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computer can process the raw data from a weighing device and make any needed corrections. Because we are looking at a piece of lumber that may weigh up to ten pounds and are concerned about weight losses of 0.01 to 0.03 pounds (equivalent to 1/2% MC), it is an extremely difficult job to accomplish at a reasonable price. Even if the calibration is found for one cell, each cell has its own calibration that must be determined. No matter how the weight of the sample is obtained, the moisture content is calculated based on the estimated oven-dry weight, which in turn is based on the moisture content of small 1-inch moisture sections. Moisture content can also be estimated by measuring electrical resistance between two needles. This relationship between resistance and moisture content has been known for a long time. Unfortunately, Nature has not put the same relationship in every piece of wood. There are substantial variations between resistance and moisture content for different species, and even for lumber from different pieces of wood from the same species. So although we can accurately measure resistance, the relationship to MC cannot always be accurately given due to the natural variation that exists. This variation results in an uncertainty of several percent MC under 20% MC. Above 20%, especially above 25%, the variation in resistance as MC changes is very small - so small, that the natural variations in resistance from piece to piece can be greater than the changes in MC from 30 to 50% MC. In essence, each piece of wood has its own calibration of resistance to MC at high MCs. If moisture sections are cut (as mentioned above), then it is possible that the resistance method of measurement could be used to determine MC. If the MC is not measured with a high degree of accuracy, then the schedules used must be conservative - that is, have a built-in safety factor. It must be remembered that these concerns apply to hardwoods intended for exacting uses where small errors in MC measurement can lead to degrade during drying or lead to subsequent manufacturing problems. For non-exacting uses of hardwoods (upholstery frames, for example), the present electrical moisture measuring systems are accurate enough. Electrical measurement of moisture in the kiln for non-degrade-prone species (such as yellow-poplar, aspen, or soft maple) can also be done with present systems and without concern for potential errors, as any errors will not be economically serious - that is, won't result in degrade. Computerized Control Today it is possible to purchase very sophisticated,yet reasonably priced control systems for lumber dry kilns. Temperatures can easily be controlled, with properly designed and functioning equipment, to within 1/4°F. The control systems are able to measure and control the moisture in the air very accurately. With computerized memory, they are able to change kiln conditions smoothly, rather than abruptly. In addition, they are able to equalize and condition as well. Research and experience has shown that these systems do save money by producing higher quality in less time than conventional equipment of the past. 49PDF Image | Principles and Practices of Drying Lumber
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