This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment 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 = http://www.woodbin.com/ref/wood/emc.htm , 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). >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. 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. Gee, whizzzz. I really don't want to go out and buy a laboratory-grade = oven. Any good ideas? 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! Along the way I found a few fun things related to MC: The first addresses "how long does it take for wood to reach EMC?" Air Drying:=20 Wood moisture eventually comes to an equilibrium with air moisture, approximately to relative humidity=20 (RH) 0 10 20 30 40 50 60 70 80 90 100=20 MC % 0 3 5 7 9 11 13 15 18 23 35=20 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=20 t =3D L=B2 / D=20 where=20 D =3D 1x10-6 cm=B2/s transverse and radial, 1x10-5 cm=B2/s lengthwise,=20 L is the length along the direction of diffusion.=20 t is the time to 1/e of the moisture change, that is to 63% of the = equilibrium change.=20 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 =3D 1 cm and the diffusion equation gives t = =3D 1x106 s, 11 days. The equilibrium MC change required is 8%, so in 11 = days you can expect 63% of 8% =3D 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 =B120% of this.=20 And another addresses "how much does wood/spruce change dimentionally = with changes in MC?" http://www.woodbin.com/calcs/shrinkulator.htm - a great little = calculator. ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/0f/28/88/7f/attachment.htm ---------------------- multipart/alternative attachment--
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