Don,
That is a vacuum gauge that measures vacuum in units of inches of mercury.
I do not think that a common engine vacuum-gauge needs to actually use/contain mercury; it is just calibrated in inches of mercury.
I might be wrong.
When I mentioned mercury, I was referring to a switch that actually uses mercury, as a cheap-and-dirty "tilt switch".
I do not know how that particular accelerometer/decelerometer makes its measurements - it might be just by vacuum?
