Rel. Humidity (was Re: Voicing question

[Tim Keenan & Rebecca Counts]

dpitsch wrote:
> 
> Yes, Damper Chasers[sic] affect the RH in the room, and in fact the home. <snip>

earlier post:
> > >  Hogwash.  Water in the air is a vaper.  All vapers disperse to
> > >all parts of the room and home.  To think that the bellyrail would keep the extra humidity
> > >from affecting the hammers and action is just not consistent with
> > >how vapers react in an enviroment.
> > >Davd Pitsch

To which Greg Newell responded:

> > Dave ,
> >         Does this mean that Damp Chasers do not in fact control only
> > the piano but in fact the entire room as well?????
> >                                 Greg Newell
> >True, to a point. However, as Dave points out, a Dampp-Chaser can not 
come close to a room- or furnace-dehumidifier in adding absolute volume 
of water to the air.  It doesn't try to, nor is that really relevant.

RELATIVE humidity (RH)n is what we are talking about. The amount of water 
which air can hold without condensing is a function of its temperature. 
RH is an expression of the amount of water in a given volume of air at a 
given temperature compared to the amount of water that the same volume of 
air could hold at saturation point at the same temperature. Yes, water 
diffuses through air quite quickly, but it moves from areas of high 
concentration to areas of low concentration.  That is the definition of 
diffusion.

In a perfect world (or a museum) it is possible to maintain an 
environment at a relatively constant RH.  In most houses, particularly in 
climates where there is a 50 C (90 F) difference between summer and 
winter temperatures, this is an impossibility.  In houses built in Canada 
before the last 15 years or so, if you tried to maintain an interior 
humidity of 45% at 21 C (72 F) when the outside temperature is -25 C (-10 
F) there would be water running down all your windows and condensing in 
your insulation, because in the boundary layer at the walls and windows, 
the temperature is a lot lower than 21C and the RH is therefore >100%.

To lower RH, in summer, you must either raise the temperature of the air, 
or remove water from it. If the R.H. outside, at say 35 C (90 F) is 85%, 
when you bring that air inside and cool it to 21 C (72 F), you are at or 
near the dew point. In a house, where humidity is usually a problem in 
summer, if at all, you don't want to raise the temperature, so you must 
remove moisture by using refrigeration to create a "sink" -- a cold zone 
where local RH is 100% so that water will condense and can be disposed 
of.  Unfortunately, unless you have air locks on all your doors, this is 
a losing proposition too, and it becomes practically impossible, or at 
least very expensive, to keep R.H. down to 45% in summer.

Wood doesn't care about what the relative humidity is on the other side 
of the room. Moisture in the wood is in equilibrium with moisture in the 
boundary layer of air around the wood. If you raise the RH, by adding 
water to the air in the immediate vicinity of the soundboard, the wood 
will reach equilibrium with that humidified air.  As long as you are 
adding moisture to the immediate environment as fast as it can diffuse 
away, you are accomplishing your aim.  Picture a cigarette burning in an 
ashtray.  The smoke will diffuse to all areas of the house, and pretty 
quickly, too--but it will always be orders of magnitude more concentrated 
at the ashtray than on the other side of the room, as long as the 
cigarette continues to burn.

Another "thought experiment" (as Einstein called them).  Imagine having a 
nice hot shower in winter with the bathroom door open and the exhaust fan 
off.  Put a hygrometer on your vanity top and another on the other side 
of the house, with all intervening doors open.  The R.H. in your bathroom 
will be 100% within minutes, and will stay there until the hot water runs 
out.  The R.H. on the other side of the house will never approach 100%, 
no matter how long you run the shower.  The wallpaper in your bathroom 
will peel, that in your bedroom won't.  That cellulose is the same as the 
cellulose your soundboard is made of.  Likely even made of spruce.

It is possible in some parts of North America to control room humidity 
within the limits which we are told pianos "like", but in many areas it 
is quite impractical, if not impossible.  I think the Dampp-Chaser system 
is the next best thing.
 
Tim Keenan
Noteworthy Piano Service
Terrace, B.C.