There are a few ways to prevent sanitizer from getting drawn back through the blow off tube from liquid contraction during crash cooling.
Users with a pressurized transfer setup and a high-resolution gauge can apply a small amount of pressure to the vessel. We recommend applying just 0.5 PSI to fill the head space with CO2. This method maintains a fully closed system and prevents oxygen contamination.
Another great way for owners of Chronicals or Brew Buckets to keep a closed system is to remove the blow off assembly, sanitize a standard party balloon, then fill it with CO2. Then attach the balloon to your blow off barb/cane. The small amount of pressure from the balloon forces CO2 back into the fermenter.