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

["Sean T. Stevenson" <ststev@uniserve.com>]

If you were to escape from your downed submersible with only a 1 ATA
lungful of air, without taking a breath from any other source, than you
are correct that you would not bend.

The difference is that free divers do not breathe from a compressed gas
source, and so the inert gas loading in their tissues is limited by the
initial lung volume upon descent.  With SCUBA (and this thread has to
do with submersible escape using SCUBA gear) the lungs are filled to
ambient pressure and as the diver breathes the inert gases then begin
to saturate the tissues (oxygen is metabolized, but interestingly
enough, has a narcotic potential equal to or greater than nitrogen). 
Helium is a much friendlier gas than nitrogen from a decompression
perspective - it ongasses rapidly, but also offgases quickly and does
not do the damage that nitrogen does, and also reduces adverse
physiological effects like narcosis.

A submarine escape scenario would likely be a complete surprise to the
operator.  Even having the presence of mind to grab the escape gas and
start breathing it, respiration at this point is likely going to be
extremely rapid, until the operator has a chance to calm down and begin
to rationalize the situation, to let the vehicle equalize and then
effect an egress.  This series of events is going to require a lot of
gas (a "spare air" definitely does not fit the bill, here).  Once free
of the vehicle, the ascent is immediately begun, but how much time has
now elapsed?  How much inert gas has been absorbed?  Even with zero
actual bottom time, at 90 meters, the effective descent time from
equalizing the vehicle and escaping has earned a decompression
obligation at this point.  While you could conceivably perform an
in-water decompression on the way up, let's get real - you have just
crawled out of a crashed submarine and you are going for the surface no
matter what.

Using heliox (no nitrogen) gives you the best shot at escaping from any
depth without permanent physiological damage.  Heliox 16 (16% oxygen,
84% helium), is the deepest breathable gas (90 meters) that is not
hypoxic near the surface, which leads me to believe that it is the best
possible gas to employ for the purpose of emergency submarine escape.

-Sean


On Mon, 06 Mar 2000 22:29:12 -0700, Jon Hylands wrote:

>On Mon, 06 Mar 2000 06:56:31 -0800, "Sean T. Stevenson"
><ststev@uniserve.com> wrote:
>
>> Carrying a bottle of heliox 16 as escape SCUBA gas would permit an
>> escape from up to 90 meters.  Of course, you'd probably sustain a bend
>> escaping from that depth, but then bends you can cure... drowning you
>> do not.
>
>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? I thought it was due to nitrogen build-up
>in the blood, which is caused by prolonged exposure to pressure. But in a 1
>atm sub, you're not exposed to the pressure at all until you exit the sub.
>
>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?
>
>Later,
>Jon