That would be the initial condition. However, the internal pressure will increase, up to the point where the amount of CO_2 being blown out the other end matches the input. That will require a _lot_ of CO_2 to be pumped in, and hence a large increase in the pressure. Franklin T. Adams-Watters -----Original Message----- From: Henry Baker <hbaker1@pipeline.com> If I remember my physics correctly, air pressure is approx. 14.7#/sq in, which is the weight of the entire column of air 1 inch square sitting on the square inch. According to a source on the internet, standard air has an average molecular weight of ~29, while CO2 has a molecular weight of ~44. So I would estimate that the same column of pure CO2 would produce a pressure of approx 22.3#/sq in. This would mean that to pump pure CO2 into the "stack", it should take a head pressure of 7.6#/sq in. ...