Bubble Gauge Calibration

[Delwin D Fandrich]

If memory serves, the Baldwin bubble gauge vial is calibrated in
increments that indicate 17 minutes of arc. I think this gauge was
designed by Harold Conklin and he set up the calibration this way to
make it easier to translate string deflection into string downforce
against the bridge. 

Using this gauge, if the bubble is zeroed on the speaking portion of the
string and it indicates a positive deflection of one gradation line (or
17 minutes of arc) on the back scale portion of the string, the
downforce of that string against the bridge will be 0.5% of the scale
tension of that string. (The length of the backscale does not matter,
only the angle of deflection.) Two gradation lines indicates 1.0%
downforce, etc. I.e., if a single string has a scale tension of 170
pounds and the gauge indicates two gradation lines of positive
deflection, the downforce of that string against the bridge will be 1.70
pounds. If the total scale tension of the piano were 40,000 pounds and
the bubble gauge indicated two gradation lines of deflection on each
string the total downforce would be 400 pounds.

I’ll leave it to the mathematicians among us to figure out what the
gradation lines of the Lowell bubble vial indicate.

ddf