DC Query Thoughts

[Farrell]

Maybe reviewing a basic concept regarding relative humidity (RH) would help
answer some of the posted questions. Keep in mind that a plastic (or
otherwise) sheet attached to the back or underside of a piano will minimize
air exchange between the room and the atmosphere adjacent to the soundboard
back. It will help stabilize the soundboard backside environment. It will be
relatively effective in doing so for temperature, and very effective for
water vapor.

If the room temperature is fairly consistent (let's say within a 10 degree F
range) and a plastic back is added to a properly designed and functioning DC
system, there will be no problem with condensation, etc. The RH at the rear
of the soundboard will remain quite constant because of limited air
exchange, with the DC system cycling a bit as temperature varies a bit. Keep
in mind that humidity does not "have to leak out". Even if absolute humidity
stays constant, the changes in temperature will appropriately change RH, or
vice versa.

The amount of water vapor air can hold depends upon its temperature: warm
air can hold more than cold air. The maximum amount of water vapor that air
of a given temperature can hold is called its absolute humidity at
saturation. Except when foggy or rainy, air seldom contains the maximum
amount of water vapor that it could. The amount of water vapor actually held
in air of a given temperature is termed the absolute humidity.
Importantly, changes in wood moisture content are keyed to changes in RH,
not absolute humidity. Relative humidity is the ratio of the actual amount
of water vapor contained in air of a given temperature to the maximum amount
of water vapor that air at that same temperature could hold, expressed as a
percent. In other words, relative humidity is the absolute humidity divided
by the absolute humidity at saturation of air at the same temperature.

I would think that if you have a fairly closed environment, you would have
be real careful with the design of the water tank (I'm assuming a full DC
system - the closed system idea is very attractive to me in Florida because
I do not use the water tank) because of the possible danger of evaporation
of water from the tank (even if the humidifier is not "on") raising the
absolute humidity to the point where the dehumidifier cannot lower the RH
enough. In that case, either RH would rise to an undesirable (catastrophic?)
level, or if you had enough wattage under the piano, temperature might rise
to an excessive level.

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

----- Original Message -----
From: "PAT A RALPH" <KENNETH.GERLER@prodigy.net>
To: <pianotech@ptg.org>
Sent: Monday, March 06, 2000 12:32 AM
Subject: Re: DC Query


> Richard,
>
> D-C has a system for verticals when there is not enough space inside to
> install the system OR the inside system is not able to do the job AND it
> uses a plastic sheet over the back of the piano.  I have not used the
system
> or have heard of anyone using it and what is does to the sound??   I am
sure
> there are enough little holes  at the back of the keyboard cavity that
> moisture can leak out and not cause any damage to the underside of the
> grands.  BUT best bet is to contact D-C directly.  I am sure they have
some
> data from tests run on their systems before offering them for sale to
> technicians.
>
> Ken Gerler
>
> ----- Original Message -----
> From: Richard Brekne <richardb@c2i.net>
> To: PTG <pianotech@ptg.org>
> Sent: Sunday, March 05, 2000 3:13 PM
> Subject: DC Query
>
>
> > Hi list
> >
> > I just installed a DampChaser system on two of our grands at the
> > conservatory here in Bergen. I have placed a hygrometer under each  and
> > kept an eye on how the system is doing and found that the humidity
> > varies within about a 15% range daily, pretty uniformly over the whole
> > soundboard. I dont know how long the humidity stays at any particular
> > level tho as the hygrometers just show the present, high, and low levels
> > each time I check them.
> >
> > The guy who runs the import of these in Norway suggests that I cover the
> > bottom of the grand with a thin plastic film, enclosing thus the whole
> > system and underside of the grands.
> >
> > It is this I am wondering about. Do any of you have experience with this
> > kinda thing ?  How does it affect the sound of the pianos ?? Are there
> > better solutions ? Couldnt this cause damageing condensation ???
> >
> > The two rooms are quite different in size, the smaller being just big
> > enough to house a 215 cm grand and the other being large enough for 5 or
> > 6 such grands. Both are on the buildings ventilation system which pumps
> > lots of dry air into each room constantly. And both systems register
> > about the same amount of variance. Each system is installed just a bit
> > back of center of panel due to beam considerations, and are as high up
> > under the panel as I could get them. The hydrostats are located just
> > forward (keyside) and one beam width apart from the humidifier, and also
> > just about as high up as I could get them. Humidity levels read from low
> > 30's to around 45-46 % in each case.
> >
> > Any suggestions will be gratefully read.. grin.
> >
> > --
> > Richard Brekne
> > Associate PTG, N.P.T.F.
> > Bergen, Norway
> >
> >
> >
> >
>
>