Reef Fish wrote:

> >Somewhere, sometime, in some context, I remember reading that water is
the
> >universal solvent, that eventually, everything will be disolved by water.
> >While this sounded odd at the time and still does, if it's true, then
there
> >are lots of containers that will work . . . for a while.
>
> Excellent response!

Glad you liked it.

> One of the reasons I threw out that bait was for the "time of exposure"
> factor regarding ALL SOLVENTS, which you correctly and appropriately
> identified.
>
> It's a matter of HOW LONG ... which takes it back to the SCUBA TOPIC
> that many discussants overlook, "time of exposure", when discussing
> the dangers of diving (air or nitrox) to  PPO2 1.4, 1.6, 1.8, etc.

Nice transition back to scuba.

> The INFO of time of exposure is CONVENIENTLY suppressed in PADI
> and other scuba manuals.   Unfortunately, the net effect is that
> SOME divers get the WRONG impression that you would be dead if you
> dive below 131 fsw, or certain ppO2 or its equivalent.  You WOULD
> BE,  if you stay there LONG ENOUGH.  But how long?   :-)

I don't think we can blame this one on any one or even a few agencies.  As
you are aware, I supplemented my somewhat quick and dirty nitox course with
a lot of self study.  Of all the things I needed to know more about before I
was comfortable with my knowledge, O2 time limits were the hardest to find,
let alone learn.  Almost everyone's first reaction seems to be that if you
go over 1.6, you're likely to convulse.  Almost nobody thought to add "if
you stay there long enough."  Once I realized that there was a CNS time
limit and a Pulmonary Toxicity time limit, and figured out which was which
and how to apply them (other than by looking at my computers), I still did
not understand why, if 1.6 was OK for a while (I'm not going to put a number
to it without my resource materials), there wasn't also an OK time for 1.7,
1.8, etc.  As near as I can tell, the best answer to my question comes in
multiple parts:
1. CNS toxicity versus PPO2 is not linear.  The reduction in tolerance
increases faster as the PPO2 increases.  There is an OK time for PPO2
greater than 1.6, but nobody's brave enough to put a number to it because,
2. At 1.6 or greater, the progressively shorter tolerance begins to get
small enough that variability between individuals and in the same individual
on different dives (including all the variables associated with different
dives) is increasingly significant relative to the predicted tolerance.  At
some unspecified PPO2, CNS risk becomes a crap shoot.
3. If you crap out, you're not likely to get a chance to apply why you learn
from your mistake.

> NOW ask the same question about ... 218 fsw, say.   Some may be
> surprised that you don't die as easily as you might think at that
> depth.    :-)   It depends on ... the TIME OF EXPOSURE.

Nah, 218 is fine.  You don't die instantly until you exceed 219.63158 (to
the tolerance of the lousy calculator in my Palm IIIxe).  This is, of
course, assuming that you start at sea level, that there is 20.9% oxygen in
the air you're breathing and that 33 feet really does add one atmosphere.
Of course, I'm joking.

You have no idea how close I have come to commenting on your deep air dives,
both from a toxicity standpoint and from other standpoints.  Only the fact
that it's your business and not mine and that it's so hard to comment
without appearing judgemental kept me from doing so.  Since you brought it
up, sort of inviting inquiry, I'll ask my questions.  I'm not trying to
convince or judge you, only understand better.  Please keep this in mind so
that I may understand without a battle I have no desire to join (or start).
I'll give my opinions (on my own diving) separately to further ensure
against being perceived as judging your choices.

1. It's clear that CNS toxicity is a function of time, partial pressure and
other criteria.  I believe it's also clear that the decrease in tolerance
accelerates as the PPO2 increases and that there are no guarantees even
within the recommended PPO2 levels.  While I realize that time applies, I
have not seen recommended "limits" for PPO2 greater than 1.6.  Have you?

2. If you haven't, how do you know when to stop?  How do you know when the
safe time at a high PPO2 is so short as to make any exposure little more
than a crap shoot?  What limit have you drawn for yourself?

3. Given that getting it wrong is likely to be a very unforgiving event, why
would you tempt fate when there are alternative methods that offer so much
less risk?

4. It is generally accepted, right or wrong, that narcosis has some effect
at 90 fsw or more.  While the severity of the effect varies from person to
person and dive to dive, it is also generally accepted that the higher the
PPN2, the greater the effect and that there is some point at which a diver
will become sufficiently impaired to be unsafe.  It seems reasonable to
assume that, at more than 3 times the depth where some effect is believed to
occur, that impairment would, at a minimum, be significant and at worst,
fatal.  Again, why tempt fate when there are alternatives that offer so much
less risk?

                            End of Questions for Bob - - - Start of Choices
I Made For My Diving

OK, here are my choices.  These are the limits of my planning.  They are not
necessarily applicable to an emergency, when I may be quite willing to
accept increased personal risk to save the life of another, particularly a
friend or loved one.  I do not recommend anyone set their own limits
according to mine.  Rather I recommend that each diver learn about the risks
they might take and decide for themselves what is OK and what is not.  May
everyone's decisions be good ones.

While I recognize the potential for more extreme depths, I have adopted
standards for my own diving that I am comfortable with.  I chose a maximum
PPO2 of 1.4 ata.  I adjust my gas mixture to that choice when the marginal
advantage of enriched air justifies the increased cost.  Outside of that
range, I breath air, so far, to a maximum depth of 175 fsw.  I chose not to
add increased risk of CNS toxicity to the already elevated risk of narcosis
I have accepted and, therefore, will not be planning dives on air below 185
fsw.  I chose a hard limit rather than a softer depth/time limit at least
partly because hard limits are easier to ensure if I happen to be narced to
the gills.  I rarely have reason to dive below 130 - 140 feet.  All my
diving beyond this has been planned and executed on a task basis, with a
specific goal in mind.  If/when I desire to dive to greater depths more
often or to more than 185 fsw, I'll take the time to learn how to reduce
both N2 and O2 risk by breathing other gases and/or mixtures (a process
already started), I'll learn how to decompress as effectively and more
efficiently from longer, deeper dives, I'll take a course to ensure I
learned correctly, I'll study some more and then I'll go do the dives I
wanted to do badly enough to go to the trouble to learn how to do them
within the level of risk I chose for myself.

Lee