I think all the temperature data would be very relative,
_unless the line voltage were noted._  It is like measuring
a light bulb everywhere, wondering why your hand burns,
yet not noting the input voltage.

Also of consideration is the ambient room temperature, and
air flow, noting stuff blocking the piano's back, etc.

Will Herzog

 [ A wattmeter will indicate proper voltage range under load:
 [ at voltages above or below normal the power consumption (watts)
 [ rises noticeably.  -- Robbie