Slightly niche compass question….

[ChrisB]

We are not really sure where the ‘air’ comes from. Possibly dissolved gas in the mineral oil. It could be that diving with them causes some kind of rapid change in pressure / nucleation event that creates the bubble. They never seem to go away and re-dissolve though.

Whatever the cause, it's not diving, as it happens to Silva compasses that have only been used above water.