Mailing List Archive Mailing List Archive
Good post, Rich. Your rationalization works for me. And, on the
BioMarine, I've got four ways to notice whether I got the right amount of
peotouze. Rod
On Tue, 1 Oct 1996, Richard Pyle wrote:
>
> Hi Pete,
>
> > Assuming the right flow rate up to 3.0 L/min (which is mil spec)
> > How do you reason that a pPO2 of 0.7 ATA does
> > not leave enough room to prevent hypoxia?
>
> First of all, what do you mean by flow rate of 3.0 L/min? Since we're
> talking setpoints, we're talking fully closed. "Flow-rate" has tended to
> be used in SemiClosedSpeak. I assume what you mean is "Can the solenoid
> keep up with 3.0 L/min O2 injection?", and I suspect that for most if not
> all of the fully-closed units about to come on the market, the answer to
> that question is a solid "yes".
>
> So, then, how do I reason that a setpoint of 0.7 doesn't leave enough
> margin for error for hypoxia? The answer stems from the fact that 90% of
> my concerns about diving with a rebreather have to do with what happens
> when the rebreather *fails* in some way. When they work, they work
> great. The situations you find yourself in when they *don't* work are
> the ones that you really need to train for. Now, if a rebreather fails
> on you, how are you gonna die? One way is from bends, but this is easy
> to cover yourself for (backup deco tables, plenty of OC bailout gas
> supply, IWR rig ready to go in the boat, etc.). Another way is from
> hypercapnia. A number of folks at the forum indicated that CO2 buildup
> can lead to blackout as insidiously as hypoxia or hyperoxia. This stance
> surprises me, because it is utterly inconsistent with my own personal
> experiences. Talking to the guys who are using BioMarines, I find it is
> inconsistent with their experiences as well (and you BioMarine guys can
> correct me if I'm wrong on that). I'm not saying that the people who
> maintain that CO2 blackout is insidious are wrong, especially in light of
> the fact that they have vastly more experience than I do. However, there
> is no denying that my personal experiences are not consistent with that
> stance. In any case, I think essentially everyone would agree that CO2
> blackout is nowhere near as insidious as hypoxia or hyperoxia, (i.e., that
> the latter two problems are likely to occur much more quickly and with
> much less warning than the CO2 problems).
>
> O.K., so that leaves us with O2 problems -- specifically either too much,
> or too little. Like I said, 90% of my concern for diving with
> rebreathers is when the things fail, not when they work. Putting aside
> user error for a moment (users will always be able to kill themselves
> pretty easily, and that's an entirely different discussion), the
> rebreather can give you a hyperoxic mix by injecting too much O2, and a
> hypoxic mix by injecting insufficient O2. Let's start with the hyperoxic
> problem. One way this can happen is the solenoid can jam open. As I
> already explained to "John Todd", this is a problem that is immediately
> self-evident to the diver, and with the right training & equipment, is
> easily overcome. Another way it could happen is by injecting too much O2
> in the loop because of a flaw in the solenoid control system (electronic
> hardware or software). From my experience, this problem is also often
> ('though not always) failry self-evident, simply because I find it easier
> to notice a solenoid that continues to fire over and over again much more
> obvious than a solenoid that doesn't fire at all. Furthermore, if O2
> continues to be injected into the loop faster than your body burns it up,
> the loop volume expands and expands until it maxes out on exhale (you
> also notice it by the fact that your buoyancy is changing
> substantially).
>
> O.K., so that's two ways a rebreather system failure can lead to
> hyperoxia, the former is extremely self-evident, and the latter is at
> least partially self-evident. Now, how can a rebreather lead to hypoxia?
> First of all, there are essentially the same two ways: the solenoid can
> stick shut, or the control system can malfunction. The difference is,
> failures of this type on the hypoxic side are much less self-evident. My
> solenoid often doesn't fire for one or two minutes at a time. Thus,
> relatively long periods of time with no solenoid injection is not
> unusual. If I am task-loaded, I have found that I am *much* less likely
> to notice a non-firing solenoid than I am to notice an over-firing
> solenoid, for several reasons. But these are not the only ways a
> rebreather can lead to hypoxia. For example, a failure in the gas
> delivery system (i.e., regulators and plumbing) might easily go
> un-noticed. The solenoid continues to fire, but no or insufficient O2 is
> passing through it into the loop. A well-trained diver will notice this,
> but it is still much subtler than the hyperoxia failure modes, and is
> therefore barely self-evident.
>
> O.K., now lets compare how "lethal" hypoxia and hyperoxia are. I think
> it's fair to say that both can lead to death in divers, and neither has
> any reliable physiologically self-evident warning. At the hyperoxic end,
> the limit is unclear. Some say 1.3, some say 1.4, some say 1.5, some say
> 1.6, the French commercial guys (I'm told) decompress routinely at 1.9.
