I regularly check my piano humidity, but lately it's been more of an every-other-day thing.
Today, I noticed something odd - the hygrometer positioned below my soundboard suddenly showed a drop in humidity that wasn't really reflected above the soundboard or at the hammers. Hmm..
In the below picture, you can see the yellow line drop below the standard trend, a little after 6pm on Jan 21, and the gap was getting wider. The blue line is also below the soundboard but positioned at the action/lyre, the pink line is above the soundboard at the hammers. The purple standing out on its own is the outdoor humidity/temp.
It's actually a bit easier to see in the temperature graph - the temperature below the soundboard popped up nearly 8-9 degrees over the last day and a half.
I thought maybe the batteries were going out so I retrieved it in order to change the batteries. But under the piano I noticed the speakers and power unit were lit up. Huh. it turns out my schmancy player system was left on and the auto-power-off feature didn't trigger properly:
Yep, turned on around 6am Jan 21 and constantly pumping 37 watts into the components under the soundboard.
I turned it off manually and the values all fell back in line.
This was of course a bit of a unique situation with the electronics package, but it also made me think - things like Piano Life Saver/Dampp-Chaser attach in the same way at the bottom, and similarly operating by pumping heat (either damp or dry) from below the soundboard. I didn't expect that doing this might result in an 8+% RH difference between the bottom and top of the soundboard. I'm sure it's fine, the difference is peanuts if it is actually maintaining a steady RH over the course of the day/week/season, but interesting to see the data.