oxygen toxicity appeared to be met with puzzlement. I couldn't tell whether the divemaster, who said he was an experienced tech diver, agreed or disagreed, but I'm pretty sure he overheard this exchange, and he didn't offer any correction.

At that point, both this other diver and the divemaster sort of shrugged and smiled at my apparent ignorance, and we all moved on to the much more fun and more pressing topic of deciding where to dive. I made a mental note to ask the Scubanati of SB about this discussion with the divemaster. 36% for Devil's Throat? Really?

 

[Centrals]

My tec training(IANTD) has been 1.4 at max depth and 1.6 for deco.
AFAIK, 1.6 is NEVER normal for the working portion of the dive ie. max depth.

 

[wetb4igetinthewater]

If you exceed the recommended ppO2 of 1.4 or 1.6 even, you won't go go into convulsions immediately. If I did my math correctly, EAN36 at 130' gives a ppO2 of 1.8. If you are at that level for a brief time, will you go into convulsions? Probably not. But this is getting to the normalization of deviance, and it is up to you to decide how you wish to address that. I know that many people before the use of trimix would dive down to 230' on air (a ppO2 of 1.7), such as the Rouses as recorded in "The Last Dive" (yes, they died, but it wasn't due to CNS toxicity). If the "experienced tech diver" routinely dives to 1.6 during the active part of his dive, I would never dive with this person.

I've never seen any information that specified the limits where one can be at various ppO2 levels, just the fixed values that we shouldn't exceed. I remember when @Doppler was in the Seattle area giving a talk, he made a comment about this. While I picked up both of his books, I haven't read them yet to see if he mentions more about this topic. Hopefully he can chime in.

If I understand this article: Alert Diver | Understanding Oxygen Toxicity, if you exceed a ppO2 of 3, sounds like you are in a bit of trouble. But there is a lot of distance between ppO2 values of 1.4/1.6 and 3.0

 

[Kevrumbo]

Well, I've done In-Water-Recompression at pO2 of 2.0 (100% Oxygen with air breaks, and comfortably relaxing lying in the sand at 9m/30' in 28deg C water); but I wouldn't be using Nitrox36 at 39m/130' at ppO2 1.8 or greater, high gas density & work-of-breathing, AND potentially exerting myself into CO2 poisoning & Ox-Tox susceptibility by fighting a potential downcurrent at the exit. . .

 

[uncfnp]

Daily Limits for CNS Oxygen Toxicity

Shearwater and the CNS Oxygen Clock - Shearwater Research

I think these two articles will answer most of your questions.

The CNS or Oxygen Clock or Oxygen Exposure Time Limits are all referring to basically the same thing. OTU's are relevant to oxygen pulmonary toxicity.

 

[Lorenzoid]

Thanks for the replies. Maybe the guy was just being macho. Come to think of it, he was also sort of condescending when we said we preferred seeing marine life over doing swim-throughs, and he automatically assumed we were not comfortable doing swim-throughs and explained how we could go around them by following the divers' bubbles. We're perfectly comfortable with them, but just don't find swimming through them as interesting as seeing living things. Maybe to this guy, diving is all about challenging oneself (and one's physiology)?

Thanks, @uncfnp, but as the happy owner of a Petrel I have read pretty much everything on Shearwater's excellent website, and I have read that post on @Doppler's blog as well. My question is not "What is the Oxygen Clock" but rather why my DM might have believed the Oxygen Clock is relevant to this particular dive, as I never had the chance to ask him, and the conversation was getting testy. Keep in mind my DM was advocating a higher percentage of oxygen than what I would think is conventional wisdom, not a lower one. If the Oxygen Clock has any bearing on this, wouldn't a lower percentage be beneficial to minimizing daily oxygen exposure? And besides, isn't the more pressing issue here with the Devil's Throat dive what happens with a single exposure--which for purposes of planning this dive I refuse to assume is "brief" merely because I can't think of a reason one might have to linger at 130+ feet--to a ppO2 of greater than 1.6 (maybe as high as 1.8)? As Kosta pointed out above, the diver is "probably not" going to go into convulsions, but as I understand it, the probability is partly dependent on how long the diver remains at 1.6, how much the diver is exerting himself, and perhaps other less well-studied risk factors such as medications a diver might be taking. Why not plan conservatively? I just can't fathom why he would advocate 36% for Devil's Throat when many of the earlier responses in this thread--including the OP--questioned whether even 32% is not conservative enough. If nobody here can venture a guess, then that's that. It's not like I'm going to track the guy down and re-hash it with him.

 

[uncfnp]

@Lorenzoid.I may be misreading your question.

I think the oxygen clock is relevant because it deals with exactly the question under discussion, oxygen exposure. Exposure is measured by both time and pressure and while in a single dive the Single Dive Limit is the only factor it is still part of the "oxygen clock." As you know, this number for the PPO2 of 1.6 is 45 minutes. But the working recommended limit is only 80% of this. So about 35 minutes would be the maximum for a dive at this depth. And the given limit is based on a "normal dive." As you have pointed out, for cold or strenous dives this limit should be reduced.

But are you asking why he was using the oxygen clock to support diving DT at 1.8? I assume he was using the concept to validate a "brief" time exposure. But you are right that there really is no justification to his approach and I agree that it was machismo at work. My understanding is that once you hit 1.8 the risk of oxto increases dramatically. Could he get away with it? Probably. Is it appropriate. No. Is there any justification or even advantage to it? No. And by the oxygen clock "book," that dive would seriously restrict or even eliminate any further diving that 24 hour day.

Edit: and I agree on the swimthroughs. Interesting a time or two then seeing the underside of the reef is just...meh. But sometimes the exits thenselves are quite beautiful.

 

[KevinNM]

People have gotten CNS Oxygen Toxicity at ppO2 1.3 (and drowned). Data collected during researched showed that the susceptibility to CNS oxygen toxicity is not consistent even by person. A person was (for example) found to go into convulsions at 2.5 after an hour one day and 2.2 after 20 minutes on another day. Don't play with excessive ppO2, you really can die.