Drying samples
November 10, 2011 | 6:00 pm CST

Q: What is the correct range of temperatures to use when oven drying sample wafers, and what might be the risk or consequences if a higher temperature is used? If a sample weighs 100 grams and then is dried and weighs 75 grams, does this mean the moisture is 25 percent MC, as it lost 25 percent of its weight?

A: The correct temperature for an oven used to dry wood for moisture content calculations is 115F to 117F. This was determined by agreement many years ago and is part of the definition for determining the moisture content of wood. If higher temperatures are used, the wood will dry faster, but you might also evaporate other materials in the wood or cause the wood to decompose, thereby giving you a false reading.

To calculate moisture content for lumber and solid wood, we divide the weight of water (25 grams in your case) by the oven-dry weight (75 grams). The answer is 33 percent MC.

However, engineers and people measuring the MC of chips, fuel and other products will calculate the MC differently. They will divide the weight of water (25 grams) by the total weight of the piece (100 grams) and get an answer of 25 percent MC.

The first method is called the oven-dry basis, while the second is called the wet-basis moisture content.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.