Cold Crashing

[Simon_]

Near freezing (without freezing)

[HomeBrewWest]

By the time the beer is due to cold crash I've usually replaced the blow off tube with a small bubbler (see below). I've found a blow off tube is not necessary on my set up once the initial fermentation has settled down, so I replace it with an airlock.

I'm currently brewing a lot of small batches in 5L plastic 'bottlejohns' with airlock on top. I find the issue you describe comes up a lot for me, not just when cold crashing, but anytime I move the bottle. The plastic tends to flex when I lift the 5L bottle and air is sucked in through the airlock.

My workaround for this is to keep the amount of liquid in the airlock to a minimum, i.e. just enough to cover the 'U bend', but not enough for the liquid to make the leap over the top and into the FV (see pic below).

Its important to always fill the airlock like this.

Atmospheric varies by as much as from 950mb to 1050mb, that's a change of 10% which means that the air space tries to expand and contract by up to 10%.

Buckets will flex a bit, but carboys won't. And even a small amount of CO2 production helps too, unless the beer is left too long in the fermenter.

[Qs]

Near freezing (without freezing)

Or as close as you can get. Leave it an extra day or two if you can't get right down.