Pat Shingleton: "Rittenhouse..."
Hygroscopic means having an affinity for water in both vapor and liquid form. The slow, silent influence on wood is relative humidity and through the constant exchange of water vapor with the air, wood absorbs moisture when humidity rises and releases that moisture when it drops. The moisture content of wood is the weight of the water it contains as a percentage of the wood’s oven-dry weight and is identified as the EMC or equilibrium moisture content. The moisture content of wood in live trees ranges from 30 to 200%. As the wood swells and shrinks it gains or loses water once it is at or below the 30 percent mark. This is referred to as the fiber saturation point. We live in an area with elevated measures of humidity and experience the shrinking and changing shapes of indoor wood. Examples include gaps between floorboards, creaking stairs, trim joints opening up, piano sound-boards changing tunes with doors and drawers that once stuck, opening easily. Of course the process reverses when higher levels of relative humidity restores moisture content to its original levels. David Rittenhouse made mathematical instruments for surveying and astronomy and in the 1700s invented the Rittenhouse Hygrometer. This weather instrument registers the relative humidity of the air by using wood as its sensor. Rittenhouse's device registered the expansion and contraction of wood detected through the wood’s grain. Wood swells and shrinks about 80 times as much around the growth rings and 40 times as much across the rings. Rittenhouse took two identically sized strips of mahogany, glued them together to complete a single slat and attached one end to a base; placing a tipped pointer on the other end. When the humidity rose, the strip swelled, forcing the slat to bend and when it dropped the strip shrank and bent. Realizing technology has advanced needed measurements, Rittenhouse’s invention is used today by designers of plywood, laminated floors and layered wood to ensure that these products remain flat as they adjust to the power of relative humidity.
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