Re: A Simpler Sub

[SFreihof <SFreihof@aol.com>]

<< So now I'm thinking again about really pared-down, ambient-pressure,
 only-partly-dry designs. I'm thinking about some sort of skeletal cagelike
 contraption on which to hang weights, batteries, motor, a seat, and a
 glassed-in box to stick my head in, open at the bottom.
 
 With this approach, I wouldn't be diving very deep or for very long, so I
 could just come up with some simple way (air compressor? snorkel tube and
 blower? A bucket?) to change the air in my "bubble" after a dive. So I'm
 wondering how to find out -- short of putting my head in a plastic bag and
 starting a stopwatch -- how long I can breathe happily out of a volume of
 about 9 cubic feet (0.25 m^3), without changing air or scrubbing CO2 or
 anything. Any ideas? >>

I wouldn't attempt it without a fresh air supply onboard, even if it's just a
scuba system.  Another possibility is surface supply, such as a Brownie Third
Lung, which is a compressor that floats in an inner tube on the surface and
pumps air down.

As promised, here are the requirements for cabin air as per NOAA Dive Manual
of January 1975. (I wish I had a more recent edition.)

Partial pressure of oxygen should be
maintained at between 0.19 to 0.5 atmospheres. 
Partial pressure of Carbon Dioxide should be
between 2 to 7 mm of mercury.  Carbon Monoxide,
as a trace contaminant, must not exceed a partial
pressure of 15 X 10(-3) mm of mercury.

Also, requirements for flow rates for cabin flush without scrubbers:
2 cubic feet per minute (cfm) for each person at rest, 4 cfm for persons not
at rest.

At that rate, 9 cubic feet of air is good for about 4 minutes.  As you can
see, that would require up to 240 cubic feet of air per hour per person.
That's not practical in most cases for onboard air supply.  The simplest way
is to use a scuba type supply.