CO2 purity?

> From: Matt VanBuskirk <j.vanbuskirk at m_cc.utah.edu>
> Date: Fri, 01 Nov 1996 17:48:40 -0700
> CO2 is not very pure?  Is there differen't grades of C02?  I have
> getting my tank filled at US welders here in Salt Lake City, Utah, the
> price seems right and they can fill it on the spot.  On the other end of
> town we have a medical supply company that also fills co2 tanks, but it
> takes a day to get the tank back.  
> Question- How does one know if the CO2 they receive is pure?  Would you
> assume it's better to have your tank filled at a medical supply company
> vs. a welding shop?
> The idea of not getting pure co2 is troubling to me, since I too have
> had similar problems as the original poster with not being able to lower
> my pH.

Sorry to give you a case of FUD (Fear, Uncertainty and Doubt).  I was
thinking more along the lines of DIY CO2 and differing reactor styles.

Compressed CO2 from bottles is very pure, whether it's food grade or
otherwise (as previously noted). 

I wonder if chemically generated CO2 (yeast generated or otherwise) is
all CO2 or if there are significant impurities in it?  When the yeast
mixture is first started, the bottle has some air in it.  This leads
me to believe that the output is not pure CO2, at least at first.  

When you use an "inverted bowl" type reactor (upside down bottle,
Tetra CO2 bells, etc), gases will diffuse out of the water just as CO2
will dissolve into the water.  If the reactor is pure CO2 to start
with, the partial presures of other gases will be greater in the water
than in the reactor and some gas will move into the bell.  If you
don't "empty" the reactor every so often, you will have less and less
pure CO2 in it.  When I was using Tetra bells, I noticed that after a
few days, only half of the gas in the bell would dissolve instead of
most of it, i.e., the water level in the bell would only rise halfway
up instead of almost to the top as the gas dissolved.