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One alternative analysis is a double grading study. In this study, the lumber is graded green and then is regraded after drying. The differences in grade between the two is attributed to normal shrinkage or to drying degrade. Double grading is especially effective with hardwood lumber. For softwood lumber, especially dimension lumber, an analysis technique is to have the lumber grader at the planer mill indicate the cause for any downgrading or remanufacturing to maintain grade. (For example, the grader might indicate that if the piece were straighter it would be a No. 2, but because of warp it is a No. 3. Or, he may indicate that the piece must be trimmed four feet to eliminate an end check, thereby avoiding downgrading the piece.) Another analysis technique is to measure the thickness of green lumber. If there is a much overly thick lumber, then drying time will be longer, energy use higher, over-drying more likely, and so on, adding up to significantly higher drying costs. Thickness analysis programs are commonplace in most state natural resource departments. As has been stated in previous discussions in this text, incorrect moisture content is probably the single, most common operator error. Incorrect MC also is the cause of as much as 80% of the furniture manufacturing defects. It is possible to sample the MC of a load of lumber with a moisture meter. When these values are averaged, then one obtains the estimated average MC for the load. But what about the spread of the data? If the standard deviation of the MC readings is calculated, and then multiplied by 1.96, then the region between [average - (1.96 x std. deviation)] and [average + (1.96 x std. deviation)] represents 95% of all the MC readings in the lumber pile. The standard deviation is easy to calculate; in fact, many small hand-held calculators have this calculation built-in. Virginia Tech also has a free IBM-PC version of such a calculation program available. After drying is completed, it is a good idea to cut several QC samples. These samples will indicate if there are any problems with the lumber, such as checks, honeycomb, stress, wet lumber, and so on. A picture of these samples is given in Figures 3 and 4. (Note: It is helpful to cut these same samples when green or partly dried lumber is received from an outside vendor. It is surprising to me how many times I am called on to analyze what a kiln operator is doing wrong, but when I arrive, I find that the problem is poor quality lumber.) 56PDF Image | Principles and Practices of Drying Lumber
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