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Thread: How much am I off gassing?

  1. #31
    RBW Member Dr. Lecter is an unknown quantity at this point Dr. Lecter's Avatar
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    Re: How much am I off gassing?

    Quote Originally Posted by kwinter  View Original Post
    If you're talking about the work by Dr. Neal Pollock that you have been involved with, keep in mind that these are looking at competing factors. Dr. Pollock is looking at bubbles and reducing DCS while Dr. Sawatzky is looking at oxygen toxicity. High O2 during deco will naturally reduce the risk of DCS, but will of course increase the risk of OxTox. Keeping pO2 to a max of 1.3 will minimize OxTox risk at the cost of either longer deco or increased DCS risk. This is why I consider them competing factors and advised the reader to understand both and make an informed decision. As I have said in the past, if you raise your DCS risk and take a hit, it will be after you have surfaced and have help available. If you have a seizure from oxygen toxicity even at 10 feet, you are probably going to die. The risk of OxTox is low, but the consequences are severe. The risk of DCS is higher but less costly. Minimize which ever cost you want or find a balance between them.


    iPhone. iTypo. iApologize.
    I must have missed it in the article you cited as suggesting running 1.3 or lower at all times is somehow different from doing 20mph under the speed limit despite ideal road conditions: is there something special about CCR that makes many, many years of successful OC deco on 100%O2 inapplicable when balancing DCS vs. CNS risks?

    Yes, running a constantly higher ppO2 on a CCR will cause the CNS clock to run faster than on OC, but that's simply a question of total CNS loading and nobody was suggesting a 1.3 ppO2 for OC dives with huge CNS loading during them. Indeed, for those of us doing deep square profiles it really doesn't make that much difference whether we're CCR or OC in terms of CNS loading--we were at or above 1.3 for almost all of the dive even on OC.
    Last edited by Dr. Lecter; 6th September 2014 at 01:22.

  2. #32
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    Re: How much am I off gassing?

    Quote Originally Posted by kwinter  View Original Post
    If you're talking about the work by Dr. Neal Pollock that you have been involved with, keep in mind that these are looking at competing factors. Dr. Pollock is looking at bubbles and reducing DCS while Dr. Sawatzky is looking at oxygen toxicity. High O2 during deco will naturally reduce the risk of DCS, but will of course increase the risk of OxTox. Keeping pO2 to a max of 1.3 will minimize OxTox risk at the cost of either longer deco or increased DCS risk. This is why I consider them competing factors and advised the reader to understand both and make an informed decision. As I have said in the past, if you raise your DCS risk and take a hit, it will be after you have surfaced and have help available. If you have a seizure from oxygen toxicity even at 10 feet, you are probably going to die. The risk of OxTox is low, but the consequences are severe. The risk of DCS is higher but less costly. Minimize which ever cost you want or find a balance between them.


    iPhone. iTypo. iApologize.
    I'm not referring to any work with Neal, but if you're implying that Neal would recommend that someone raise their PO2 and risk a hyperoxic seizure, just to reduce bubbles, I think you're mistaken. Dr. Lecter summed it up nicely. Divers doing huge dives go well past the CNS limits set by the NOAA tables. For the rest of us doing baby dives, it's pretty arbitrary.

  3. #33
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    Re: How much am I off gassing?

    Quote Originally Posted by Tienuts  View Original Post
    I'm not referring to any work with Neal, but if you're implying that Neal would recommend that someone raise their PO2 and risk a hyperoxic seizure, just to reduce bubbles, I think you're mistaken. Dr. Lecter summed it up nicely. Divers doing huge dives go well past the CNS limits set by the NOAA tables. For the rest of us doing baby dives, it's pretty arbitrary.
    Of course Dr. Pollack would not recommend anything to bring on O2 seizures. Don't be ridiculous. These are academic studies dealing with 2 competing factors. I am not recommending one approach over another. Merely stating the obvious that understanding both and the relationship they have to each other is better for making an informed decision. You do what you want and I am not going to interfere. I know plenty of folks who use their CCRs as oxygen rebreathers at the end of a dive starting at 6 meters. That is their choice. But you can not dispute the fact that they are increasing their risk of OxTox by doing so. The risk in their minds is so small that it is insignificant to their dive plan. Fine. I am not holding to any arbitrary limit or using NOAA tables to define a cliff that you will fall from if you exceed it.

    I am saying that the literature and some recent studies confirm that there were no (or possibly only one) cases of oxygen toxicity at pO2 of 1.3 while there are documented cases at pO2 higher than that. Dr. Lecter is correct that the reasoning is for the increased O2 exposure during CCR dives compared to OC dives. He is also correct that with a square profile the difference is minimized, but it is still real based on the average CCR dive being longer than OC dives. So even though the OC diver in a perfectly square profile dive might be diving at 1.4 for the entire bottom time while the CCR diver would be at 1.2 or 1.3, the CCR diver's total O2 exposure is often higher. Now add to that a little CO2 elevation from retention or bypass or WOB being higher and you have substantially augmented the likelihood of an oxygen induced event.

