Since a rotary piano pump can suck 80 to 100 or more inches water column, with no flow, it is indeed impractical to make a manometer long enough to never be sucked out in any circumstances.  20 or 30 inches is enough for most player adjustments.

Making a water trap for a manometer is simple in concept; one just needs some sort of sealed chamber in the tube from the piano to the manometer, where the water will end up, if the manometer is sucked too hard.  The picture shows one way to do it.  The details are unimportant, as long as one keeps in mind some basic principals:

The water must fall into the trap, rather than going all the way into the piano.  The chamber must hold all the water in the tube, with plenty to spare.  A 30" x 1/4" manometer holds less than two ounces of water.  But try it first to make sure the bottle is big enough!

Sucking out the manometer is bound to happen.  So it should be easy to unhitch the chamber; and lift it and tilt it so the water will run back into the tube when this happens.  When this occurs, some of the fluid will stick to the tubing and the bottle, and the manometer zero will end up 'lower' on the scale.  A sliding scale makes it easy to 're-zero' the reading when this happens.

Richard Vance
23 Jun 2001 13:50:09 -0400