Wood is dimensionally stable when the EMC (equilibrium moisture content) has been reached and the relative humidity and ambient air temperature are no longer changing. When these conditions are reached, shrinkage and warping will not occur.
However, when the wood absorbs or loses moisture below the fiber saturation point - around 25-30 percent - it will swell or shrink. How much, depends on the species, orientation of the growth rings/woodgrain and the moisture content. Most shrinkage occurs in the direction of the growth rings and boards with similar woodgrains will have similar tendencies. For example, species such as oak, beech and hickory tend to shrink more than teak or cedar. The orientation of the growth rings also plays a role in determining the amount of shrinking.
Indoor and outdoor conditions are also critical. For example, at 70F and 75 percent relative humidity, the moisture level will reach 14 percent. At 70F and 35 percent relative humidity, the moisture level will reach 7 percent. And at 70F and 20 percent relative humidity, the moisture will reach 4.5 percent. Since plant conditions are usually between 30-45 percent relative humidity, the recommended moisture content for stable woodwork is 6-8 percent.
Moisture meters should be used to determine the wood’s moisture content. There are two basic types: pin and pinless.
Pin meters measure the electrical resistance between the two pins; this resistance changes with the moisture content of wood. Moisture should be measured down to the core and as the pins are hammered towards the core, consecutive measurements indicate if the wood is dried.
If the product is already finished and the moisture content needs to be checked, a pinless meter could be the better choice. Pinless meters use electromagnetic wave technology, which measures the density in a 3-D field underneath the measuring pad and indicates the average. While both types of meters must take into account the wood species, the wood’s temperature does not affect the measurements in the pinless meters, therefore no correction is necessary.
One of the advantages for pinless meters is that a large number of boards can be scanned in a short time and the measurements leave no visible marks, i.e., no pinholes. However, pinless meters require a somewhat flat and smooth surface. The measuring area should be at least as big as the measuring pad on the meter. Also, the measuring depth is crucial. Moisture values are too low for materials thinner than the measuring depth.
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