S-type Airlock question

[MedievalForester]

Now I know this is a dumb question, but should the red cap be left on the airlock during fermentation, or taken off? I was looking over it and can't decided if leaving the cap on would build up air pressure or if it is designed to release the gas with the cap on.

[Angus]

In terms of pressure, it really makes no difference, although having it on prevents things from entering the airlock that could, given the right environment, grow and increase the danger of contamination. My advice therefore is, always have the red cap on.

Angus