Mailing List Archive Mailing List Archive
I *know* I am going to regret sending this. But it happens to be a slow
day, and after skimming through the recent archives, I see some threads
of discussion that starve for rational thought. Let's look at a few of
them briefly:
1. Death of Hennie Pretorius
I knew nothing of this incident until I glanced through the archives
today. First, Kudos to Dennis Harding for maintianing a rational tone
despite irrational attacks. Getting to the point: The bulk of this
discussion seems to focus on nit-picky little details that may or may not
warrant heated (or otherwise) discussion, but do not in any way (even
from the most convoluted perspective) have any bearing on the cause of
his death. From what I have read, I have absolutely no idea what caused
the observed symptoms. However, it is clear we can eliminate a few of the
going hypotheses:
Did he die because the instructor was from one agency but not another?
Did he die because the instructor was a smoker?
Did he die because he was 30 lbs overweight?
Did he die because he went to 105msw instead of 90msw?
Did he die because he breathed 32% O2 during deco?
Did he die because he breathed 80/20?
I challange anyone on this list to present a rational explanation of how
any of these factors would have cause the observed symptoms (i.e., sudden
and dramatic neurological symptoms upon surfacing, after relatively long
deco with no apparent indications of a problem).
The only possible considerations I can see are that he had a history of
neurological barotrauma, and that the chamber treatment may have been
suboptimal.
As to the former, although I believe that a valid case can be made that
history of neurological barotrauma should be considered a contraindication to
deep diving; if we all followed that advice, many of the more outspoken
individuals on this list (myself included) would have an assortment of
rebreathers, scooters, lights, double 104's, and other scuba gear to
sell.
As to the latter, regardless of the wisdom of the chamber treatment
administered, that still doesn't explain the sudden onset of severe
neurological symptoms in the absence of any gross decompression
violations.
Why is this important? When I was a kid, my dad would always hit me with
the "I hope you learned somthing from this bad experience" line. I
understood where he was coming from and all, but the problem was, because
I was such a brilliant little kid, most of my bad experiences were of the
"sh*t happens" type, rather than "gosh, that was stupid of me - I won't
do that again" type. As much as I would want to learn something from it,
there really wasn't anything of practical value to learn, because the
circumstances leading up to the bad experience were either
ultra-particular to that specific event, or were cryptic and
unidentifiable. Don't get me wrong - whenever something bad happens to
anyone, we should all try to learn as much from it as possible, so that
the collective body of all these bad experiences will ultimately reveal
trends to teach us how to reduce our chances of getting hurt.
However, in this particular case, until we can get a better handle on
what actually caused or contributed to or otherwise led to his death, we
don't really have much to learn other than "sh*t happens"; which we
already know.
2. 80/20
I've personally never breathed 80/20 on an open-circuit dive. However,
the arguments I've seen on *both* sides of this debate range from
"stretching it" to "truly rediculous". Guys: it's a gas. It happens to
contain 80% O2, 20% N2. It is neither the cure to bad buoyancy control,
nor a notarized death certificate. Neither side of this argument will
ever "win", because there is not enough substance to argue about. The
only point that makes sense to me is the one raised by Joel that you can
get more total gas molecules in a given cylinder without a booster, which
might convey logistical advantages for a 30-foot stop. (Joel doesn't get
credit, though, because the tone of his message makes it sound like he
intended this as an example against 80/20, when it looks to me more like
an example in favor of it).
3. Fudge Factors in deco calculations.
Since before Abyss ever existed - when it was just a topic of discussion on
the CompuServe scuba forum - I have been arguing with Chris about the
"meaninglessness" of all these super-hyper complex fudge factors, and how
there was no real basis for the math behind any of them. I invented the term
"titanium doorstop" in reference to the following analogy: why go to the
trouble and expense to use high-performance machining technology to
manufacture a wedge of titanium to within 0.0001" tolerance -- so it can be
used as a doorstop? Especially when a random chunk of wood will do the job
just as well (if not better). The point here is that decompression is,
always has been, and almost certainly always will be a very "fuzzy",
imprecise practice at its core; so why try to micro-manage the numbers with
all these gee-whiz features that mean nothing?
Chris' response has generally been twofold: 1) These fudge factors only
affect the deco slightly; and 2) in most cases, they lead to longer
decompression times (which presumably means lower probability of DCI
symptoms). After giving it more thought, I finally stopped arguing with
Chris for these reasons, and for the simple reason that even though the
fudge factors don't necessarily reflect physiological reality, neither
do compartment-based deco models in general....so what's the diff?
