An optical detector on a spirit tube can work.  I have
done some preliminary experiments with a spirit level tube, light
source and photo detector.  One can pick up the bubble though my
initial test setup was crude to say the least.

	I wonder if a capacitance probe against the glass of a spirit
tube would pick up the difference when the bubble went past or was
under the probe?  The advantage to that would be that ambient light
wouldn't interfere with the detector.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group
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