Hygrometer

[Ron Nossaman]

>Yes, I believe it is. That's why I am confused. It seems that the intent 
>is to dry down the board to a specific moisture content. Since this is the 
>case why is an instrument that measures the moisture content of the air 
>being discussed? Again, respectfully, what am I missing?
>
>Greg Newell


The connection Greg, the connection. If you know the temperature and 
relative humidity, you can compute the equilibrium moisture content of a 
piece of wood that has been in that temperature and humidity long enough to 
reach equilibrium, moisturizationally. The rate at which the MC in a piece 
of wood changes is relative to the difference in MC of the wood, and in the 
surrounding air. The greater the difference, the faster the moisture 
exchange between the two. Wood left in 70°F air at 40%RH will eventually 
stabilize at about 7.7%MC. But you don't know at what rate. If you are kiln 
drying and want an absolute minimum cycle time, you don't have time to sit 
around with a yard full of green 16/4 wood waiting for the stuff in the 
kiln to reach EMC, so you jack up the temperature to provide a greater 
moisture potential differential and measure it directly with a probe from 
time to time until it hits the target MC and pull it out before it passes 
on to ultimate mummification. In a piano shop, drying out an 8mm thick 
spruce panel, the drying time is considerably less and you probably aren't 
processing 10,000 board feet at a time, so you can afford another day for 
it to stabilize in it's own time in the box. That being the case, if you'll 
keep an eye on the temperature and RH% in the box, and compare the MC 
computation from those measurements with your target EMC, you just have to 
leave the sucker alone and wait for the wood to get to that MC all by 
itself. You can leave it in there for two weeks, justifiably assuming that 
anything that thin that hasn't reached equilibrium by then probably never 
will, or you can use your handy homemade faux soundboard MC indicator 
instead of a probe to get an adequate indication of the MC of the panel 
without poking holes in it with a $200 instrument who's calibration is most 
likely in doubt anyway, most especially at low MC levels.

Didn't I send you a copy of my Excel MC compuationalizationating worksheet?

Ron N