Soundboard Drying and MC

[Farrell]

There was a discussion (s) a while back asking at what rate does a soundboard loose/gain moisture. Related, I think the was also a discussion about how long does it take to dry a panel down for ribbing. The have also been discussions about how to determine whether a soundboard panel has reached equilibrium moisture content (EMC).

I ribbed a soundboard panel today for a Steinway M. The soundboard was partially redesigned by Del Fandrich. I built the panel and laminated/radiused ribs. Below are data documenting the panel drying process from shop conditions in which the panel wood had equilibrated with for the past couple years (on 3/31 - just prior to putting the panel in the hot box), through the drying process over a week, and ribbing which - well actually, I'm in the middle of it - six of ten ribs have been glued to the panel.

      Date
     Temp
     RH
     EMC
     Width
     
     
      03/31
     70
     50
     9.2
     1136.0
     Shop
     
      04/01
     98
     33
     6.4
     1135.0
     Hot Box
     
      04/01
     100
     25
     5.0
     1134.0
     Hot Box
     
      04/02
     83
     30
     6.0
     1133.4
     Hot Box
     
      04/03
     83
     33
     6.4
     1134.0
     Hot Box
     
      04/04
     87
     31
     6.0
     1133.4
     Hot Box
     
      04/05
     85
     30
     6.0
     1133.3
     Hot Box
     
      04/06
     85
     31
     6.2
     1133.2
     Hot Box
     
      04/07
     86
     29
     5.9
     1133.2
     Hot Box
     
      Rib
     70
     35
     6.9
     1133.2
     Shop
     


I determine whether the panel has reached EMC by measuring across the grain at the widest area of the panel. The width units are millimeters. As you can see, the panel shrunk 2.8 mm while its MC dropped from 9.2% to 6.0%. I use a good quality tape to measure and I am estimating the tenths of millimeters. You can also see that after about two days in the hot box, the panel had shrunk down to 1133.4 mm - almost consistent with a 6.0% EMC - the hot box conditions got away from me a tiny bit and RH went up the next day and thus EMC, but quickly went back down with a little more heat. You can also see how the panel shrinks with lower EMC conditions, and also grows with increasing EMC conditions (4/2 to 4/3).

When you raise the EMC conditions in the hot box and observe that the panel increases in width a bit, you know the panel previously was at a lower MC than the new EMC - so you can be very sure the panel had equilibrated with an EMC lower than your new EMC.

I normally keep my shop between 45% and 50% RH, but when I rib, I crank up the AC and the dehumidifier - as you can see, I have the shop at 35% right now - I can spend quite a few hours ribbing a soundboard and not worry about the panel picking up any significant moisture.

BTW, I use a good quality certified hygrometer that is documented to be accurate to within 0.2% RH.

Anyway, gotta go glue. Just thought some might be interested.

Terry Farrell
Farrell Piano

www.farrellpiano.com
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