Straight out of Felder and Rousseau, "Elementary Principle of Chemical Process"

1atm = 33.9 ft H20 at 4 deg C.

Jake, I disagree. If the pressure (vacuum) gauge reads lower (less) than 0 psia then you are exerting more than 1 atm of pressure on the item in the bag. The benefit of the autoclave is that you can reach the high activation temperatures required for prepeg AND put it under pressure.