The barrel-style airlock contains an outer barrel and free-floating inner dome that sits overtop or the inner stem. The third piece is a perforated plastic lid to keep debris out of the barrel and minimize evaporation.
Insert the inner dome over the stem; fill airlock barrel with filtered water to the fill-line (1 1/2 tablespoons water). Snap the lid overtop of the barrel. Top up the airlock with water when the water level falls below the fill-line. Failing to do this will break the airlock, allowing air inside the ferment vessel. Appropriate for all jar sizes. Can be swapped out for the mini airlock after the room-temperature phase, before transitioning the ferment to cooler storage.
Allows a visual passage of gas pressure to escape but allows no air to enter.