Re: [PSUBS-MAILIST] life support method?

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Sean T. Stevenson" <ststev@uniserve.com>
To: <personal_submersibles@psubs.org>
Sent: Thursday, August 03, 2000 9:28 PM
Subject: Re: [PSUBS-MAILIST] life support method?


> On Thu, 03 Aug 2000 16:27:14 -0400, Chris Horne wrote:
>
> >I think he's just trying to automate it so that he doesn't have to do it
> >himself- it takes care of itself; I tend to agree.
>
> This approach can be taken too far, however.  Despite automation, you
> should always be aware of the oxygen level, and be prepared (and so
> equipped) to control it manually if you need to.
>
> >However, I've been doing some thinking and a little research on
rebreathers. It
> >seems that basically the way a rebreather works is to use an absorbant
(namely
> >Sonfalime, or whatever it's called) to absorb CO2 from the air, assuming
that
> >most of us mortals breath in O2 and exhale CO2. Thinking about all of
this has
> >led me to several questions:
> >
> >1) Does the absorbant actually absorb CO2 and not return anything into
the air?
> >2) If not, does it just grab off the carbon atom and return O2, but just
not at
> >100% efficiency?
> >It seems that if it absorbed CO2 only, and we simply added a little O2
every
> >once in a while to compensate for the loss, then the system should stay
> >balanced- but is this true, given a 1atm environment? (I don't know- I'm
asking)
>
> The scrubber materials typically used in rebreathers (Sofnolime,
> lithium hydroxide, etc.) have a high affinity for CO2.  They do not
> break it down into C and O2, but rather capture the CO2 as it is passed
> through it.  The greater the surface area of scrubber material that is
> exposed to used air, the more effective it will be.  This is why
> scrubber canisters with radial paths (gas enters at center, exits at
> circumference) are used in many systems over linear path scrubbers.
> Two things that will reduce the scrubber effectiveness are moisture (in
> the case of lithium hydroxide, moisture is REALLY bad... I'd stick with
> Sofnolime) and loosely packed material, which can permit channels to
> form which allow gas to pass through without being scrubbed.  If
> everything works as it should, then with passive or correct manual
> addition of O2, theoretically the system will stay balanced in 1 atm or
> any other pressure.
>
> >3) Why does everybody watch partial pressures, and do we need to if the
sub is a
> >1atm environment? I assume it has to do with percentages changing as the
> >pressure changes (in a typical diver/rebreather configuration), and thus
they
> >cannot rely on just percentage of O2.
>
> In 1 atm, gas fractions and partial pressures are equivalent
> expressions.  Partial pressures are used as they can be applied to the
> widest circumstances.  Surface air has an oxygen partial pressure of
> about 0.209 ata.  At rest, the human body can typically endure oxygen
> partial pressures from 0.16 ata to well in excess of 2.0.  In the
> presence of a pressure differential across the body (such as a diver in
> the water), or where synergistic effects of other gases are concerned,
> these limits tend to tighten up a bit.
>
> >4) Can you get pure O2 from a consistent source without having to jump
through
> >hoops?
>
> I used to get O2 from Praxair, until I realized just how badly I was
> getting hosed on the pricing.  I now deal with Air Liquide for both
> oxygen and helium.  If you want a half decent reference on dealing with
> gas companies to get what you need, take a look at the "Oxygen Hacker's
> Companion" from Airspeed Press.
>
> >-chris
>
> Hope this helps.
>
> -Sean
>
>
>