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Tiny Bubbles?

All our dive compasses are liquid filled, not with champagne, but with mineral oil to dampen the movement of the compass card and also protect the compass case from pressure at depth. The liquid is sealed in these compasses with a bubble of air. Liquids are considered incompressible; the air bubble provides an balancing expansion space because air is very compressible. A bubble will prevent the case from breaking when the liquid expands if heated. The bubble will change in size or even disappear entirely depending on temperature and pressure, including altitude and weather. Because the compasses are liquid filled with no compressible air space other than the insignificant tiny expansion bubble they are unaffected by pressure at depth and do not have a depth rating.

A small bubble is normal, but compass manufacturers recommend replacing the compass if at sea level and room temperature a bubble larger than { 0.25 in | 6 mm} develops. A larger bubble is indicative of a defective compass.