Sitka EMC

[Farrell]

I am in the process of making a sitka spruce equilibrium moisture content (EMC) gauge as described by Nick Gravagne. Briefly, it is simply a 6 x 48-inch wide swath of soundboard panel with a thin rib or two on the back (ribs parallel to long axis and grain perpendicular to long axis). The gauge will indicate MC by the amount of curve in the panel. 

Part of the process will be to calibrate the gauge. To do that I need a sample piece of wood that I can accurately weigh to calculate its moisture content - the gauge will be calibrated with this sample piece. I took a 91.71 gram square piece of old soundboard panel and dried it down to 0% EMC in oven (weighed 84.55 grams when dry). Before going in oven the EMC was 8.5%. It required 7 hours in the oven between 220 F and 250 F (much longer than I thought would be required). As of an hour ago, the sample had been out of the oven for 29 hours - it is in my shop at 72 F and 46% RH. The MC has only risen to 2.45%. 

I will keep monitoring this sample until it reaches EMC with the shop (about 8.5%). I strongly suspect reaching EMC will take a good 5 days to a week. I don't know why I find this so surprising, but I do. I thought it would take up water much more rapidly. I think it is going to be an excellent exercise for understanding just how long the panel will need to be in the hot box to dry down to any target EMC. I was assuming you would toss the board in the box for a day or so - I suspect it will require quite a bit longer.

Nothing really hard here - just sharing my amazement. Also, from what I have seen so far - all this talk of taking a board out of the hot box and the need to rib it in 12 minutes or it will soak up tons of water seems as though it may be hogwash (as long as you are not doing the ribbing outside in the Amazon jungle). I'm going to play with this a bit. Interesting stuff.

I bought a good hygrometer and am making this EMC gauge. I was thinking that having both is kinda redundant. But now I realize they are not. The hygrometer is required to set the hot box temp & RH to a target, and the EMC gauge is required to verify that the wood has reached EMC with the box environment.

Terry Farrell