Re: Pressure regulation

[SFreihof <SFreihof@aol.com>]

Hi Dan;

In a message dated 98-03-13 17:47:03 EST, you write:

<<  I am designing an all acrlylic sub. I have a small plastic fabrication
 shop that I build aquariums in but I'm not set up to vacu-form veiw port
 shapes yet I'll let you all know when I am. 
 
 As for air supply- since my sub will be designed for relatively shallow
 depths less than 30m I thought I would use a low voltage air compresser
 and a metering valve to expell air at a constant rate and a SCUBA type
 compressed air supply fed through a diaphragm valve to maintain a stable
 air pressure.
  >>

I have several comments to make, first concerning the basic design of your
sub. First, let me say that I think an acrylic sub would be REALLY unique!
Maybe I misunderstood your design intentions, but I assumed you were
attempting to build a 1 atm (pressure hull) sub.  Using acrylic as a
structural material though, I would worry about trying to make it a 1
atmosphere (or pressure hull) submarine.  So I suppose you are considering
either an ambient pressure design (either semi-dry or dry), or a wet design.
Either ambient or wet design will eliminate most through-hull stress, since
internal and external pressures are balanced.

The design I have chosen is ambient pressure dry, which is pretty simple
because it's not much more complicated than inverting a glass and submerging
it.  The air can't get out, so the water won't come in.  Both dry and semi-dry
ambient pressure subs work this way.  In semi-dry ambient pressure subs, the
cabin volume is actually the ballast tank, and bouyancy is controlled by
controlling the water level in the cabin.  You sit in water with your head in
a bubble, so to speak.  Not much advantage over a wet sub, in my mind.  You
probably already know all this.

The drawback to ambient pressure subs is that you are under pressure with
depth, so the normal limits of scuba diving apply regarding depth and time.
The deeper you go the less time you can stay without requiring decompression.
The operator becomes the limiting factor.  The advantage is that the hull
strength requirements are not nearly as severe as a pressure hull design, with
a tremendous weight savings which can be used to carry loads like batteries
and/or equipment.

The design I am working on has two hatches (redundancy is good)!  One is on
top for entry and egress on the surface, and one on the bottom for
entry/egress when parked on the bottom, so it serves as a diver delivery
vehicle as well, while still a 'dry' design.  Of course ballast tanks are
required since the cabin volume is constant in a dry sub, and therefore cannot
serve a dual purpose as a ballast tank.

I have opted not to use a scrubber because of cost and inherent dangers.  Most
CO-2 absorbants create toxins when wet (correct me if I'm wrong), and are
expensive because of the need to monitor and regulate CO-2 and O-2 levels.
I'm also not sure how pressure affects the efficiency of the absorbants.
Consequently I haven't done much research on scrubbers.

That leaves two options.  Either you must 'flush' the cabin with fresh air, or
you must provide a breathing air supply directly to the occupants.  Flushing
the cabin with sufficient fresh air to maintain an adequate O-2 / CO-2 level
becomes prohibitive because of the flow rates required.  You would either have
to carry a hellofalotta air, or you'd have to substantially limit your dive
duration.  I can provide you with specific info on required flow rates... it's
in the NOAA Dive Manual.  Consequently, I have opted to use personal supply to
the operator.

Also, remember that in an ambient pressure dry design, cabin volume is
critical because of the bouyancy it creates... something like 64 lbs. per
cubic foot depending on whether you're diving in salt or fresh water.  The
bigger the volume, the heavier it has to be to sink the bubble.

Maintaining the cabin volume is a simple matter of maintaining cabin pressure
equal to water pressure.  That's exactly what a scuba regulator is designed to
do, and that's exactly how I intend to do it.  Anybody have any comments on
that approach?

All sub designs interest me, but the ambient pressure dry design is what I'm
working on, and therefore is the focus of my interest.  I'm seeking comments
and input.

Down bubble!

Stan