Different question about heater bars

[Richard Brekne]

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Farrell wrote:

> 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.....
>

You isolate the system far too much, and over simplify the situation as such...
grin... Number one.. you cannot discount the back side of the piano.. or the
exterior of the piano otherwise... Equilibrium will always take the easiest
path. And the easiest path is air ... from wherever it leaks in...

That being said..... you will have to some degree the affects as you describe
them below... But on the other hand, you will also have the affects as I
describe them. Raising the temperature does indeed also lower RH in the interior
of the wood... whats good for the goose is usually (and in this case as well,
good for the gander.

Your Kiln example is a much more isolated system to begin with. And if it was
completely and totally isolated... where on earth do you think these water
molecules would migrate to ??? You put a panel into an oven that is already dry
inside... thats how it draws out water.. this isnt exactly the same process as
damp chaser creates.. tho admitedly some of the same functions are at work.

I think you would aggree that takeing a panel and puting it into a kiln which
has the same moisture content in the air as the panel does in the wood, sealing
the kiln and turning it on wouldnt have exactly the same effect.. now would it.

This humidity thing is always fun... and dont worry... even if I am blowing air
out of my better end,,, I still enjoy the conversation.. grin.

>
> 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
> >
> >
> >

--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway


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