Length of cold crash?

[TechBrew (Richard)]

@ynotbrusum

Crash pretty much as fast as you can, but realize that the vacuum created by the drop in temperature in a sealed vessel needs to be accounted for and CO2 applied to equalize that - and as to blow off tubes/airlocks on unsealed vessels, you will get suck back on them.  Pretty much a given and most of us provide for CO2 to avoid that, but just thought I should mention it for “clarification” (pun intended).

I have two holes in the (4-inch) cap for my fermenter. The first one has an airlock. The second one starts with a stopper in it, but after fermentation has been going long enough that I feel that all the air has been blown out of the headspace, I put in a piece of stainless tubing connected to a standard “Happy Birthday” mylar baloon. When the balloon is full of CO2 and generates some backpressure the airlock begins bubbling again. When I cold crash the balloon provides more than enough gas to keep anything else from getting sucked back in. Why buy CO2 when you are already making it?