Climate Control; DC thoughts and concerns

[Robert Scott]

Sarah Fox wrote:

 >>>>
...Heat would escape the soundingboard from the topside, and so the
effective temperature of the board would be roughly the numerical average of
the air temperature above and below the board.  Thus, the board would not
really be kept at its target MC...
<<<<

I raised this very question with Bob Maier, who told me that their direct 
measurements of MC in the soundboard show closer control of wood MC than 
the "numerical average" analysis would lead you to believe.  I can imagine 
several explanations for such a fact.  For one, the air currents below the 
soundboard are being driven by a fairly large temperature gradient.  Above 
the soundboard, there is not so big a temperature gradient, so the air is 
more calm, which means the top the soundboard is not being washed by 
ambient air as much as the bottom is being washed by DC discharge 
air.  This would tend to bias the MC of the board closer to what is being 
forced by the DC discharge.  Another factor is the location of the 
humidistat.  If the humidistat is located directly between the DC and the 
board, then the DC's immediate discharge is what is being controlled.  But 
if the humidistat is located a little off center so that it sees air that 
has already been partially diluted by ambient air, then THAT is what is 
being controlled, which means the immediate DC discharge will be beyond the 
target humidity by some amount.  This may be just the amount necessary to 
compensate for the fact that the board is seeing DC air from the bottom and 
ambient air from the top.

You also mentioned DC cycling.  That is not a real concern.  The frequency 
of this cycling is bound to be quite high, compared to the MC response of 
the board.  The humidistat is a fast-acting plastic strip, while the board 
needs much longer exposure before it significantly changes its MC.

-Bob Scott
  Ypsilanti, MI