Re: [PSUBS-MAILIST] PSUB Fatalities...

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Jon Hylands" <hylands@ibm.net>

> I don't understand this...
If you're in a 1 atm. sub, and you do an emergency exit, and free ascent to
> the surface, why would you bend?

Hi Jon,

Let's say you have a buoyancy problem that puts you on the bottom at 200
feet in a reasonably intact hull.  You don't have SCUBA.  You open a flood
valve in the bottom of the hull, the water starts to rise inside as you
equalize the pressure on the hatch.  Depending on the size of the valve,
this could take time.  As that pressure equalizes, the air bubble trapped
inside your sub is being compressed by depth pressure, so you are no longer
at 1 ATM.  Now you're breathing depth-compressed air.  Depending on depth,
time, and how fast you surface while blowing expanding gas, you could get
bent.  Or, take a big lung full of that depth-pressure compressed atmosphere
inside your hull, open the hatch to escape; try swimming to the surface
while holding your breath; and boom!  Instant pneumothorax!  (Your lungs
explode.)  So you've got to be breathing something.  SCUBA, NITROX, HELIOX,
a Steinke Hood,  all give you that option; but you've still got to do it
right, or you're toast.  Sean's point about minimal nitrogen being desirable
is a good one; I can see advantages over compressed air, and if there's any
possibility you might have to escape deep, HELIOX could be a wise choice
SCUBA is still better than nothing, though.  Getting buzzed or bent is
better than busting your lungs or drowning.

> The guys who do those crazy stunts where they hold a weight and
> free-descent down a rope to like 300 feet, and then release and
free-ascent
> up to the surface, all without any SCUBA gear at all, they don't bend...So
what gives?

They're not breathing compressed air.  The lungful they take at the surface
is only one lungful of air molecules.  As they descend, they get squished
real good by depth pressure, but they can turn around and head back up
without worrying too much about expansion because they've still only got one
lungful of air molecules inside.

Here's a REALLY simplified explaination of what's happening here.  If it
takes, say, 10,000 air molecules to fill your lungs at the surface, it will
take that much more for each additional atmosphere of pressure encountered
with increasing depth.  At 33 feet it would take 20,000 molecules; at 66
feet it takes 30,000; at 99 feet it takes 40,000; and so on.  The diaphragm
in your second stage regulator is balanced by water pressure to give you the
right amount of air molecules; and you don't feel the difference in your
chest (much) because outside waterpressure on your chest balances it out.
But if you take a 40,000 molecule lungful at 99 feet, hold your breath, and
head for the surface, (you'd die before you got there, but) by the time you
got to the top, your 10,000 molecule capacity lungs would be holding 40,000
molecules WITHOUT DEPTH PRESSURE TO COMPENSATE THEM.  Blammo!  You bust your
lungs.

IMPORTANT!!!  I don't want to mislead anyone into thinking it takes 99 feet
to cause Pneumothorax.  You can hurt yourself  by taking a breath of
compressed air, holding it, and rising only a very short distance.  This is
why you NEVER NEVER NEVER hold your breath while breathing compressed air
underwater.  (Yeah, I know.  There's guys out there saying "I skip-breathe"
all the time to stretch my downtime; and if I told you I haven't done it I'd
be lying.  But we all know what can happen, especially midwater if we get
disoriented, "nitrogen high",  and lose track of our depth.  It doesn't take
much, and it's just not a safe practice.  So I stopped doing it a long time
ago; and it's generally accepted that divers just shouldn't do it.)

Try this right now, sitting at your computer.  Take a breath; exhale
forceably through your mouth, and then "cut it off" with that thing in your
chest called the epiglottus.  Now relax, and let the rest of the air out.
Easy, right?  Maybe not so underwater.  If you take a lungfull of compressed
air at depth and bolt for the surface while holding your breath, the
expanding air in your lungs can force your epiglottus shut from within just
like you just did; but under these conditions of unequal pressure as you
ascend, you won't be able to get it to relax; the air will continue to
expand with no means of exit, and you could rupture your innards, lungs,
epiglottus, and all.  I've heard people say you might overcome this at the
onset by hitting your purge valve or going back down; but I don't want to
mess with it.  "Ounce of prevention / pound of cure."  Breathe naturally
underwater, and don't hold your breath.  Safest way to go.

To those thinking about SCUBA training, may I say that I hope the point
being made here is that breathing underwater is a little more complex, and a
lot more potentially dangerous, than it might look at first glance; and it
is, therefore, extremely important to get the best instruction possible
before using any form of breathing apparatus underwater.  "Good training not
cheap; cheap training not good."  When your life is at stake, do you really
want to trust the lowest bidder?  Come on!  You're worth more than that!

Dive to survive!

Pat