Re: [PSUBS-MAILIST] question for technical divers

[Phil Nuytten <72020.572@compuserve.com>]

Hi, Carsten:
        We called this escape system the 'Newt-chute' , (although that is
also the name of a rescue system for the occupants of an entangled
diving-bell in deep water (600ft.+ -200M) that I designed and built for
Oceaneering in 1973 - see 'Diving-bell Escape systems - observations and
test results' OTC proceedings Houston, 1973) 
        Essentially, the Newtchute is a polyurethane-coated (internal)
ballistic- cloth nylon cylinder, closed at the top and open at the bottom.
The diameter depends on the size of the subs hatch - it should, of course,
be smaller than the CLEAR diameter of the hatch and/or trunk. Once the
diameter is determined, the length can be calculated so as to give 5 to 6
cubic feet- (140 - 170 liters) of volume to generate 320-385 lbs( 145-175K)
of lift, less the volume of the wearers head and shoulders ( in practise,
the head and shoulder area can be discounted because of the vent bubble
area immediately below the Newtchute) This is an amount of lift that
optimizes the speed of ascent - thereby minimizing gas uptake - with the
requirement to carry a large blow-down gas supply  (usually Heo2). Even 6
cubic feet is a large requirement - remember that's a volume of 6 cubic
feet at depth - not 6 SCF - This is an area that seems to be difficult to
understand for some, so bear with me if you already know all about it!. If
you have a square container, 12 inches by 12 inches, it will contain one
cubic foot at one atmosphere absolute (sea level) - for every 1 atmosphere
of pressure that you compress into it, it will contain an additional
STANDARD cubic foot  ( sometimes wrongly called a SURFACE cubic foot -
altho' for our puposes it could be called that!) At a thousand feet, the
ambient pressure is 445 PSI or about 75 Atmospheres- so if you pressure
your square bottle (!) to 445 PSI, you will have 1 cubic foot of volume
available to you at one thousand feet. But we need more - so double the
pressure to, say, 1000 PSI ( for sake of round numbers) now you'll have
double the volume or 2 cubic feet. Still not enough, so double the pressure
again, to 2000 PSIG, now you'll have 4 cubic feet - just about there - how
'bout 3000 PSI  to give you 6 cubic feet. and so on - don't trust my math (
I never do!) but you see the basis and you see that you have to carry a
large amount of gas for such a system. You can get by with much less, say,
half of that or 3 cubic feet, but it gives you a slightly longer 'dwell' at
depth - where the ascending gas expansion is so much less than at shallower
depths. If you cut it down to 3 cuic feet, a standard SCUBA bottle will
about do it for you. ( note that I've ignored PSIG versus PSIA so the
numbers are off by that distinction)
        Sorry to belabor the point - I had a guy at a TEK conference try to
convince me that his SCUBA bottle held a 100 cuic feet even when it was
depressurized -  I said " Jeez! must be annoying to have to use a crane to
get in and out of the water  - and - Man! what a weight belt to sink that
sucker!"
        Back to biz: internal webbing harness to go around under your
armpits, once head and shoulders are in the Newtchute - fixed strap across
the back - snap buckle across the chest - just like a BC.
         Now, how do you get free of the pressurization bottle? ( you do
not want to take it with you) - You rig a QD into the fill hose with a loop
in the hose - small loop, part of a full bend - clamp a non-stretch cable
or light chain to the quick-disconnect slide collar. Hatch opens - you're
gone - hose comes tight - pulls QD slide- QD disconnects instantly.- Scuba
bottle stays in sub.(We use this all the time for different auto
disconnects - we first used it with the JIM ADS in the Canadian High
Arctic- in the early 70's.) slick, huh? 
        Sorry to get so lengthy, Carsten . . .you can see how this goes, I
think.
Phil Nuytten