Scary dives in Coz last week

[Charlie99]

I was taught to respct a recreational scuba soft bottom of 1.4, which for 31% would be 116 feet.

I'm playing devil's advocate a bit here, since I agree with you that EAN28 would be a better choice, but it's surprising how many divers have hard and fast rules about ppO2 levels without taking into consideration the exposure time.

Doing a square profile dive to 1.4ppO2 is IMNSHO more dangerous than a dive that is mostly 1.2ata or less, with a brief excursion to a bit under 1.6ata ppO2.

Ignoring exposure time is kind of like making blanket statements about the cumulative N2 loading on an 80' dive vs. a 60' dive without knowing the time at depth for each dive.

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Relating this back to the initial post, you might ponder whether or not you would go retrieve your buddy from 160' if you are on EAN32 (about ppO2 of 1.9 ata). The easy copout is that you shouldn't have let him get down there in the first place, but it's a good question to think about ahead of time, so that you have an immediate, instinctive reaction were that horrible situation occur.


Charlie

 

[gj62]

Charlie - I understood that CNS toxicity had little bearing on the O2 exposure - it could occur whenever the ppO2 reached your body's threshold. Pulmonary toxicity is based upon exposure... Could be all wet (wish I was in fact)...

 

[DandyDon]

Relating this back to the initial post, you might ponder whether or not you would go retrieve your buddy from 160' if you are on EAN32 (about ppO2 of 1.9 ata). The easy copout is that you shouldn't have let him get down there in the first place, but it's a good question to think about ahead of time, so that you have an immediate, instinctive reaction were that horrible situation occur.

Nope - not at all the way I saw it! All too often, I have been on EANx 32% paired off with an Air-Breathing boat-pick bud I didn't know, and each time I made sure I got the fellow's attention so I could explain - hopefully with witnesses: "I'm on Nitrox (pointing at the label on the tank) 32%, and that 111 MOD written there means I cannot go below 111 feet safely; therefore - please do not go below that yourself, but if you do - please understand that I will not be coming to get you. You'll be on your own down there, bubba!"

Then I got the Pony Bottle, which I carry with Air-Only. Now I would switch to it at 111 feet to retrieve a buddy, if need - if the 19 cu ft would last me long enough. It won't last long at that depth, with me excited about a situation! hyeah20:

Charlie - I understood that CNS toxicity had little bearing on the O2 exposure - it could occur whenever the ppO2 reached your body's threshold. Pulmonary toxicity is based upon exposure... Could be all wet (wish I was in fact)...

Yep, that's the way I understood it...?! Those are two different aspects, aren't they? I'll have to get my Padi & NOAA books out again. Someone else know for sure...? :blink:

don

 

[gj62]

Your opinion is not uncommon for those who are uncomfortable with the idea of sharing air. This is exactly why practicing such skills is a good idea; to eliminate the fobia surrounding sharing air and become comfortale with itq, for both parties. Since it is a skill which all divers should be prepared to utilize, it is a good idea to practice these skills in a non-threatening environment. In the 665 dives I have completed...

I'm quite comfortable sharing air. It is not a phobia. I also have well in excess of 665 dives (when I stopped counting in the late 80's, it was in excess of 1,500).

I will also go on record that it is an *excellent* idea to practice sharing air with your buddy as often as you need so that *both* of you feel comfortable.

My comment was - allowing a buddy to extend past the *agreed upon midway point* of a dive by sharing air is not a good/safe practice. The midway point, in terms of PSI, is reached as soon as someone gets there - not when the everyone gets there when sharing air... I hold by that statement. Think of a dive with a hard overhead...

 

[Charlie99]

Guys, this is very basic stuff that you should understand before diving nitrox. To the best of my knowledge, it is covered in the nitrox class of ALL of the cert agencies.

All too often, I have been on EANx 32% paired off with an Air-Breathing boat-pick bud I didn't know, and each time I made sure I got the fellow's attention so I could explain - hopefully with witnesses: "I'm on Nitrox (pointing at the label on the tank) 32%, and that 111 MOD written there means I cannot go below 111 feet safely; therefore - please do not go below that yourself, but if you do - please understand that I will not be coming to get you. You'll be on your own down there, bubba!"

