[CAUT] Reading low humidity (was seasonal SB failure)

[Fred Sturm]

On 3/2/06 12:52 PM, "Greg Newell" <gnewell@ameritech.net> wrote:

> Accurate reading have always been somewhat of a
> concern to me but the question I keep coming back
> to is "Does it really matter how much relative
> humidity is in the air or should we be more
> concerned about the moisture content in the
> wood?" I'm somewhat familiar with using the
> charts and determining the moisture content based
> on how long the wood has been in the specific
> environment, etc. but I wonder if soundboards,
> specifically, are holding true to that equation
> since their also coated wood. Seems like that
> would have some effect. I don't know how to
> factor that in other than to take a moisture
> content reading from the wood directly. Perhaps
> this has all been covered before and just hasn't
> sunk in yet.    .......wouldn't be the first time.
> 
> best,
> Greg
Hi Greg,
    It's mostly a matter of prediction. Given a certain pattern of humidity
change, you can expect certain effects, based on experience. For instance,
we just had a trace of rain last night, first in months. RH rose virtually
instantly inside the building from 13% to 25%. I know from experience that
by this evening  the concert grand will have F2 at about +2 cents and out of
tune unison, the notes in the octave above it will be sharp to lesser
degrees, most of the mid to high treble will have unison drift with a
pattern of right string 0.5 cents sharp, left string 0.5 cents flat, etc. If
the spike in RH disappears in a few days, the piano will go back to target.
So if I have a concert tonight, I want to tune as close to it as possible.
If I don't have an important one for a few days, I'll let it slide back in
tune.
    I'd like to be accurate about just how dry is dry for purposes of
gathering data/experience for the future. This has been a very dry winter
for me, and I've been seeing a lot of effects, like shrinking keybeds (I've
had to raise a lot of glides and shim down keyblock guides) and dowel glue
joint failures (loose hammerheads, loose shanks and catcher dowels in Kawai
upright butts), not to mention pitch falling through the floor. I'd like to
quantify it for future reference, to be able to say with reasonable
certainty that X happened following four months below X% RH.
    Now, you are right to say that it is the moisture content in the wood
that matters, in a fundamental sense of cause and effect. But it is RH,
together with temperature and airflow, that causes the wood to arrive at the
moisture content. The coating on the wood slows the transfer process, but
not by that much. Wood with very low EMC will absorb ambient high RH very
quickly. It's slower to dry out, and it takes higher temps and airflow to
speed that process (read institutional conditions <g>). I can measure RH
easier than EMC, and RH is good at predicting what will happen in the near
and long term future. So that's why I have been obsessing about RH for many
years. It takes a lot of the guess work out of the job.
Regards,
Fred Sturm
University of New Mexico