Re: [PSUBS-MAILIST] life support method?

["Sean T. Stevenson" <ststev@uniserve.com>]

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