Different question about heater bars

[Farrell]

----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, October 18, 2000 2:59 AM
Subject: Re: Different question about heater bars
snip
> > I have been of the understanding that the heater bars used by
Dampp-chaser
> > are used to decrease humidity.  This is done by heating up the air
around the
> > heater bar.  Warm air holds more moisture than cold air.  So, moisture
is
> > drawn out of the piano and into the air... right?
> >
>
> Actually, what happens is that warming the air around the piano expands
the airs
> capacity to hold moisture, while the actual moisture content of the air
remains
> the same. That is to say the absolute humidity remains the same. This
combination
> results in lower relative humidity.

Absolutely correct.

> Moisture is not exactly "drawn out" of
> the piano... rather the relative humidity is reduced.

No and yes. Assuming that the air inside the piano is at some elevated
reletive humidity and the soundboard is in equalibrium with those
environmental conditions. Now turn on the Dampp-Chaser heat rod.The relative
humididy in the interior of the piano is indeed reduced. That sets up a
non-equilibrium condition between the reletive humidity in the piano
interior and the moisture content of the soundboard wood. Because the air
has less reletive humidity and can now hold more humidity, there will be a
net movement of water from the soundboard to the air. So, it is reasonable
to say that "moisture is drawn out of the piano and into the air.

Terry Farrell
Piano Tuning & Service
Tampa, Florida
mfarrel2@tampabay.rr.com