CO2 use/rates/etc.

I am a real diy CO2 novice, but I thought I would share some
experiences.  First, getting the yeast going was easy, as I thought it
would be with the baking experience I have.   

Second, I think the yeast is a pretty inefficient converter of carbon
to CO2, since there is so much "waste" alcohol in the spent culture. 
Anyone have a use for this?   I think only about 1/3 of the carbon
ends up as CO2, so that means it takes about 3# of sugar to make one
pound of CO2.  Where I am the sugar costs about $1.20-1.50 and the CO2
costs $.80-$1.00.  So the yeast is not only messy, it is expensive in
running costs.  Comments?

Third, I started with an airstone as everyone seemed to indicate plain
bubbles were not enough.  This was hard to keep going with the culture
at just the right pressure to produce a nice stream of tiny bubbles. 
So I made a diy pressure regulator.  It worked (mostly), but taught me
that I needed to use large chambers to last overnight.

Then I took an abandoned CO2 fire extinguisher and ran it into a
reactor that was a 2l Coke bottle with the top cut off, then inverted
in the tank.  Since I could easily see the gas level and time things,
I was able to determine that the reactor delivered about 1.5l/day to
the tank.  This would be about 3g/day or about 150 days per pound of
CO2.  I have no test kit so I do not know what the CO2 level is yet. 
My pH meter is still in the mail too.

If anyone has any info or advice, please fire away.   Thanks.   Dave
Dave Gomberg, Experimenta      San Francisco CA      gomberg at wcf_com