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

[Greg Newell]

Fred,
         I gather that RH and air flow do indeed 
affect the wood and that the wood's coating has, 
perhaps, less of an effect than I originally 
thought. I'm still, however, nagged by the last 
paragraph you wrote in which you said,

"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."

         I gain from this that wood reacts very 
quickly on the intake and very slowly on the 
release. Since this is the case I still wonder 
how useful or accurate your readings and 
subsequent assumptions from RH are. You are very 
specific in your second paragraph about what you 
expect to face when dealing with your concert 
grand. While I'm sure that will be useful 
information to you I wonder how this relates to 
the real world environment in which we find many 
of our private customers pianos. I do some 
concert work but not on the scale or frequency 
with which you obviously do. This is a CAUT forum 
and your info can be quite useful in that realm. 
I guess I was just hoping to port it over to the rest of the world too.

best,
Greg



At 11:35 PM 3/2/2006, you wrote:


>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
>
>
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Greg Newell
Greg's piano Forté
mailto:gnewell@ameritech.net