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MEASURING MOISTURE CONTENT It’s obvious that wet wood has more water than dry wood, but there’s a more quantitative way to define moisture content. Chemical industries define moisture content as the ratio of the water weight to the combined weight of water and dry substance (wet basis), but that’s not how it’s done in the wood industries. It’s confusing at first, but the way that wood moisture content is calculated in the solid wood and composite industries is based on the ratio of the water weight to the dry wood weight (dry basis). You won’t be selling wood to the pulp and paper industry, but I’m going to show you a couple of examples to illustrate what I’m talking about. WET-BASIS MOISTURE CONTENT. First, let’s consider some wood chips bound for a pulp mill. Pulp mills are run by chemical engineers, so they’re going to define moisture content as a percentage of the original weight: 𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑪𝑴𝑪𝑴𝑴𝑪𝑴= (𝑶𝑴𝑴𝑶𝑴𝑪𝑶𝑶𝑾𝑴𝑴𝑾𝑾𝑴𝑴𝑶𝑾𝑴−𝑶𝑶𝑴𝑪𝑾𝑴𝑶𝑾𝑴𝑴𝑾𝑾𝑴𝑴𝑶𝑾𝑴) 𝒙𝟏𝟏𝟏% [1] 𝑶𝑴𝑴𝑶𝑴𝑪𝑶𝑶 𝑾𝑴𝑴𝑾 𝑾𝑴𝑴𝑶𝑾𝑴 DON'T use Equation [1] to calculate moisture content for lumber. This equation isn’t used by the wood industry. Ovendry weights are determined by drying wood samples to a constant weight in ovens that are at least as hot as boiling water at sea level (212°F). According to Equation [1], if you dried ten pounds of wood chips fresh from the mill in an oven at 212–215°F and evaporated all the water to get five pounds of ovendry wood chips, the moisture content would be calculated as 𝑴𝑴𝑴𝑴𝑴𝑴𝑴𝑴 𝑪𝑴𝑪𝑴𝑴𝑪𝑴 = (𝟏𝟏 𝒑𝑴𝑴𝑪𝑾𝑴 − 𝟓 𝒑𝑴𝑴𝑪𝑾𝑴) 𝒙 𝟏𝟏𝟏% = 𝟓𝟏% [2] 𝟏𝟏 𝒑𝑴𝑴𝑪𝑾𝑴 meaning the wood chips were one-half water (by weight). DRY-BASIS MOISTURE CONTENT. Equation [1] might make perfect sense to a chemical engineer who deals with tanks full of fluids, but because the water content of a piece of wood fluctuates with the environment it’s impossible for people working with solid wood products to know what the original weight of a piece of wood actually was. Instead of making the calculation based on the original (wet) weight of the wood, then, people in the lumber and composites industries make moisture content calculations based on the ovendry weight of the wood: 26PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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