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    Author(s): Samuel V. GlassSamuel L. Zelinka; Jay A. Johnson
    Date: 2014
    Source: USDA Forest Service, Forest Products Laboratory, General Technical Report, FPL-GTR-229, 2014; 37 p.
    Publication Series: General Technical Report (GTR)
    Station: Forest Products Laboratory
    PDF: Download Publication  (5.07 MB)


    The Forest Products Laboratory (FPL) has provided equilibrium moisture content (EMC) values of wood for given temperature and relative humidity (RH) conditions in various forms over the course of its history, primarily for practical purposes related to drying lumber and controlling moisture content. The FPL EMC data have been widely cited and reprinted, not only in literature of a practical nature such as the Wood Handbook, but also in textbooks and journal articles as a basis for scientific discourse on the thermodynamics of water vapor sorption in wood and evaluation of physical models. Using the data for such scientific purposes presupposes that the methods by which the data were acquired are well documented and accepted. This report questions previous assumptions about the historic EMC data and attempts to uncover and evaluate the original data sources. It also addresses a number of related topics, including how the presentation of data has evolved in the literature and whether the data are practically applicable to all wood species. We find that the data are unreliable for scientific purposes, such as thermodynamic analysis and testing of physical models, for three reasons: lack of proper documentation of methodology; the unsolvable problem of knowing which values are determined from direct observations and which are interpolated; and the absence of definitive measurement error analysis. However, we affirm that the data are indeed useful for practical applications, such as lumber drying, conditioning of wood specimens prior to testing physical or mechanical properties, and modeling of moisture content of wood members in buildings. We show that a number of mathematical models provide adequate fits to the data for practical use.

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    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.


    Glass, Samuel V.; Zelinka, Samuel L.; Johnson, Jay A. 2014. Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory. USDA Forest Service, Forest Products Laboratory, General Technical Report, FPL-GTR-229, 2014; 37 p.


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    Equilibrium moisture content, hygroscopicity, sorption, Wood Handbook, kiln drying, air drying, seasoning, conditioning, mathematical model

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