Had an idea so I figured I would share my experience. When crash cooling with a better bottle, the beer shrinks significantly and will suck the liquid in your air lock / blow off tube back into the beer. I have an idea on how to combat this. Right before setting my chamber to crash cool, I install a Schrader valve (similar to the valve on a car tire), see picture 1, in my blow off tube, see picture 2. Then I tape down the fermenter bung, just in case it wants to pop out, see picture 3. I installed a pneumatic quick disconnect on my CO2 tank for travel purposes so I can disconnect it from by regulators/manifold and install a normal tire filling tool to the quick disconnect, see picture 4. I turn the pressure on the regulator down to 2psi and purge the tire filling tool with CO2 and then slightly pressurize the better bottle. No more than 2 psi (these things probably aren't meant to be pressurized), you will hear the gas stop flowing as the pressure normalizes between the regulator and better bottle. Then I set the fridge to cool. The next morning, the sides of the bottle will probably still be sucked in but at least you did not suck the backwash back in. I then add more CO2 until the sides are popped back out and let everything settle before racking off.
