Moistureproofing shop - now stage box

[Sarah Fox]

Hey Thump,

> Speaking of which: how do Dampp Chaser bars
> dry out piano parts, as increased heat in a closed box
> such as piano case only allows more moisture to reside
> in the air?

Well, air that's warm can hold more water vapor.  Relative humidity doesn't
refer to how much water vapor is in the air, so much as the percentage of
the maximum water vapor the air can hold at a given temperature.  As
temperature rises, the maximum possible water vapor content also rises, so
the water vapor present in the air is a lower percentage of the maximum
value, hence lower RH.

The RH is a good measure of how well the air will carry off or deposit
moisture.  Consider a piano at equilibrium at a given lower temperature,
meaning that moisture evaporates from the soundboard just as fast as it is
deposited from the air to the soundboard (resulting in no net change in MC).
If the temperature is elevated from this equilibrium state, then it becomes
more "dry" and can accept more water vapor.  Similarly, the soundboard is
warmer, and so water evaporates from the soundboard more easily.  When the
soundboard is more "willing" to give up water, and the air is more "willing"
to accept, the soundboard is dried out somewhat, until a new equilibrium
state is achieved.

The ability of a heater bar to dry a "closed box" depends on the box.  If
there is not much total moisture in the box, then the temperature can be
elevated sufficiently to drive the moisture from the wood into the air to
achieve whatever MC is desired.  However, if there is a lot more water in
the wood than air to absorb the water vapor, it will be difficult to achieve
a high enough temperature to keep the RH and MC at the desired levels.  It's
at that point that the box needs to leak moisture.  How?  Well, the
soundboard can be heated slightly and evaporate moisture directly to the
(unheated) outside air from the other side.  Also, the box leaks air, so
cooler air with lower moisture content enters, is heated to produce a lower
RH, draws off water vapor from the wooden parts, and leaks to the outside,
carrying the moisture with it.

I hope that helps.  ;-)

Peace,
Sarah

PS Everyone, I'm waaaay behind in my emails.  If I haven't responded to some
of your emails in private, it's because I'm swamped with home repair work
right now.  :-(