Also, I trust everyone realizes that the practice of deep stops - which
seem to be gaining wider and wider acceptance these days - is a similar
"unfounded" fudge factor. There are theoretical reasons why it should
help, but my particular method for doing it is an off-the-cuff SWAG.
There are also theoretical (and even empirical) reasons why smoking,
obesity, cold-water conditions, and age all may affect one's disposition
to experiencing DCI symptoms; and Chris has developed his own
off-the-cuff SWAG methods for modifying the deco accordingly. In fact,
if I'm not mistaken, the option of including deep stops is yet another
one of the "unfounded" fudge factor options included in Abyss. If these
factors don't give you a warm & fuzzy feeling, then don't use them. If
they do, they probably won't help - but they even more probably won't
hurt.
4. Constant PO2 deco
This one is always a riot to me. There are two issues: "jacking up PO2
on deco", and "constant-PO2 deco". The former is something that people
who dive with fully-closed rebreathers seldom do. The latter is
something that people without fully-closed rebreathers are not capable of
doing (at least not within any practical logistical limits). So, the
"have-nots" tend to lump the two together to justify their choice of
diving equipment, because they can make a valid case about the former,
but have absolutely no experience whatsoever with the latter.
Here's the score, guys:
I have done a lot of deep decompression diving with air only (all the way to
the surface). I have done a lot of deep decompression diving with
open-circuit mixed-gas and nitrox & O2 on deco. I have done a lot of deep
decompression diving with constant-PO2 mixed gas.
The air diving/deco thing really sucks. I never knew how much it sucked
until I started using 100% O2 on deco after deep air dives. I *really*
started to understand how much it sucked when I started breathing helium
deep with nitrox & O2 on deco. However, I only actually grasped the full
magnitute of how much air diving/deco sucked when I started diving with
a constant PO2 rebreather.
The weenie approach to this topic is to bust out some deco model and
compare deco profiles for OC trimix/nitrox/O2 and CC constant PO2 diving
and say "hey look - there really isn't much difference!" Folks, the
difference is not in the numbers that a deco model spits out. The
difference is in how you feel after a dive. The CC-divers out there
already know what I'm talking about, but the rest will just have to
either take my word for it, or ignore me. If you started diving deep air
and then switched to trimix & elevated PO2 on deco, you probably noticed
the dramatic reduction of post-dive fatigue and other associated
symptoms. Well, when you switch to constant PO2, you'll notice the same
level of improvement.
As for the argument that constant PO2 will cause you to convulse from O2
toxicity, you'll have to show me the numbers of all those divers
convulsing at 1.4 or 1.3, which is where I keep the PO2 for the entire
dive.
5. CNS%
I have tried and tried and tried and tried to discover the source of this
concept, and so far it seems to fall into the "high PN2 causes RBC
rigidity" category. You don't see me publicly bashing the concept,
however, because people who choose to believe in it will tend to lower
their operational PO2 values and introduce low-PO2 breaks during long
exposures (both of which I believe to be wise practices). So, like the
Deompression Program Fudge Factors topic, while I don't see any merit in
it, I see more good than harm - so I leave it alone.
That's all I'm going to whine about today. I haven't even had the courage to
read any of the messages with "Helium" in the subject line, for fear of what
I might find.
The general themes I'm seeing throughout all these threads are "titanium
doorstopism" and "precision causation delusion". I already described the
first. The second is in reference to the all-too-frequent knee-jerk
conclusion (delusion) that Accident "X" was a direct result of Cause "Z" (or
the variation: Practice "Z" will lead to Accident "X") In almost every such
conclusion (delusion) I've seen, the scenario seems to be that the person
forming the conclusion (delusion) probably read somewhere or was told by
someone that circumstance "Z" might in some way indirectly affect outcome "X"
in a few situations. Unfortunately, without the basic understanding of the
factors involved (rather than rote memorization of stuff written in books and
spoken by "experts"), the delusion becomes conclusion. A case of "a
little knowledge is a dangerous thing".
Kind of like the "HFS" dive instructor who once told my 5'7", 110-lb
rock-climbing wife that she was more likely to get bent than he was, because
women, on average, have more body fat.
'Nuff said.
Now...don't make me come back here again! :-)
Aloha,
Rich
Richard Pyle
Ichthyology, Bishop Museum deepreef@bi*.bi*.ha*.or*
1525 Bernice St. PH: (808) 848-4115
Honolulu, HI 96817-0916 FAX: (808) 841-8968
"The views are those of the sender and not of Bishop Museum"
--
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