Calculating the equilibrium moisture content for wood based on humidity measurements

WOOD PRODUCTS NOTES Department of Forest Biomaterials Campus Box 8003 North Carolina State University Raleigh, North Carolina 27695-8003 Summary: If given enough time the moisture content of wood will come to an equilibrium with the surrounding environment. The temperature and relative humidity of the surrounding air will establish equilibrium moisture content (EMC) conditions, and the moisture content of the wood in that environment will approach the EMC of the air. This issue of Wood Products Notes introduces the "EMC Calculate" Excel spreadsheet that will calculate an estimate of the EMC based on one of the following pairs of data: • Relative humidity and dry- bulb temperature; • Wet-bulb temperature and dry-bulb temperature; or • Dew-point temperature and dry-bulb temperature. Phil Mitchell Wood Products Extension Campus Box 8003 Raleigh, NC 27695 (919) 515-5581 Phone phil_mitchell@ncsu.edu https://research.cnr, ncsu.edu/bloes/wo Distributed in furtherance of the acts of Congress of May 8 and June 30. 1914 North Carolina State University and North Carotma A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national onpn. religion, sex. age. or cfcsabtfrty In addition, the two Universities welcome all persons without regard to sexual orientation North Carolina Stale University. North Carolina A&T State University. U S Department of Agriculture, and local governments cooperating Calculating the Equilibrium Moisture Content for Wood Based on Humidity Measurements By Phil Mitchell North Carolina State University Connecting Wood Moisture Content to Relative Humidity The connection between the percent relative humidity and the corresponding wood moisture content has existed (at least) since an early Forest Products Lab publication (1 91 9). Koehler and Thelen (1 926) state “There is a definite relation between the moisture content of wood and the relative humidity of the atmosphere at any given temperature.” One of the earliest uses of the term “equilibrium moisture content” was by Jenkins (1 934). The moisture content of wood will, if given enough time, come to an equilibrium with the surrounding environment. The temperature and relative humidity of the surrounding air will establish equilibrium moisture content (EMC) conditions, and the moisture content of wood in that environment will approach the EMC of the air. Although the relationship between relative humidity and EMC is not linear, there are some simple methods to relate the two. One method to get a very rough estimate of EMC is to divide the percent relative humidity by 5 (McConnell, 201 6). Thus for a relative humidity of 75%, the estimated EMC would be 1 5%. A better estimate is provided by Table 1 which presents selected points along the EMC versus RH curve. For the given relative humidity, EMC is approximately ±1 percent EMC of the stated value at temperatures between 35°F to 100°F (Wengert, 1988). Table 1. Approximate relationship between %RH and %EMC valid to + 1% EMC from 35° F to 100°F. % RH % EMC 20 4 30 6 50 9 66 12 75 14 80 16 Stale University A&T Stale University COOPERATIVE EXTENSION Empowering People ■ Providing Solutions