Originally Posted by TXBeowulf
What keeps a big enough pressure change from sucking water up into your beer?
CO2 is escaping pretty much constantly applying positive pressure until it slows to a trickle/none. So there never is negative pressure.
Temp changes CAN create "suck back". I personally have NEVER had a violent enough temp/barometer swing to cause one, but I use glass carboys to ferment in.
The most common cause of "suck back" is useing a plastic bucket/ale pail with the lid. When the bucket is lifted, the bottom sags just a little bit causing the suck back. This is SUPER easy to avoid, just simply take off the tube or airlock prior to lifting it.
When I use the airlock I fill mine with whiskey because if a little whiskey gets in my brew...who cares? It will simulate aging in a whiskey barrel if you can taste it at all...