Airlock liquid disappeared overnight.

[jivex5k]

Read that it could be a crack in the airlock causing this. It happened once on this batch so I switched airlocks to another one that's never had an issue. Happened again last night.
Any way to stop this? Don't like the idea of star san solution mixing directly into my beer.

 

[flars]

Are your airlocks the 3 piece design or the S style? You may have the air lock filled to full if it is the S style. To full will allow a siphon action emptying the contents into the fermentor. I have never used the 3 piece style so can't comment on how they work.
For this to happen though your fermentor would have to be experiencing large temperature or pressure changes.
How are temperature controlling the fermentor?

 

[jivex5k]

It's in a cool brewing bag, was keeping it around 64 using frozen water bottles. It's a sour mash that got too low, yeast never kicked off so I let it warm out as a last resort. Have the 3 piece airlock, filled to the line.
Apparently it got cold during the night and created a vacuum according to another forum. Going to throw some brett in it from my other sour mash that got to low but is actively fermenting.

 

[unionrdr]

You just answered your own question. When the temp drops, it creates a partial vacuum in the FV which "sucks back" the airlock liquid. Such a small amount of Starsan is OK. Plain water would be a problem, however. Nasties in the water sucking back can infect the beer. I use a 50/50 mix of 80 proof white lightnin & Starsan now.

 

[hunter_le five]

That's why I prefer the "S" type airlocks over the 3 piece. When used properly, the "S" airlocks do not allow the liquid to be sucked back into the fermenter.

Rapid cooling of the wort will cause suckback due to the pressure change.

Edit:

When I do use a 3 piece airlock or blowoff tube, I use vodka for the liquid so that if it is sucked back, it won't affect anything really.

 

[unionrdr]

I've gotten suck backs with both types & blow outs as well. The S-types are harder to clean the the 3-piece imo. I was using vodka in'em. But now use a 50/50 mix of 80 proof white lightnin & Starsan.

 

[hunter_le five]

If you're getting suckback on an S-airlock, then you're probably putting in too much liquid. But I agree, they are a bit harder to clean if you get gunk in there, but (usually) nothing a good PBW soak can't fix. Fortunately at $1.50 they're almost disposable.