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In both of these cases, you run the risk of your boards drying faster than the rest of the charge. Depending on how bad the problem is, you might advance the kiln conditions too quickly for the load, causing drying defects. The biggest reason to coat sample board ends with B.O.S.S.® is to make the sample board moisture loss rate mimic that of your full-length boards (whether end coated or not). Even so, if you start noticing lumber drying defects starting to develop but your sample boards aren’t drying at a worrisome rate, start investigating. INTERMEDIATE MOISTURE CHECKS. The only way to find out if your sample boards are at the moisture content you think they are is to perform an intermediate moisture content check. You might be able to use a moisture meter, but the best thing to do would be to cut a new moisture wafer from the sample board and dry it to find out how close your calculations have been. Try this: If the end coat on your sample board failed it’s possible that there’s a moisture gradient between the ends and the middle of the board. Cut a new moisture wafer roughly one-third of the distance from one of the board ends to the center. In other words, if the original sample board was 30” long, then cut a new moisture wafer 5” from one end. A wafer taken at that location ought to represent the average moisture content of the board. If you don’t have a moisture gradient then it wouldn’t matter which position the new moisture wafer came from, but that’s still a good location. Slather the cut end of the remaining sample board with B.O.S.S.® and reweigh it. Ovendry the wafer at 215°F until it reaches a constant weight and redo your moisture calculations for the ovendry weight of your shortened sample board. If the sample is drier than you thought, the ends of your sample board probably dried out faster than the rest of your sample. This means that the remainder of the sample board isn’t typical of the rest of the load anymore, and you need to disregard that sample for the remainder of the drying run. You might decide to cut and prepare new sample boards from other boards in the charge. If you do, take as much care in picking new sample boards as you did earlier. Quartersawn boards are still probably the slowest-drying boards in your load, but it’s hard to predict whether sapwood boards (that started out wetter) or heartwood boards (that started out drier) have the higher moisture content at this point. Remember to think about where the wettest boards might be in your kiln, and try to resample from that location if possible. “FIXING” A SLOW-DRYING LOAD OF LUMBER (ACCELERATING A DRYING SCHEDULE). If the drying rate is slow but you’ve proven to yourself that your equipment is working correctly and that your sample boards are adequate, what options do you have? Probably the best thing to do would be to adjust the schedule a little. You might raise the dry bulb temperature by a few degrees for example (though this weakens the wood and might cause problems if your moisture content is high). Raising the dry bulb temperature increases the rate of heat transfer, but you have to mind the wet bulb setting as well. If you keep the size of the WB depression the same, raising the dry bulb temperature will increase both the RH and the EMC–and since the EMC controls the drying rate, the kiln will actually run more slowly. (You can see this for yourself by referring back to Table 3.) You’re going to need more information than is found in other dry kiln manuals to make these changes, so I calculated the RH and EMC values for combinations of dry bulb temperatures from 115° to 160° and for wet bulb depressions from 20–50°. Table 11 shows only a portion of this table (DB from 120°–130°F). RH values are integers and are shown in bold type, and the EMC values are given to tenths of a percent moisture content and are shown in italics. Only the even dry bulb temperatures are shown, but you will find that there is seldom a significant change in EMC due to a 2°F dry bulb change. You could always interpolate if you think it’s necessary. (See Appendix E for the table displaying the wider temperature range.) 95PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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