I was recently reading an article in BYO about plastic fermenters and their oxygen permeability. I know that plastic fermenters do have a degree of oxygen permeability, but I'm having doubts about it during fermentation. What I mean is this: If beer is fermenting or has been fermented, we assume approximately .5 - .8 (from what I remember reading somewhere) volumes of CO2 have been absorbed in the beer. If nothing is disturbed (meaning if the blowoff tube or airlock is not removed) then the contents of the plastic fermenter will remain under a positive pressure. I realize it is not a high degree of positive pressure, but as long as it stays above ambient air pressure, then how does oxygen get in? Again, if the beer is moved into a plastic fermenter and pressure/carbonation is lost, then I understand. Just a thought that I felt was worth discussion.