Different question about heater bars

[Farrell]

Gee whizz Richard. You of course know that it pains me to an unimaginable
extent to suggest that my thoughts might be more valid and someone else may
be, er, a, less than correct, BUT.....

Lets say the RH of air in vertical piano interior is 70%, temperature is 70
degrees F and soundboard moisture content is 10%. Equilibrium conditions
exist (water molecules are evaporating from the soundboard surface at the
same rate that water molecules from the air are imgrating into the
soundboard surface). Turn on heat inside piano. Raise temperature to, say,
90 degrees F. The RH in piano interior immediately drops to perhaps 50%.
Immediately, the air inside the piano is capable of holding much more water
vapor than previously. Water molecules will then migrate from the soundboard
surface from within the wood and evaporate into the air inside the piano at
a rate exceedingthe migration of water molecules migrating from the air into
the soundboard wood. Hence there will be an net decline in the number of
water molecules in the soundboard (and a net increase in the amount of water
molecules in the air within the piano - untill it leaks out and mixes with
room air). That is what is meant by soundboard moisture content. That is the
absolute amount of water "trapped" within the wood. It is not a mere
artifact of heating the wood and the air wihin the wood pores having a lower
RH. A net volume of water will continue to escape from the soundboard until
a new equilibrium is established between the RH of the piano interior and
the water content of the soundboard. (We are or course neglecting the fact
that the soundboard is also exposed to the outside air on the backside, but
this does not change the course nor end point of this discussion.)

This is why when one dries a soundboard before installing into a piano or
before shimming, the soundboard shrinks - because the board has an lower
absolute volume of water within it. You can prove this to yourself by
weighing a piece of wood before and after heating for a long time. It is why
 a freshly cut log is real heavy, but one of the same size and species that
has dried for a year is much lighter - because water has been "drawn out"
from the wood!

I think. I'm having fun watching language at work. I truely hope you are
also. :-)

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

----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, October 18, 2000 2:33 PM
Subject: Re: Different question about heater bars


>
>
> Farrell wrote:
>
> > > 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.
>
> Grin... How fun it is to watch language at work. I'll buy your
interpretation of
> the words "moisture is drawn out of the piano and into the air" in this
> connection. Its actually what I was saying. Still, from the other
standpoint..
> absolute humidity remains the same within the wood as it does within the
air..
> expanding the air surrounding air, and thereby decreasing Relative
humidity does
> indeed create the non-equilibrium you describe, but as to whether or not
one
> describes the equalizing of this in terms of "drawing out humidity" or
rather
> that the soundboards air content also expands, retaining the same volume
of
> absolute humidity is ...grin.. up for discussion... I would choose the
> latter.... for ..... clarity...hehe..
>
> Keith.... sorry to be so picky...:)
>
> >
> >
> > Terry Farrell
> > Piano Tuning & Service
> > Tampa, Florida
> > mfarrel2@tampabay.rr.com
>
> --
> Richard Brekne
> RPT, N.P.T.F.
> Bergen, Norway
>
>
>