Re: [PSUBS-MAILIST] Life Support Revisited

["Cliff Redus" <dr_redus@devtex.net>]

For background on this discussion, reference Phil Nuytten's excellent, paper
titled " Life Support in Small One-Atmosphere Underwater Work Systems"
presented at the 3rd International Conference on Life-Support and Biosphere
Science held at lake Buena Vista, Florida, January 11-15, 1998.

As described in the paper, for a 1- atm, system in which CO2 is scrubbed
from the air, the cabin pressure is  reduced and if pure make up O2 is added
to bring the pressure back to 1 atm, then the original mole distribution of
gaseous components of the cabin air will be restored. So theoretically, you
would not have to have an O2 sensor.  But from a practical point of view,
there are many reasons you want to have the sensor.  First, if you are
designing to one of the engineering standards such as ABS, they require a
measurement of the partial pressure of O2. Besides, this is pretty cheap
insurance if some part of the life support systems goes wrong, such as the
barometric pressure sensor being out of calibration, content of makeup O2
tank being different from what you expected, leaking of air or O2 fittings
that would cause a slow pressure build in boat, and scrubber absorbent being
saturated. Anyone doing work on a 1-atm psub should test every design
decision and every system from a safety perspective with the goal of making
the boat as safe as possible.  Should you have O2 and CO2 sensors, on a
1-atm psub, absolutely from my perspective!

Cliff Redus



----- Original Message -----
From: "Jonathan Wallace" <jon@psubs.org>
To: <personal_submersibles@psubs.org>
Sent: Saturday, August 02, 2003 11:12 AM
Subject: Re: [PSUBS-MAILIST] Life Support Revisited


>
> >The simple fact that there has NEVER (in recorded history) been a
survivor of
> >a "compartment fire" in which the PO2 was greater than 25%!
>
> I'm not doubting the fire hazards involved with a higher than normal PO2.
> I'm trying to understand the relationship between pressure in the vessel,
> metabolizing O2, and adding O2 based upon a pressure standard.
>
> Let me see if I can clarify what I'm looking for with another hypothetical
> example.  If I close the hatch and note that I have an internal pressure
of
> 30.00 (inches mercury) and a P02 of .2, I consider that my "dive
atmosphere
> standard".  If everything worked perfectly, when I came back to the
surface
> the internal pressure of my vessel would still be 30.00 and the PO2 would
> still be .2 regardless of how many hours I had spent in the dive.
>
> Now, I breathe and metabolize O2, and exhale C02.  The PO2 in the vessel
> drops from .2 to .19999 and the CO2 finds it's way to the scrubber and is
> collected causing the pressure in the vessel to drop to 29.999.  The very
> sensitive barometric valve in this case, opens and allows in enough O2 to
> bring the pressure in the vessel back up to 30.00.  Let's assume I hold my
> breath for arguments sake so that I haven't altered the atmosphere any
> further.
>
> Is the PO2 now back at .2 or is it higher because I've bled pure oxygen
> into the cabin?
>
>
> Jon