Dear Mr. Wizard:
I brewed a batch of lager (a three-gallon batch in a five-gallon carboy) and instead of the normal CO2 venting out of the air lock, a vacuum formed. I used a wort chiller and pitched the yeast at 72° F. At 18 hours the cap was sucked down onto the vent tube with liquid (vodka) suspended in the vent tube (wort temperature 64° F sitting in my basement). I moved the carboy to a refrigerator sitting in a cold garage and the wort temperature dropped to 54° F over the next 18 hours. The same vacuum was present at 36 hours. Finally after 48 hours a slow fermentation was going with the cap raising, but it was three-plus days before a strong fermentation was present. Could the rapid temperature change or poor yeast start have caused the vacuum? What happened and what can I do to prevent this in the future?
Mr. Wizard replies: What happened to your fermenter was due to the relation between gas temperature, volume and pressure. A sealed container will exhibit a decrease in gas pressure if the container is cooled and conversely its pressure will increase when heated. Fermentation airlocks allow gas to escape and a cool carboy that is heated will not build pressure. However, a warm carboy that is cooled will exhibit a drop in pressure. This vacuum will suck liquid from the airlock into the carboy. Eventually the airlock will be empty and the vacuum will be relieved. If your fermentation took off more quickly and/or your wort was cooled to the same temperature as your basement this would not have occurred. This phenomenon is easy to avoid early in the process, but if you cool your airlocked carboy after the primary fermentation is complete, the vacuum will return. I prevent any liquid from an airlock being sucked into my beer by removing the airlock and covering the carboy with aluminum foil. After cooling, I replace the airlock.
Vacuum is not a major problem for homebrewers, but it is perhaps the most devastating force present in large tanks. Big tanks are designed to withstand some sort of pressure, but are rarely designed to cope with vacuum. Big tanks can be sucked in like a tin can if they are not treated properly, for example rinsing a large tank with cool water after a hot wash. If you keep in mind that a vacuum is created when you go from hot to cold you will easily prevent this problem.
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