<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=windows-1252"> <META content="MSHTML 6.00.2800.1106" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY> <DIV>I've been playing around with seeing how accurately I can determine = spruce moisture content for drying a panel prior to ribbing. I am = determining MC by oven drying samples in my kitchen oven. A discrepancy I have been unsuccessful in resolving is that the Forest Handbook (and = other sources) has charts that clearly suggest that wood at EMC in my = shop at 68 degrees F and 40% RH should be 8.3% (from <A href="http://www.woodbin.com/ref/wood/emc.htm">http://www.woodbin.com/r= ef/wood/emc.htm</A> , which also has the Forest Handbook data). When I oven dry samples I am = pretty consistently getting an initial MC about 1% greater than the tables = suggest: my last sample came out to 9.55% EMC in my shop (the environment in my shop = has been steady for weeks).</DIV> <DIV> </DIV> <DIV>From what I can gather, greater than 1% uncertainty is not good. = That would mean I would have no idea whether I am reaching an EMC of 6% or 5% - = very different goal for very different soundboard designs, and to the best of = my knowledge, not interchangeable.</DIV> <DIV> </DIV> <DIV>I think part of why I am getting greater initial MC values may = be from the release of volatile organic compounds during the oven = drying process in addition to water. A couple papers I have found on the topic = suggest that this can become a factor with some VOC-rich woods - and I think = spruce and other softwoods would fit this bill much more readily than most = hardwoods. Additionally, my oven cycles between 215 degrees and 250 = degrees, the high temperatures, I suppose, encourages a loss of VOCs, and thus additional weight.</DIV> <DIV> </DIV> <DIV>Gee, whizzzz. I really don't want to go out and buy a = laboratory-grade oven. Any good ideas?</DIV> <DIV> </DIV> <DIV>I know, I know. Quit worrying about it and build a board. Well, I'm = a perfectionist, and I have not yet received my spruce order. So until = then I'll just have to spaz out a bit!</DIV> <DIV> </DIV> <DIV>Along the way I found a few fun things related to MC:</DIV> <DIV> </DIV> <DIV>The first addresses "how long does it take for wood to reach = EMC?"</DIV> <DIV> </DIV> <DIV><FONT color=#0000ff>Air Drying: <BR>Wood moisture eventually = comes to an equilibrium with air moisture,</FONT></DIV> <DIV><FONT color=#0000ff> approximately to relative humidity = </FONT></DIV> <DIV><FONT color=#0000ff>   (RH)  0  10  20 =  30  40  50  60  70  80  90 =  100 <BR>MC %  0   3    5     7    9 =   11  13  15  18  23   35 <BR>At MC lower = than the fiber saturation point (usually 35%), moisture change takes place by = diffusion within the wood. The standard diffusion equation may be written as <BR>t = = L² / D <BR>where <BR>D = 1x10-6 cm²/s transverse and radial, 1x10-5 = cm²/s lengthwise, <BR>L is the length along the direction of diffusion. <BR>t is the time = to 1/e of the moisture change, that is to 63% of the equilibrium change. = <BR>So, if you have a piece of seasoned wood 2 cm thick that is at the 15% MC of = typical outdoors storage here, and you want to estimate how fast it will come to = equilibrium in your workshop at 30% RH (7% MC) if exposed to air both = sides, L = 1 cm and the diffusion equation gives t = 1x106 s, 11 days. The = equilibrium MC change required is 8%, so in 11 days you can expect 63% of 8% = 5% = lower MC, that is 10% total MC. That leaves 3% to go, and you can expect 63% of = that 3% to take place over the next 11 days, to 8% MC. So, 3 weeks should be enough = time for 2 cm thick wood. If your wood is 4 cm thick, it will take 3 months, = 6 cm thick, 7 months. According to the literature, most seasoned temperate = woods change moisture at a rate within ±20% of this. </FONT></DIV> <DIV><FONT color=#0000ff></FONT> </DIV> <DIV><FONT color=#0000ff><FONT color=#000000>And another addresses = "how much does wood/spruce change dimentionally with changes in MC?"</FONT></DIV> <DIV><BR></DIV></FONT> <DIV><A href="http://www.woodbin.com/calcs/shrinkulator.htm">http://www.woodbin= .com/calcs/shrinkulator.htm</A>  -  a great little calculator.</DIV> <DIV> <SPAN = id=__#Ath#SignaturePos__></SPAN> </DIV></BODY></HTML>