Originally Posted By: Tjin
Put your compass in hot (not boiling) water will reduce or even remove air bubbles.


Only temporarily. Expose the compass to low temperature and/or low air pressure (at high altitudes) again and the bubble will grow back IME.

Nevertheless, it is a quick fix for a little while at least. Also, that's why it's a good idea to carry a compass close to your body on winter treks and in the mountains - keep your compass warm and there's less chance of a bubble forming inside the capsule.