Re: [PSUBS-MAILIST] life support method?

[John Brownlee <jonnie@cobweb.scarymonsters.net>]

On Thu, 3 Aug 2000, Chris Horne wrote:

>1) Does the absorbant actually absorb CO2 and not return anything into the air?

	Yes. It is a binary chemical reaction between the CO2 and the
absorbant  most of the time. It is important to note that this is
different from a molecular seive (i.e. zeolote bed) that absorbs the CO2
into it's crystal structure without changing the chemical composition of
the absorbant in the bed.

>2) If not, does it just grab off the carbon atom and return O2, but just not at
>100% efficiency?

	Nope. CO2 is a pretty stable molecule and does not divide easily.
Absorbants are not, de facto, 100% efficient though. The reaction
rate-laws will greatly favor driving the absorbtion of CO2 into the
metallic oxide to form another solid, hence removing the CO2 gas from
circulation.
	The classic reaction here from a textbook is often an alkali
metal oxide reacting with CO2 to create a carbonate:

	X O  + CO2  --->  X CO3

	It's not always as simple as this, but it's important to get the
idea of it. Most of the time the absorbant material is undergoing a
(sometimes non-reversible) chemical reaction that takes the CO2 gas and
turns it into a solid, thus removing it from the atmosphere.

<PLANETARY SCIENCE>

	Complete aside: Earth and Venus have very different atmospheres,
but very similar bulk compositions. One idea is that the CO2 from
volcanism on the Earth is largely not in the form of a gas, it's fixed in
the bottom of the ocean as limestone by wee critters. If you take the bulk
of carbonate rocks and trip them of their CO2, you get a very similar
atompshere to Venus! Thus, fixing carbon dioxide is essential to the
survival of life, and yet living things seem to be the center of that
mechanism. Earth supports life because it has living things on it to begin
with! CO2 absorbtion isn't just for subbers!

</PLANETARY SCIENCE>

>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)

	Kind of. There is actually a slight difference between the volumes
of O2 and CO2 we are taking in and out, largely because our bodies are
complicated chemically. It is easy to think that the reason our gaseous
wastes aren't totally balanced is because we produce other kinds of waste
(solid, liquid, other gasses, etc.) This often doesn't come into play with
psubs, though.

>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.
	
	Well, for starters many sensors are easily finagled into
outputting partial pressures. Also, the chemical reactions we are
concerned with are sensative to partial pressures (Boyle's law? Been a
while since I had to put names to this stuff) as well as total pressures.
>From a design perspective, it can be easier to measure the total pressure
with one method (i.e. mechanical altimeter) and partial pressures of
gasses we care about (CO2/O2) by another (electrochemical cell). With
rebreathers you have the added complication of ambient pressure, which
makes the question of partial pressure the more sequitur. Straight
percentages don't convey this, and a partial pressure of O2 that is fine
at one pressure can be lethal at another. You let on to this in your
mention of dive tables above.
	In a malfunction a 1atm sub can quickly become 'not-1atm.' It's a
good idea to keep this in mind when designing a life support system.

>4) Can you get pure O2 from a consistent source without having to jump through
>hoops?

	Yes. This is what regulators are for, as long as the pressure in
the bottle is higher than the pressure on the outside of the regulator you
get a constant stream at a given pressure. It's up to you to then turn
this into the rate problem of balancing the atmosphere for partial
pressure.

	Someone please correct me if I'm out to lunch here,

							John

John Brownlee
Chief Systems Administrator
Scary Monsters Network
jonnie at scarymonsters dot net