Compasses made for orienteering have the needle in a sealed
transparent plastic capsule, which is filled with a light oil
for damping. Occasionally bubbles appear in the oil, and
interfere with the movement of the needle, which is very
annoying.
Sometimes the bubbles are due to tiny leaks. These get bigger
and bigger, and eventually you have to buy a new compass.
Sometimes, however, the bubbles disappear by themselves, and
orienteers on the orienteering list are asking what causes these
to appear in the first place.
It is true that trips to high altitude often seem to cause the
bubbles to appear, but I would rule out change in atmospheric
pressure as the cause, since I would expect the pressure change
inside the capsule to be negligible.
On the other hand, a significant temperature drop could cause a
big drop of pressure inside the capsule due to volume
contraction of the oil. I would have said that bubbles of
vapour of volatile components of the oil could form, but these
bubbles take some time to go away once the temperature returns
to normal, as if they are gas bubbles that need to redissolve.
The extent to which air dissolves in oil is outside my range of
experience. Does anybody know if this is a reasonable
hypothesis?