I have always used either a blowoff tube or a three-piece airlock (one with the small, floating cap on the exhaust tube) for fermentation. Today's batch is my first using the two-piece (S-shaped) airlock. I filled both chambers to the max line marked on each tube and noticed that, after several hours, the sanitizer from one of the chambers has almost completely transferred to the other side, with only a small amount of sanitizer remaining in the U-bend beneath the two chambers. Is this normal for two-piece airlocks? Should I pull it out and refill again with sanitizer or vodka so both sides are equal in volume?


  • Instead of putting sanitizer in the airlock, i put a little bit of vodka in it. So if I am moving the fermenter around and some of the liquid gets dumped into the fermenter, its no problem. Sep 13, 2011 at 14:10
  • If its all on one side, how do you know when it's done fermenting? Jun 10, 2019 at 23:58

2 Answers 2


It's behaving exactly as it should. Notice that the max-fill line is halfway up the tubes: that leaves plenty of room for the gas to fill one side without the other side overflowing with sanitizer.


The gasses escaping from the bucket (mostly CO2) push the liquid from one tube to the next. The liquid is pushed until the gas can float upwards through the liquid to the exit of the airlock.

A similar thing happens in the 3 piece airlock, only is a little harder to see. The level of liquid inside the area under the cap is pushed downwards, until the gas can escape around the outside of the cap.

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