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navindra why is it the relative humidity that's important as opposed to the absolute humidity?
Ooh good questionā¦ I do think absolute humidity is important, as in, itās not completely irrelevant.
But my understanding is that moisture moves around (for lack of a better term), and itās RH that determines how it movesā¦ if the RH is high, itās easier for moisture to āmove intoā the piano ā¦ i.e., be absorbed by the wood and cause the wood in the piano to swell (and we all know why thatās bad).
If the RH is low, itās easier for moisture to āmove out ofā the piano, i.e., for the wood to dry out, which causes the wood to shrink and in worse cases, crack.
When the RH is around 40-50%, thatās when moisture moves to or from the wood the least.
Itās different for other materials (say, fabric for example). So I think this is why itās RH that matters, not just absolute humidity. Because as the temps go up and down, and the RH moves accordingly, it changes the ability of the moisture (thatās already there, the absolute humidity) to move into and out of materials in the room.
Hereās another interesting (and I believe relevant) tidbitā¦ in the paperwork for my Venta humidifier (note, not a de-humidifier), it explains that when you first turn on the unit and start putting more moisture into the air, the first thing that happens is that soft materials in the room soak up that moisture ā curtains, plush furniture, carpeting. During this time, hygrometers may show little or no increase in RH. But once these items have soaked up enough moisture, then the additional moisture coming from the humidifier starts to stay in the air more and thatās when youāll start to see the RH go up on your hygrometer.
So thereās probably a better, more technically accurate way to explain why itās RH that matters, but hopefully this helps a little bitā¦