"Glen, what you said makes sense, but what would the bubble be composed off otherwise? I am pretty sure the bubble wasn't caused by a leak, as I watched it shrink and disappear after allowing the compass to sit on a laptop power brick for a while."
Bubbles can form in several ways. Poor quality control and manufacturing. Excessive heat causing a leak. Low pressure (think aircraft hold) causing a void (bubble) to appear. An extreme temperature drop, causing the fluid to contract to the point that a bubble forms. The latter is the most common.
Don't forget that watchband compasses have very small capsules and the case is fairly rigid (some larger compass capsules are flexible enough to serve as a kind of expanding/contracting diaphragm). My guess (and it's only a guess) is that when these small compasses are filled, probably at a medium temperature, it doesn't take much of a temperature drop to cause a bubble. Otherwise, why should so many develop bubbles even before they are sold off the rack?
Bubble issues are less common in fullsize baseplate or orienteering compasses. Though they can occur, they are usually small and insignificant in my experience (certain Brunton Eclipse models excluded, seen a few whoppers there, though they've always disappeared after warming). Don't forget that liquid-filled baseplate compasses are issued worldwide, especially by nearly all the Northern European and Scandinavian military forces (Sweden, Norway, Finland, Germany, Denmark, France, United Kingdom, etc, etc.). Also Canada and the German Alpine Association. The original Suunto liquid-filled compass was invented in Finland by an officer in the Finnish Army who was unhappy with issue air-filled individual compasses and the tendency of needles and pivots to freeze up. The new Suunto was intended to be used in the field, in Finnish winters, and many have seen exactly that type of service.
"I dropped by this shop..and picked up a "Virginia Outdoors" water-filled lensatic compass, which looks like a Cammenga clone. Construction is metal, and quality seems decent, though the movement of the card is slower than my Suunto Clipper, though no less accurate. Does anyone know anything about this compass, e.g. where it's made in, quality, etc?"
If you're willing to accept an educated guess, it's one of the many Chinese- or Taiwanese-made lensatic compass clones. They come in recent (Cammenga) and older round ('engineers') lensatic-style compass housings, except that virtually all use the same liquid-filled shallow compass capsule (shallow meaning it's easy to ground the compass card if you tilt the compass even slightly while taking a bearing). Most have zinc or plastic outer housings instead of the issue aluminum. Quality can vary widely, as well as accuracy (some of these have compass cards marked every degree and can be read to 1/2 degree, theoretically). Believe it or not, I have a $5.00 Wal-Mart example with a plastic housing that is accurate to 0.5-1 degree and has NEVER suffered from a bubble in all kinds of temperatures, but I have seen several that don't approach this level of accuracy. It is cheap though, and I did have to add a copper tab to the housing to keep the compass dial from freely rotating in its housing and to keep a bearing locked in!
"In any case, I think I'll get the Cammenga M27. Won't be doing any jungle training myself, and there's the one-year warranty if it comes to that."
Understood. There are very few compasses that don't have some type of drawback, so I thought you'd want to know about the air-filled models.
"As an aside, I just found out the black prismatic compass issued to me in the military was a British Francis Barker M73. I would've been a lot more careful with it had I known it retails for US$390 new! It nevertheless took all I threw at it (or rather threw it at), and it worked great in Singapore's humid rainforests (80+%)"
Not to be a contrarian ALL the time, but I have one of these, and I have to tell the truth - although the M73 is fitted with a diaphragm, and was recently reconditioned, it still regularly forms a bubble at low temperatures (always disappearing on return to room temperature). The M73 (even recent production) is in reality quite an old design, and uses alcohol as the liquid dampening fill, much like some marine compasses. My theory is that the alcohol is less suited to large, radical changes in temperature than the light oils which now predominate as the liquid of choice in modern compasses. But it's only a theory.
Worst bubble I ever saw (and which has never disappeared) was on a Plastimo Iris 50 marine handbearing compass. Never even made it to sea before a huge bubble formed - it was stored at room temperature on a shelf away from extreme heat and extreme cold, though the difference between daytime hi and nighttime low could be up to 40 degrees F. Plastimo says the compass' diaphragm should have prevented any bubbles, but it didn't for me.