    And to use Dr. Lecter's analogy of going 20 mph under the speed limit, I would say it is more like the natural tendency to reduce speed when you have precious cargo such as small children in the car. Folks that routinely speed at 20 mph ABOVE the limit while alone will generally not do so when the kids are in the car. It isn't because of the increased fear of getting a ticket. It is because even though the risk of something going wrong is minimal (and no different than when he is alone in the car), the consequences are much more severe.

    If the cost of staying at 1.3 during deco is a few extra minutes in the water, then that is the price I am willing to pay. If circumstances require a shorter time in the water, I may choose to cut deco short and increase my DCS risk rather than boost my pO2 and increase my OxTox risk. That is a personal choice. I can always stay on 100% O2 once I am on the boat or dry land.

    Call it ultra conservative if you like. I say live and let live. Fair enough?

    BTW, it is important to note that we are effectively talking about only the 6m/20ft stop where you are pumping up to 1.6 and I am at 1.3. Once we move up to 3m/10ft we are both at 1.3 on pure O2. So how much are you shaving off your deco?
    Last edited by kwinter; 9th September 2014 at 04:21.
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  4. #34
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    Re: How much am I off gassing?

    Quote Originally Posted by kwinter  View Original Post
    I am saying that the literature and some recent studies confirm that there were no (or possibly only one) cases of oxygen toxicity at pO2 of 1.3 while there are documented cases at pO2 higher than that.
    Can you point to this literature?
    My understanding was that there were effectively no cases of otherwise healthy divers toxing at PPO2 under 1.6 - particularly while at rest on deco.

    Even if there are half a dozen instances, that's 6 cases divided by;
    10 000 rebreather divers doing 50 dives a year for 10 years = 5 million dives. Gives a risk in the magnitude of 1 in 1 000 000, just to put it in some form of wild arsed guess level of perspective... And that's based on there being more cases than I'm aware of.

    Whereas I'd suggest that the chance of my rebreather leaking is around 1 in 100, and the chances of a significant niggle something like 1 in 500...

    Certainly I'd suggest that anyone NOT running pure O2 from 3m upwards is an idiot, but that's another topic.

    (Oh - FWIW I personally start pushing the PPO2 up to 1.5 ish as soon as I get to stops of 5 minutes or so duration and start getting bored. And as a guinea pig in a mates experimentation I know I show no signs of pulmary toxicity or O2 induced myopia even after 10 days of 4+ hours a day of this practice)

    Mike

  5. #35
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    Re: How much am I off gassing?

    Quote Originally Posted by Mike  View Original Post
    Can you point to this literature?
    My understanding was that there were effectively no cases of otherwise healthy divers toxing at PPO2 under 1.6 - particularly while at rest on deco.
    Look here, but take the source into account.
    http://www.rebreatherworld.com/showthread.php?t=47460
    Ken

    Quote Originally Posted by Dsix36  View Original Post
    Just remember that listening to an idiot such as myself may very well get your arse dead.

  6. #36
    RBW Member Dr. Lecter is an unknown quantity at this point Dr. Lecter's Avatar
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    Re: How much am I off gassing?

    That's all well and good, but dives--both OC and CCR--getting CNS into the high hundreds or low thousands of % are not exactly uncommon. So I don't give any weight to the claims that CCR dives result in longer exposures to fixed PPO2s...simply put, almost no CCR divers are doing anything special in that regards as compared to traditional OC tech diving.

    The increased CO2 issues inherent in CCR give some additional theoretical weight to CNS toxicity, I admit. But do we have any evidence about just what kind of increase we're talking about in a properly functioning unit? We all know CO2 is bad for ox-tox and that narcosis may be good for it...but that's not exactly concrete information from which to evaluate risk, even if we had data about how much more CO2 a CCR diver might retain versus the same diver on OC.

    Personally, I think slowly dropping the PO2 every 20-25 minutes at the 20' stop (and on deeper stops where there's enough accrued time to get into that kind of issue) way down for the CCR equivalent of an air break is good practice. Experience has shown it works for managing massive CNS percentages for decades of tech diving. Beyond that, though, it's all a crap shoot...saying people should be aware of the competing risks and chart a personal-preference course between them isn't particularly helpful when nobody knows what the risk increments are.

  7. #37
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    Re: How much am I off gassing?

    Quote Originally Posted by kwinter  View Original Post
    Look here, but take the source into account.
    http://www.rebreatherworld.com/showthread.php?t=47460
    Thanks - Would be nice to see more detail of the instances Dr Mitchell cites, and to get a feel for any contributing factors in any case of siezures between 1.2 and 1.6 PPO2.
    Not that I'd discount Dr Mitchell's opinions, I'm just curious.
    Clearly he needs to be conservative in what he recommends, but while it is logical that risk increases with increasing PPO2, my personal view is that the increase is not significant below 1.6ish PPO2.

    I also have seen it suggested that the beneficial effect of breathing pure O2 is greater than would be expected based simply on the PPO2 increase.

    I'm fairly comfortable with the content of the paper he references as well - the author is happy to share his knowledge.

    Mike

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