Accumulated 02 following a large number of repetitive Nitrox dives over 3 days.

[TrimixToo]

I suspected the computer was providing erroneous information and it turns out I was correct.

You don't have to guess wrong very many times underwater. It's a self-limiting behavior.

The more I learn about this stuff the more conservative I become.

Famous last words? "It'll be OK this time." "I can make it! Hold my beer and watch this...."

 

[chillyinCanada]

Hey, I have an excuse and I'm sitting in it.

FIFY

 

[chillyinCanada]

Astran, in future will you set your computer at 1.4?

 

[Astran]

Astran, in future will you set your computer at 1.4?

Not going to change a thing, other than perhaps dive a bit more conservatively if and when my Oceanic tells me I'm over the 02 limits. For example if I'm sitting at 60' I probably wouldn't drop to a 100' to check out a different part of the wreck or whatever and if the dive isn't all that interesting I wouldn't be opposed to ending it early, especially if there are more repetitive dives planned.

Hmmmm... interesting conclusion.

But... even if you thought the computer was wrong, wouldn't it be a better practice to end the dive and investigate the computer when you're out of the water? I mean, just due the remote possibility that the computer was... right?

Yes, my decision to ignore the 02 warning based on nothing more than "intuition" was probably rather shortsighted even if ultimately correct.

I'm glad everything worked out but just think about how you would feel if your dive buddy has a serious issue during that dive? Could you still justify taking that risk of ignoring 3 computers and not feel guilty?

Yes that occurred to me, which was one of the reasons I started this thread. I feel much better now.

The following post sums up my takeaway from this thread rather nicely.

Personally I think you did the right thing but maybe for the wrong reasons, but now you know that these computers are stupid in this respect so can ignore them. If you do the numbers properly it is really hard to get limited by CNS toxicity doing no stop dives.

 

[GJC]

I have never heard this, I will have to read the NOAA dive manual

The NOAA dive manual does not have that much info on how they got their limits. The US Navy dive manual has more info on limits under the rebreather section. I learned the most from the links on this thread.

From what I read, the OTU 24 hr limit before symptoms occur, minus the exposure from a table 6 chamber exposure, leaves 300 for diving and that is the basis for the NOAA limit.

 

[jgttrey]

Do you download your dives? You can probably, in retrospect, figure out your CNS loading for some of your past dive days giving credit the 90 minute half time, and also your cumulative OTUs. I can do that on Diving Log. I doubt you have a problem there, but it might be interesting. This would give some hard data and might provide comfort, or caution, for future diving. You can see just how close you came, if at all, to CNS limits on the other methodology. OTUs would be interesting too as keeping those in check is a good thing if you have have to hit the chamber, though I doubt you have an issue there.

Just keep in mind that oxygen toxicity in divers is not well understood. The "oxygen clock" is not a precise description of a known physiologic process. It is an attempt to rationalize empirical data and provide guidance for planning. It is a rough tool for something that we don't fully understand.

Much of the data comes from other applications, like hyperbaric therapy and anesthesiology and is applied to diving in the hope that it is instructive. But, it is not perfect. Consider:

• The exact mechanisms for why humans seem to tolerate much higher partial pressures of oxygen in a dry chamber versus immersion (which is a fact) are not fully understood.
• It is also known that we tolerate high PO2 better during resting portions of the dive, like decompression. This is why technical divers usually apply different PO2 limits for different parts of the dive.
• Air breaks on deco seem, empirically, to be more useful than the models would suggest, so we do it. But, the models don't really explain why it is effective.
• We also do not fully understand all the other risk factors for CNS problems -- CO2 retention, medications, nitrogen loading, physical fitness, etc. nor do we have a firm measure of how much of a role exertion plays, though we know it does.

Technical divers push or exceed 100% CNS all the time, but usually on deco in shallow water, where we know it is far better tolerated. They also push PO2 to 1.6 routinely at gas switches and at 20 feet on O2. But, they would never do that during the "working" portion of a dive.

All that is a long way of saying that caution is appropriate when borrowing experience from other divers, diving different profiles, etc. What is "safe" (relatively) on the deco bar is considered unacceptable at 100', particularly if you are swimming against a current and working hard. It is a very complicated subject.

 

[Astran]

Do you download your dives? .

I do. Here's the profile of the dive where I got the 02 warning.
View attachment 543497

 

[JackD342]

I found this thread to be a good opportunity to review what I know, (and what I have forgotten), especially regarding the PADI EAN course using tables rather than computers.

