CO2 purity?

On APD #289- George commented on a few possibilities on why someone's
tank pH wasn't lowering, despite the use of CO2.  Number five really
caught my attention.
> 5. Your CO2 source is not very pure.
> George
> in Relatively Calm Colorado Compared to Yesterday's 100 MPH Breeze

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.

Matt Van Buskirk
Where the Salt Lake Valley skies are filled with the beautiful grime of
exhauling cars!