Carbon_Liberator said:
Say Batten, I'd be curios to see if you come up with that 6% error consistently, or if it is a variable thing.
Well, my microwave experiments are over for the season, so I can't answer that. :roll: :lol:
Sorry, that was a low blow aimed at moisture meters in general, but I didn't tell the whole tale. I never said it was an "error", just that there was a difference. Truth is, though, it would probably be a pretty consistent finding. The meters are just a bunch of resistors and whatnot, with a digital readout. As long as the batteries are fresh, there should be pretty good precision among readings. The problem is that the MM only reads about 1/8" into a given surface, but the oven-dry method accounts for every bit of water that was originally in the wood.
I split that locust round in half and used the Harbor Freight MM to take a reading from an inner face, just like everyone has been recommended to do here on this forum. However, drying wood actually has a moisture
gradient acrosst its thickness. That is, in fact, how it dries. As air movement carries away water molecules from the surface, the outside gets drier and internal water then diffuses along this gradient to the outer surfaces until the wood eventually reaches the
equilibrium moisture content (EMC) determined by the relative humidity it is stored in. With the oven-dry determination, I cut a slice across the grain, about 2" from the end and about 1" thick. This slice included all the drier wood on the outside, plus the wetter wood on the inside. As long as there is a still a diffusion gradient in the wood, the oven-dry method will
always show a lower MC than using a MM on a freshly split inner face.
Then, there is the lack of discrimination between wood types in the cheaper meters. All MMs are calibrated using doug fir as a reference, otherwise, they wouldn't agree with each other. The expensive meters have programs that allow you to compensate for the differences in electrical resistance (usually small, but larger at the higher MC range) between wood types. There are tables on the Internet that allow you to convert your readings, but first you have to be sure of the species you have. Seems lots of folks aren't so sure what they have. I knew I had black locust, but I never went to the table to do the conversion because I really didn't care about MC, I was trying to determine the specific gravity of the locust to compare it with the average SG quoted in the literature. Most sources say 0.69, I got 0.67. Pretty damn close, especially considering I was impatient and scorched the wood pretty bad, releasing at least some of the non-water weight (lots and lots of dense smoke) into my kitchen, and putting a crack in it as well (which, naturally, is filled with air only). Except for those two things, I think I might have gotten it spot on at 0.69.
This brings up another interesting use for microwave drying for folks who aren't sure what species that have on hand. It'd be very informative to know the specific gravity to compare with the known range of SG for the species you suspect (or hope) it is. If the SG turns out to be 0.45, for example, well... it ain't ash, oak, hickory or locust for sure.
A big help would be to dry it in the oven or microwave until it stops losing weight, then figure out the volume. Easiest way at home is to cut it into a block with a sharp saw and then multiply the face width x face depth x face length (in millimeters). Now weigh it on the kitchen scale (in grams). Divide the volume in millimeters by 1000 to get the volume in centimeters, then divide that volume by the weight in grams to get the specific gravity. Easy-peasy once you know how to do it. ;-)