The image below shows the moisture meter which is used in the sound board panel drying box to indicate when panel's moisture content is ready for the gluing of the ribs. The arm deflects to indicate its state of dryness. The spruce-arm cross section is made of two halves which are glued together, with the grain running along the axis of the arm for one half width, and cross-grained for the other. Since the linear expansion and shrinkage of spruce is about 1% along the grain and about 4% across the grain (when quarter cut), the arm will deflect in response to its overall moisture content. The thickness of the arm is approximately the same as the sound board panel, so it will tend to take up and lose moisture at a similar rate. Therefore the meter can be relied upon to indicate the moisture content of the panel when it is placed in the drying box with the panel.
Clearly, the calibration of the Overs Pianos Sound-Board-Panel-Moisture-Meter remains at the masking tape and pen stage at this point in time.