When lagering.....

Should I use an airlock or foil over the top of the carboy? I'm asking because my local brew shop told me barometric pressure can cause the sanitizer in the airlock to be sucked into the lagering beer. Is this true?

Search for "suckback" or "suck back" and you'll find threads. I use an s-type airlock and don't worry too much about it.

Yup - it happens. If you have a lot of headspace in the vessel, a drop in temp (like during lagering) will cause the gasses in the headspace to contract, creating a vacuum. I you put foil over the top, you're just going to be sucking in the air from the room. I've never used an s-shaped airlock (do they really prevent suckback?), but when lagering I always limit the amount of headspace by racking to the appropriate size vessel - no headspace, no suckback.

If you are using a three piece airlock, there is a good guide/example posted in the forums on how to properly set them up.. Search will turn it up.

Also fill it with vodka or similar alcohol so no harm with come with a suck back. Just a little more % alcohol.

I use vodka so if some gets in it is no problem.

You could also rig it like a blowoff tube. I coil mine into a loop and fill it. It would take a serious vacuum to pull that much liquid that far up the tube.

JLem said:

I've never used an s-shaped airlock (do they really prevent suckback?)

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They reverse-bubble, so they suck in air that travels through the liquid in the airlock. Not the greatest thing to be introducing more O2 at that point, but its an easy solution and impetus to drink up once its kegged.

I don't worry too much about it, but you could do a couple of things: (1) put sterile water or vodka in your airlock for the first few days/week of lagering so any "suck back" doesn't harm your beer. After that I think you are safe putting k-meta or starsan in your airlock as you are unlikely to get further suckback merely as a result of barometric pressure changes. If you are worried only fill your airlock halfway or whatever. (2) Re-flood your headspace with CO2 at that point to get rid of any air that might have sucked back into your carboy/better bottle. This is a bit over the top but may give you extra peace of mind.

I have a foam stopper that I sanitize and use when lagering.

I've always used a 3/16" blowoff tube going into a beer bottle with about 3" of sanitizer in it and have never had a problem with any suck back

Like others, I use and suggest vodka. Use any cheap bottom shelf crud not worth drinking. If any gets sucked in, hey, more alcohol. And its such a minute amount you most certainly won't pick up on any off flavorings.

Done. I am now using an S airlock and vodka. Thanks for all the help! Happy New Year to all.