A relative humidity of 40% for _all_ pianos works best for me.  I have
had great success keeping the room where the piano is located at 40%,
yet it requires a humidifier in winter, de-humidifiers and air
conditioning in summer.  The cost is well worth it.

I am not fond of electrical devices attached directly to pianos that
are supposed to add and subtract humidity directly to or within the
piano.  Usually the people using them have no concept of what the
devices are supposed to accomplish.  They add water in summer when the
humidity is 95% and turn on the heat in winter because they think the
piano is cold!  (I finally gave up trying to explain it to them --
experience will teach them!)  Anyone with a bit of scientific knowledge
knows that moisture or heat applied to one side of a piece of wood, and
not the other, will cause the wood to warp.

It is far better to control the humidity of the _entire_ room, where
the piano is located, even if it means keeping the doors and windows
closed and keeping a de-humidifier and air conditioning going during
the humid seasons.  During dry seasons use a room humidifier when
artificial heat is in use.

Higher humidity will cause metal rust, swelling of wood, tuning
instability, and possibly mold and mildew. Constant changes in humidity
cause the wooden parts in a player to intermittently swell and shrink,
eventually causing leaks, loose tuning pins, splits and cracks in
wooden parts and loose glue joints.  An even humidity at all times is
best for a piano.

By all means, purchase a gauge that measures humidity in the room and
watch it carefully, make adjustments accordingly.

Bruce Clark