> I've been exposed to higher than 7.0 in a chamber, 3.5 in heavy work
> situations underwater, hours at 2.8 in a chamber, etc. - all with zero
> signs of CNS symptoms. It's a time/dose thing, where the time factor is
> usually at least a few minutes. In any case, the limit is at best a grey
> zone of probabilities, with a range measured in tenths of an atmosphere.
> Hypoxia, on the other hand, has a relatively sharp limit - somewhere
> around 0.08-0.12 or so for blackout (range measured in hudredths of an
> atmosphere). There is very little margin for error here - when the PO2
> gets low we black-out -- period. The time facter is always very short.
> Furhtermore, hyperoxic symptoms (the CNS ones, anyway) are not especially
> damaging per se - it's the subsequent drowning that gets you. With a full
> face mask and an atentive partner, you can survive an O2 convulsion
> relatively unscathed. With hypoxia, however, we're talking major loss of
> brain cells if we're lucky, and death in any case that the problem is not
> immediately identified and corrected.
>
> I am fairly convinced that hypoxia is 1) operationally less self-evident,
> 2) is more "absolute" in when it happens, and 3) is less recoverable than
> hyperoxia is on a rebreather. Do we agree on that?
>
> If so, then we would put hypoxia above hyperoxia on the scale of
> insidious nasties that can happen on a rebreather dive.
>
> Now, in my over-use of the word "insidious" above, I am emphazizing the
> non selef evident nature of such failures. The more time between the
> occurence of a rebreather failure and the point at which the failure leads
> to death, the more likely a diver is to "catch" the failure and correct it.
>
> On the rebreather I use, I find it takes about 30-45 minutes for my body
> to burn a loop full of gas at a PO2 of 1.4atm down to about 0.12atm -
> lets' say that's about 0.1 atm every 3 minutes or so. If my setpoint is
> 0.7, that means I have only about 15 minutes to "catch" a failure that
> leads to hypoxia. My diving partner John burns O2 at least twice as fast
> as I do, so if he was running a setpoint of 0.7, he would only have about
> 7 or 8 minutes instead of 15-20.
>
> Alright, so I've wasted a lot of bandwidth and haven't exactly spelled
> it out very clearly, so let me try to summarize:
>
> Rebreather failures are what get you. Hypoxia is scarier than
> hyperoxia because it kills you directly, is harder to notice
> operationally on a rebreather, and is not really a function of
> increasing probability (more of an absolute cut-off). The best thing to
> do is to choose a setpoint that maximizes your probability of survivial
> for both hypoxic and hyperoxic failures. Given the relative
> characterisitics of each type of failure, I prefer to leave a wider berth
> between my setpoint and hypoxia, than my setpoint and hyperoxia.
>
> For me, the best midpoint between the two failure extremes is in the
> range of 1.2-1.5atm. As stated in my original post, 0.7 does not allow me
> enough margin for error on the hypoxia side.
>
> > It's odd to find myself advocating more O2 - or
> > a higher pPO2 - but I count 0.7 ATA as three
> > times what you are breathing in Hawaii right
> > now. (This assumed you're not doing a 100% O2
> > surface break to get over your jet lag ;-)
>
> If the rebrether was 100% immune to failure, I'd be inclined to agree
> with you. For me, it's not about finding the phyiologically optimal PO2
> to inspire it's about maximizing the probability of surviving the dive.
>
> > Seriously, the Navy upper limit of 1.3 ATA was
> > just that. Remember that your reason for going
> > higher than 0.7 or 1.2, better nitrogen load, has
> > not been validated as a means to prevent DCI.
> > (Remember the look on Ed Thalman's face).
>
> Rememebr, lower PN2 was only part of my reasoning. Besides, I'm not sure
> if you and I got the same message from Thalman. To me, he was saying
> that the models don't reflect what's going on - a message I have been
> preaching for years. He was not saying that there is no correlation
> between inspired PN2 and probability of DCI on a given dive profile.
>
> > While an O2 hit can occur almost instantaneously
> > on exposure to a high pPO2, most occurred after "some
> > minutes. So I reason that if it's like other phsyiology,
> > exposure to 1.3 for several hours is a lot more risky than
> > say a 20 min deco at rest on 1.4.
>
> But hypoxia does occur almost instantaneously on exposure to low PO2 in
> essentially *all* cases, so that's what you REALLY need to guard against.
>
> > I think I'll stick at 0.7 for sport stuff and
> > leave the hotter pPO2's to you guys in the
> > Twilight Zone :-))
>
> That's your call. ;-)
>
> Aloha,
> Rich
> --
> Send mail for the `techdiver' mailing list to `techdiver@terra.net'.
> Send subscription/archive requests to `techdiver-request@terra.net'.
>
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]