Water in Wood - Water forms a continuum throughout the sapwood. The cell walls of wood in sapwood are saturated with water. A term fiber saturation point is used to denote the amount of water in the cell walls of sapwood. There are at least 5 ways to measure the fiber saturation point (FSP). The FSP is defined as that point of moisture concentration in cell walls that when reduced, physical changes occur. It is not so simple to say that FSP is that moisture concentration when the cell walls can no longer hold water. The FSP of wood is measured as that point where physical changes occur when the cell wall moisture drops below that point; 27% ± 3% depending on the method of measurement. Moisture concentration is measured on a volume weight basis in trees. For example if a cube of wood 2 centimeters on each side were cut from a tree and weighed and oven dried until all water was driven off, and weighed again the dry weight of wood over the weight of the water x 100, equals the percent moisture while volume remains constant. If the block weighed 100 grams wet and 50 grams was wood so the wood had a 100% moisture concentration. It is possible for wood to have a 200% or even 300% moisture concentration. What this means is that while volume is constant, the weight of water may be twice or three times the weight of the wood. In sapwood of most trees the moisture content is about 60 to 80%. The moisture changes greatly when we consider some heartwoods and discolored and decayed wood. In heartwood, or false heartwood, the moisture contents may be below the fiber saturation point. In discolored and decayed wood the moisture concentration may be very high; over 200 and 300%.
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