Originally Posted by emgesp1
I filled up the airlock with sanitized water to the line and left it alone, a couple hours in I checked on it and noticed the water was under the line
Is this a 3pc (disassembleable) lock? When the primary cools the floating cup in a 3pc gets sucked down to the bottom o fthe lock. Further vacuum will pull the airlock liquid down into the primary. Assuming the liquid is clean and orthodox this should cause no problem.
In situations where negative pressure is suspected (ambient cooliing, container flexing while moving) an S-type bubbler will allow the air to move backwards without sucking the liquid through.
Note that when your primary is outgassing lots of CO2 it is possible to lose liquid out the other end due to evaporation, overflow, splashing, etc. Evaporation is accelerated with volatile liquids like vodka and heavy outgassing as with ApfelWein. Luckily Ed warns us about this in advance.