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Definitions Related to the Air Around Wood EMC (Equilibrium Moisture Content) The EMC is a property of the air. It is defined as being numerically equal to the MC of a piece of wood in equilibrium with the temperature and humidity of the air. The values of EMC and RH at various temperatures (an excerpt is presented in Table 1) are based on oscillating conditions of RH for spruce that was being dried for the first time. The values were intended to be used for dry kiln operations. Although the original data, which are now used throughout the world, were quite limited, the EMC data are extremely useful and are applied to all species with little loss of accuracy. (See also How Dry Should Wood Be? on Page 6.) EMC is an especially important variable in drying in that the shrinkage of wood is directly, linearly related to EMC. RH (Relative Humidity) The relative humidity is a measure of the moisture in air. It is the ratio of the amount of water in the air at a given temperature to the maximum water that the air can hold at the same temperature. Relative humidity is an important variable in drying in that the drying rate is directly related to the humidity. That is, at a given temperature, Dryingrate=k(100-RH ) where k is a coefficient that depends on species, MC, thickness, air temperature, and so on. Dry-Bulb Temperature The ordinary temperature of air is called the dry-bulb temperature. The term originates from the fact that the measuring sensor used to be a long rod, called a bulb, and this sensor was dry, compared with the sensor used to monitor RH which was wet. Wet-Bulb Temperature The temperature of a sensor covered with a wet muslin wick (using distilled water) with air flow across the wick of 600 feet per minute is the wet-bulb temperature. The wet-bulb is always cooler than the dry-bulb (or equal to the dry-bulb if the RH is 100%). The basic method of determining relative humidity is to measure both the dry- and wet-bulb temperatures of the air and then, using a published table developed over 75 years ago, look up the RH. Some electronic methods of directly measuring RH, such as those using a salt treated or solid state sensor, are not reliable enough, especially over the long term, in the acidic, high humidity environments in the dryer, to be used for drying. Measurement of the EMC by measuring the resistance of special pieces of wood or cellulosic materials, however, has been shown to be reliable. The RH and wet-bulb temperature, if needed, can then be looked up when the EMC and dry-bulb temperature are known. 18PDF Image | Principles and Practices of Drying Lumber
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