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more than I usually need, but too much capacity is never a bad thing to have and it’s good to have a backup. Even if I weighed sample boards on this balance, the uncertainty wouldn’t likely affect my moisture calculations by more than 0.25% or so. Before you start your moisture content determinations, clean up your samples–knock off any sawdust or loose splinters. The next step is to weigh them using the appropriate balance for the sample. I often record the weights by writing on the samples themselves with a fine point permanent marker while I’m standing at the balance, and I recopy the original and the final dry weights into a spreadsheet when my drying is complete. Copying these numbers into a spreadsheet will make a good contribution to your recordkeeping. Recordkeeping might seem like a bit of extra work sometimes, but having numbers to look back on can really help when you’re trying to troubleshoot a problem. After you’ve recorded the original (“wet”) weight of your samples, check the oven temperature to be sure it’s hot enough. Oven temperatures might drift as ovens get older, controls get bumped and so forth, but you have to remember that your test results won’t be correct if your temperature isn’t at least 212°F; lower temperatures will never dry wood samples completely, no matter how much time you let them sit in an oven. You won’t know that you’ve completely dried your samples until you get two or more identical weights, so take your samples out periodically to reweigh them and record the weights each time. Keep the oven door closed as much as possible and don’t leave the samples out in the open any longer than necessary, because they can change weight more quickly than you’d imagine. If your oven and the balance are in different locations, it’s a good idea to put the samples into small plastic bags to minimize air contact. Once you’ve gotten two identical weights in succession (two hours apart) then it should be safe to use those weights in the moisture content calculations. Not every sample will dry at the same rate, so you will have to exercise a bit of patience. The amount of time you will need to dry samples will vary according to the species, the moisture content, the size and so forth. You’ll get a good feel for how long it will take for your samples to dry after you’ve done this a few times but it’s often going to be in the range of ten to twenty-four hours. MICROWAVE OVENS. Some people say that you can use microwave ovens to dry wood for moisture determinations if you use a low power setting and a carousel tray, but I’ve encountered problems with this method. In one of my tests, it took only a moment of extra time in the oven before yellow-poplar and oak samples started to smoke and burn from within, even at 270 watts (3/10 power in a 900 watt oven). The smoke isn’t a problem, because mass loss due to smoke is negligible, but it’s rather unsettling to see smoke pouring out of the oven nonetheless! Low density samples (yellow-poplar) were more forgiving than high density samples (oak, knots); denser wood seemed to absorb the microwave power more quickly and started to burn sooner, or perhaps these samples just dissipated microwave energy less well. The biggest problem I had was identifying the end of drying. I microwaved some samples for a minute or two at a time until I achieved successive weight measurements that were identical, but when I afterwards placed these pieces in an laboratory oven at 215°F to verify that I had achieved total dryness I sometimes found that I had not actually dried my pieces completely in the microwave. The calculated MC difference was usually several percent. For this reason I cannot recommend using microwave ovens for moisture content determinations, though microwaving could be a useful way to partially dry the samples and shorten the subsequent drying time in a convection oven. 30PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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