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It’s impossible to talk about pianos, piano tuning, and piano care without talking about humidity.
A first step in understanding how humidity is effecting the piano, is knowing what the humidity is in the home or room where the piano is located.
If you can find one, this is a hygrometer I recommend: Taylor 5565 Hygrometer/Thermometer . It can be calibrated, is easy to read, and never needs a battery replacement.
Unlike digital hygrometers, this one can be calibrated to assure accurate readings of both the hygrometer and the thermometer. Calibration can be done using a sling psychrometer which I always carry with me for checking the calibration while I’m there servicing the piano.
I am not a fan of most ‘digital’ hygrometers because they don’t have any calibration screws or adjustments on them.
I’m sure there are accurate digital hygrometers out there. Most HVAC technicians carry one that costs about $400, and of course, and it checks out just fine. But most of us don’t need that much of a hygrometer!
Some digital hygrometers can be reasonably accurate in the midrange, but their accuracy can really fall off in both extremes of dryness and wetness.
!Make sure your hygrometer can be calibrated! If you can’t find one of these Taylor 5565’s, make sure the one you choose has calibrations adjustment screws on the back of it.