Bubbles in Compass -- Help
Foobar T. Clown
> I've got a Vion/Morin 2000 Mini-compass (the famous old hockey puck
> hand bearing compass) with some air bubbles. I see what looks like
> a wax seal over a fill hole and a "thin" fluid.
I'm sorry, I don't have the answer, I just have the same problem.
I've had it twice now, which is the same number of times I've taken a
bubble-free hockey puck on a commercial flight to the Carribean in my
carry-on. (It was brand new the first time, so I took it back to the
store and exchanged it after the cruise, but the warranty was history
when it happened again.) Both times, the bubble appeared after two or
three days aboard.
I took the rubber armour off it when I got home, and now that I know
how they're constructed, I won't buy another. Below is a crude, ASCII
cross section showing how the bottom plate, (the X X X part) snaps
into the body. The "OO" is an O-ring seal.
+----------------------
| +-----------------
| | - - - - - -
+--+ | - - fluid - -
| __| - -space- -
| |OO - - - - - - -
| |+-----------------
| || X X X X X X X X
| |+-----------------
|____/
My wild-ass guess is that the floating back plate and the O-ring seal
act like a one way valve. Fluid blows past the seal when the pressure
inside rises above ambient (e.g., when the compass is taken aloft in a
plane), but the back plate clamps down on the seal and doesn't let
anything back in when the plane comes back down. So now, we've got
this compass with the fluid inside still at 10,000 feet, and we bake
it in the Carribean sun. Heat elevates the vapor pressure of the
fluid, and as soon as it's higer than the reduced pressure inside the
chamber, a vapor bubble forms.
Since the housing is transparent everywhere, it's pretty easy to see
that these compasses don't have expansion chambers.
I once got rid of a vapor bubble in a cheap, hermetically sealed
hiking compass by subjecting it to some extreme temperature (I forget
whether it was hot or cold), but I won't do the same thing to my Vion
compass because I fear that the O-ring will just leak more fluid.
Maybe someday I'll drill the plug (it's too hard to poke a needle
through), re-fill the compass, and re-seal the plug hole, but not
today: I'll wait until the compass is unuseable.
West Marine sells a similar type of compass. I didn't buy it the
first time because it's smaller, and you can't easily use it as a
steering compass. (My hockey puck is essential when I charter from a
certain company whose custom cockpits don't allow the helmsman to see
the compass if he or she sits down.) I don't know who makes the West
Marine compass, but someday I hope to buy one just to find out if it's
any better.