Unless you were just using hyperbole to get the guy's attention, this is the sort of misunderstanding that is all to common. If you took the PADI course, then you should have a card from DSAT that has equivalent air depths on one side, and on the other side a table titled "Oxygen Exposure Table for Enriched Air (nitrox)" That table is for CNS toxicity, not pulmonary/whole-body toxicity. The entries on that DSAT table (45 minutes at 1.6ppO2, 150 minutes at 1.4ppO2, etc., are what NOAA calls the single dive limits. There are two other NOAA CNS Oxygen toxicity tables. One is simply the limit for a 24 hour period limit. The 3rd NOAA CNS clock table is the "exceptional exposure" table, which is the one that I use as a guideline as to what I'm willing to do in the case of signficant danger to another diver.

Pulmonary (sometime also called "whole-body") toxicity is also a ppO2-and-time exposure calculation, but with a significantly different curve/table.

You don't have to take the above on faith. A simple review of your nitrox course material or a bit of googling will quickly bring you up to speed.

Charlie

 

[crispos]

This has become an interesting thread. But off topic.

Anyways, coppermaus survived.

I am thinking of moving to Coz, I bought a house down there and am going down March 17 for an extended period. I am interested to find this divemaster and see what is what.

P.S. I am bringing down double 130's so I plan to have enough gas for Devils Throat.
The problem is, they won't fit through the tunnel.

 

[gj62]

Guys, this is very basic stuff that you should understand before diving nitrox. To the best of my knowledge, it is covered in the nitrox class of ALL of the cert agencies.

Not only have I looked back at the PADI materials, I also checked DAN and UHMS.

While there is a time/exposure listing for up to 1.6PO2 (45mins - which is what Don & I were aware of), there's no mention in PADI of reasonable limits beyond that. There have also been reported cases of seizures based on limited exposures over this level, which are believed to relate to a variety of factors - CO2 loading (exercise), cold, immersion depth, personal limits, etc. (UHMS 2002 Symposium notes). In fact, in reviewing these sources, there was a much wider range of exposures/tolerances than were found in other exposure-based events (such as DCS).

The key appears to be that O2 loading happens *much* more quickly with the higher PO2.

According to tables in ZPlan (taken from NOAA up to 1.7, and adopted based upon the Repex method (Dr. B. Hamilton)), at 1.6, your PO2 loads at 2.2% per minute (hence the 45 minute limit). However, for 1.8, your PO2 loads at 50%/min! That gives you 2 min to 100%, and seizures can occur within the 100% threshold. Admittedly, this is conservative, but hey - who wouldn't want to be?

I will look further - mostly because I am interested, not because I plan to push it. As a final note, it appears that a minor PO2 exposure violation can have much more risk associated with it than running over the 45 mins at 1.6.

Charlie, if you have other sources that indicate this is incorrect, can you post the links?

 

[Charlie99]

Not only have I looked back at the PADI materials, I also checked DAN and UHMS.

Good. We are much closer to being on the same page now. Much closer than when you first posted: "I understood that CNS toxicity had little bearing on the O2 exposure - it could occur whenever the ppO2 reached your body's threshold. Pulmonary toxicity is based upon exposure"

As I had noted before, CNS toxicity is not due to ppO2 solely, but to the combination of high ppO2 and exposure TIME.

Yes, the standard NOAA CNS clock accelerates signficantly once you get beyond 1.4ata ppO2, but passing that limit, or even the 1.6ata limit does NOT mean instant death.

Now that we are on the same page, we can rationally review the CNS clock that results from a Devil's Throat dive on EAN32. Keep in mind that the only portion of the dive below 111' (ppO2=1.4ata) is just the very end of the swimthrough, and that unless you have poor buoyancy control upon exiting, you will never exceed 125', and definitely not exceed 132' (ppO2=1.6ata).

Hopefully, your conclusion will be something different than

"Devil's Throat on nitrox ? Crazy if you ask me. Too deep unless you like convulsions.:

Charlie

p.s. a few other tidbits:

1. Yes, CNS oxtox problems are much more unpredictible than even DCS problem. Because of this, the CNS oxtox limits have been set rather conservatively.

2. Almost all oxtox incidents reported have been from someone breathing a mixture other than what was intended. Mismarked or unmarked tanks, breathing a deco bottle while at depth, an improperly mixed tank that gets used without being tested ---- those are the real life dangers that most often kill people.

 

[gj62]

Thanks Charlie - yes, my prior post used poor wording - rereading it now, your interpretation was better than my intent - I must have been narc'd

 

[ScubaTexan]

Anyway...if you are a fan of tunnels and the abyss like I am, there's just no way to describe it. And those who don't like the deep blue, will never understand the attraction.

I never said I didn't like the deep blue -- all I said was that Devil's Throat didn't sound all that appealing to me. Going through swim-throughs and seeing the abyss in Cozumel are two of my favorite experiences there.