What causes–and how do you prevent–air bubbles from showing up in liquid filled compasses? Just about every liquid filled compass I have used usually gets a small air bubble in it.
First off, Mark, never fear: a small bubble in your compass will not affect its accuracy. Bubbles can form from high elevation or exposure to cold temperatures, which cause the liquid to contract, leaving an air bubble behind. Usually, the bubble will disappear when the compass is returned to sea level and/or room temperature. If it doesn't, place it in a warm spot-like a sunny windowsill-so the liquid can heat, expand, and return to it's normal volume.