Temperature Change affecting pitch

[Richard Brekne]

Hmmm... looks like we need an in depth understanding of why and how relative
humidity changes.

You have to understand that there is something called relative humidity and
something called absolute humidity. AH is exactly how much humidity there is in
a given volume of air. RH tells us what percentage of maximum AH there is in the
air at any given time. The reason for RH is that the maximum amount of AH
changes with the air temp. Hot air can "hold" more humidity then cold air...
ok...

now think about this example. Nice partly sunny day outside.. with outside
temperature of 90 % and an outside RH of 75 %. (Early summer / late spring in
Iowa ??? grin) You are sitting in your house and get to feeling to warm so you
turn on the airconditioner which takes the outside air as it is... humdity and
all and cools it down while pumping it around inside your house. The air gets
compressed with cooling yet there is still the same amount of humdity per volume
unit (AH) stuck in the air. This means that the air now has a higher RH factor
as that same AH amount now relates to a lower temperature and thus represents a
higher percentage of the maximum amount of humidity the air at this lower
temperature can hold. You cool this outside air down enough and its going to
start raining somewhere in your house...(grin... overdriven point dont you
know).

The same thing happens in reverse in the winter. Warm air expands, and unless
you add humidity to that air... then the AH stays the same but the air can hold
more.. so RH goes down. Remember. RH only tells us what percentage of the
maximum amount of humidity the air can hold at any given temperature. It does
not directly tell us how much humidity there is in the air itself.

Think this through... it can be difficult to sort out at first... but it makes
sense enough after a while. In your example RH would go up if and only if
nothing is done to remove some of the humidity from the air after / during the
cooling process. Some airconditioners do this well, some dont, some too well. It
all depends on how its installed, and what kind a unit it is, and what the
conditions its operating in are like.

Really tho... thats why we have these handy gadgets like RH readers. All you
really need to do is place that by the piano and see what it reads. And if you
are interested in seeing the affect of the inner environment (in a rough sense)
on that RH reading you get... go outside for a few minutes and check what the
outside RH is. If its different... then something inside the house is either
drying out / or adding damp to the air.

Farrell wrote:

> Now why would turning on the AC make a piano go sharp? The cooler temp.
> would make the plate and case contract (albeit very slightly), thus lowering
> pitch, and the lower humidity (because any properly operating AC unit will
> remove water from the air, thus lowering relative humidity) would tend to
> make the soundboard contract, again lowering the pitch. Why in the world
> would it go sharp?
>
> Terry Farrell
> Piano Tuning & Service
> Tampa, Florida
> mfarrel2@tampabay.rr.com
>
> ----- Original Message -----
> From: "pryan2" <pryan2@the-beach.net>
> To: <pianotech@ptg.org>
> Sent: Tuesday, March 28, 2000 11:02 PM
> Subject: Re: Temperature Change affecting pitch
>
> > I learned the hard way that temperature has a immediate effect on pitch.
> > When I go into one of the showrooms to tune a piano, I click on the air
> > conditioning (I'm in South Florida), proceed to strip mute a piano and
> begin
> > tuning middle strings with the SAT.  After getting half way through the
> > piano, I go back to where I started and re-check my work. All the red
> lights
> > are spinning sharp now. What do I do now? Start over or let them go?
> What
> > I do now is turn the AC on in advance and let the showroom and pianos to
> > cool down before starting.
> >
> > Phil Ryan
> > Miami Beach
> > -----------------------------------------------------
> >
> > ----- Original Message -----
> > From: "Larry J Messerly" <prescottpiano@juno.com>
> > To: <pianotech@ptg.org>
> > Sent: Sunday, March 26, 2000 7:15 PM
> > Subject: Temperature Change affecting pitch
> >
> >
> > > Just for my own information, when I came into my store this morning I
> > > took pitch readings on two grands at 61degrees F. then turned on the
> > > heating system and raised the temperature to 68 degrees.
> > >
> > > The 6' Kranich and Bach dropped pitch 3.2 cents initially and then when
> > > (I presume) the plate temperature rose ended up 1.6 cents flat of where
> > > it had begun the morning.
> > >
> > > The 5'3" George Steck initially dropped 0.4 cents then continued to fall
> > > to 1.2 cents from where it had been.
> > >
> > > They have not changed any more over the last hour.
> > >
> > > No real problem or question here, just thought it was interesting.
> > >
> > > Larry Messerly, RPT
> > > Phoenix/Prescott
> > >
> >
> >

--
Richard Brekne
Associate PTG, N.P.T.F.
Bergen, Norway