Breathing Bung, #10
The silicone breathing bung can function as an airlock for extended aging of beer and wine. Made of natural white silicone, the bung has vent holes through the center, and a silicone "flapper" that fits tightly against the holes. This allows gasses to escape by pushing up against the flapper, and then the valve shuts protecting the product from contamination.
#10 breathing bung is recommended for use in PET carboys.