Q. We are confused about the best and most accurate way to measure the MC of dry lumber that we purchase and that we also dry ourselves. Can you help?
A. For purchased lumber, buyer and seller need to discuss the exact moisture measuring technique and equipment that will be used.
The oven test (heating a small sample at 215F) is certainly the most accurate, if done properly, but it is a destructive test that requires perhaps 24 hours. A microwave oven will speed up the drying process, but the technique is critical to avoid charring or smoking the samples. It also tells you the MC of the wood at one small location and does not tell you if the core is wetter than the shell. So, the most practical method is likely a moisture meter...there are two types.
To speed up the measurement process, pinless-type moisture meters can be used. They are not greatly affected by wood temperature. They give instant readings and can be used to scan any location on the lumber’s surface. They can measure as low as 4 percent MC.
If the dry lumber has been exposed to high humidity just before measuring, the surface moisture can skew the reading. As these meters respond to differences in wood density by giving differences in MC readings, they might have a percent or two inaccuracy on pieces with a density variation from normal. Nevertheless, their speed and ease of use are awesome.
The pin-type moisture meter has been used on wood for over 75 years. They are quite accurate when species and wood temperature corrections are made. When insulated needles are used, the moisture is measured at the depth of tip of the needle. This allows a moisture gradient, shell to core, to be measured.
For an average MC, the insulated pins are driven to 1/4 of the thickness of the lumber. Slight surface moisture regain does not affect insulated needle readings. Three concerns are that these meters measure the MC in just one spot, that they take time to drive the needles, and they cannot read much under 6.5 percent MC (over-dried lumber). Even so, they are the workhorse of our industry.
The best operations when checking the MC of lumber will have both meters, using the pinless to quickly scan a lot of pieces, and using the pin meter to double check any pinless readings that are too wet.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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