First, I was happy to find that I was able to jump right into using the tables correctly per the course, with no review necessary.

Now, using that method, which admittedly has an inherent "conservative" factor not only because of the limited number of rows and columns in the tables and the assumption of a square profile, but also a compounded effect due to multiple "round ups" as you move from chart to chart, I applied it to the OP profiles as listed in two different earlier posts. (Also, there are 5 dives that were described with start times but only as "about 90 feet for about 40 minutes." So more approximations there.)

Significant notes from that exercise:

• Of the 13 dives described across 2 days, the O2 exposure calculation for each dive was either 25% or 30%, except for the final Sunday dive that was 20%.
• Five dives started with a 24 hour cumulative O2% greater than 100%.
• The dive in question (#2 on Sunday) started with an O2% of 180%.
• One dive as described exceeded the MOD for 1.4 ata. (#1 on Sunday immediately preceding the dive in question, @ 1.45 ata)
• The last two dives on Saturday began with surface intervals less than the minimum of 1 hour recommended in the PADI course.

Yes, of course I know that a dive computer does a much better job with more precise calculations, and this is probably a great example of the benefits of a dive computer. But it was still interesting and enlightening to work through the profiles with the "manual" method.

Other items of note, from the PADI EAN course manual, rev. 05/07:

• "Some individuals retain carbon dioxide, which is believed to contribute to oxygen toxicity. Some tests show that a 1.4 ata limit reduces oxygen toxicity risk for those few individuals who retain carbon dioxide."
• "Divers have had oxygen toxicity while breathing oxygen partial pressures at or near 1.6 ata."
• Regarding Pulmonary Oxygen Toxicity - "...which results from prolonged lung exposure to high oxygen partial pressures. This is highly unlikely within the oxygen limits you learn in this course. …This is mainly a concern for commercial or technical dives that require long decompression stops using pure or high amounts of oxygen (50% or more)."
• From guidelines for using the Oxygen Exposure Table: "Although these NOAA limits don't specify minimum surface intervals, and there's no measurable surface interval credit, it's recommended that you allow a surface interval of at least an hour between enriched air dives, whenever possible, especially if you exceed more than 50%."

 

[yle]

Not going to change a thing, other than perhaps dive a bit more conservatively if and when my Oceanic tells me I'm over the 02 limits.

Yes, my decision to ignore the 02 warning based on nothing more than "intuition" was probably rather shortsighted even if ultimately correct.

Suggesting that you were "correct" to ignore your computer is like someone playing Russian roulette, pulling the trigger and getting only a "click"... and then claiming there were "correct" to pull the trigger because the game is apparently safer than they were told.

Your computer algorithm deals in probabilities, not absolutes. Just as you have that one in six chance of ending your life playing RR, but a 5 in 6 chance of surviving, you probably have a better than even chance of ignoring your computer and getting away with it unscathed. But you pull that trigger frequently enough and it won't take long to find the bullet.

Play whatever game you like, but I have one suggestion for you and one important request:

The suggestion: why bother with a dive computer? Its only function is to give you advice (well, and to collect data that we'll be able to use to determine what happened to you), and you've already decided that if you don't like the advice you're going to ignore it. Just use a depth gauge and watch, old-school. The gauge and watch will never offer advice, so you won't have to even worry about whether or not you want to ignore it.

The important request: on behalf of the scuba community, please refrain from advising new divers. Unfortunately they often look to divers like yourself... those they perceive to be experienced and commensurately knowledgeable ... for guidance. A new diver might not be so lucky dodging bullets if they're encouraged to play your game.

 

[yle]

Don’t bother arguing. The only thing more sacred than a Shearwater UI round here is the limits imposed by an Oceanic dive computer, especially their NDLs.

Personally I think you did the right thing but maybe for the wrong reasons, but now you know that these computers are stupid in this respect so can ignore them. If you do the numbers properly it is really hard to get limited by CNS toxicity doing no stop dives.

What I have learned is there is a whole universe of people taught to do this in a simplistic way.

Ken, if ignoring the computer's warning is the right thing, and there is a better way to do what the computer tries to do (i.e. calculate exposure to various risks to provide reasonable advice to the diver), or if the way it is done is too "simplistic"...

can you share with us the right and/or better way? I think the entire community would be grateful for this knowledge.

I ask only because it seems your post is agreeing with the OP that the computer was wrong, but the OP needs to know what is right.

Thanks!