Re: [PSUBS-MAILIST] AIR TIME

[Ray Keefer <Ray.Keefer@Sun.COM>]

Hi Lucas,

Being not expert and off the top of my head.

You start with the air in your cabin you had when you shut the hatch.
Which is about 80% N2 and 20% O2 with some CO2 mixed in.

As you breath the O2 gets used and CO2 gets created. The N2 you breath
in and out but it neither gets used or created. N2 is inert. It is just
there.

As the quantities of O2 get used up and an equal quantity of CO2 gets
created the air slowly becomes more and more toxic. Either come to the
surface and ventilate or scrub the air. Scrubbing the air is half the
answer of extended and/or the 72 hour emergancy life support systems.

It uses a CO2 assorbing material that will pull the CO2 out of the air
to keep the CO2 toxicity in check. At the same time it also reduces the
pressure inside the hull due to drawing the CO2 gases out of the air
into solid form in the assorbing material. When the hatch was closed
the air pressure was at ambient pressure at the surface or 1 atm, as
the CO2 gets assorbed the pressure decreasses.

At this point we have two remaining problems. The O2 quantity is being
used up and the air pressure inside the cabin is decreasing.

The solution is to have a tank of pure O2. Which is also the second
half of the typical extended and/or the 72 hour emergancy life support
system.  From this tank you bleed enough O2 into the cabin to replace
what has been used. A quick and dirty method is to bleed enough O2 into
the cabin to bring the air pressure up to where it was when the hatch
was first closed. This replaces the O2 that was used and keeps the air
pressure constant.

Now compressed air can be used to a point but it isn't very useful for
extended life support. Problem with compressed air is for any set
quantity you let into the cabin you get 20% O2   AND   80% N2. Yes, you
get O2 to breath but the extra N2 will increase the cabin pressure.

Increasing cabin pressure has four concerns.

First, once the cabin pressure increases over 2 atm you have to worry
about N2 assorbtion into body tissues. You will need to limit your time
and/or decompress as you come back to surface ambient pressure of 1 atm
or you may get the bends.

Second, once the pressure builds up to about 160FSW equivelant, or
about 6 atm, Nitrogen Narcoses sets in and starts to cloud your
judgement.

Third, O2 toxity. This is where I am getting fuzzy. At some point O2
can get toxic. At 1 atm it is around 23% of partial air pressure. That
changes as the air pressure increases. I'll not say more till I review
my diving physiology book (PADI).

Forth, is the design considerations of your hull. If say the interial
is at 2 atm and you surface into 1 atm air. Can your hull handle a
higer interior pressure then outside? Will you windows popout? Will you
hatch pop open? If either happens while you are almost to the surface
your cabin will flood and your sub will sink. Taking you with it. You
need to design strength into the closures and/or have a over pressure
relief valve to bleed off any excess pressure over ambient at that
depth.

Another possible solution regarding increasing cabin pressure might
be to compress the cabin air to bring it down to 1 atm. All that
would take is power to run a compressor.

As you can see the topic has a lot of facets. Study hard, design well,
build with care, operate conservatively and live long.

Regards, 
Ray





> Date: Sat, 11 Aug 2001 15:48:26 -0230 (NDT)
> From: Lucas Gray <lgray@exa.engr.mun.ca>
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] AIR TIME
> 
> 
> I have a question about air. is it possible to use air in a closed system?
> 
> If you use oxygen, it goes in, gets used, turned into CO2, which then gets
> scrubbed and "leaves" the system. conservation of mass.
> 
> If you use air, oxygen, and a whole lot of nitrogen goes in, as seen
> above the O2 gets used, but what about the N2. It would seem to me that
> you would need an additional scrubber to get the N2 out, or the pressure
> would keep increasing with time as more and more N2 entered you boat.
> 
> Is this right?, does anyone know the deal?
> 
> lucas
>