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Table 2. EMC values for wood. At 70°F and 40% RH the EMC will be 7.7% EMC. (Source: Wood Handbook, Table 4-2) A similar table in the Dry Kiln Operator’s Manual 9 is based on the equipment and controls that come with your dry kiln, so it ought to be more useful for operators of small kilns. It has the advantage of being set up so you can determine the RH and the EMC from the Dry Bulb Temperature and the Wet Bulb Depression-you don’t have to know the RH before you can determine the EMC like you have for Table 2. The dry bulb temperature is the ordinary measurement of the ambient air temperature, while the wet bulb temperature is measured by a thermometer with a wet cotton wick over the sensor bulb. Air flowing over the wet wick cools the thermometer, and since drier air will absorb more evaporated water than humid air a wet bulb will cool more in dry air-thus indicating the relative humidity. The difference between the dry bulb and the wet bulb is called the wet bulb depression. This table also has smaller gaps between the temperatures so it should be helpful if your equipment has difficulty precisely reaching or maintaining a desired temperature or RH condition. What exactly is a wet bulb, anyway? It’s nothing more than an ordinary thermometer or sensor with a wet piece of cotton wrapped around the bulb. Think about the cotton bandanna that we wrap around our sweaty foreheads when we work outdoors. When a breeze comes along the sweat evaporates, making us feel cooler, and the amount of evaporation is related to the relative humidity in the air. There’s less evaporation on a muggy (or rainy) afternoon than there is on a dry afternoon, because the air is more nearly saturated with water already. The ability of the air to take up additional moisture is really what relative humidity means, anyway, so the difference between 9 Simpson, William T., Editor. 1991. Dry Kiln Operator’s Manual. USDA Agricultural Handbook Number 188. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 274 p. See Table 1-6. https://www.fpl.fs.fed.us/documnts/usda/ah188/ah188.htm 42PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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