Moisture Content of Wood
The moisture content of wood is measured as the ratio of the weight of water in a given piece of wood, to the weight of wood when it is completely dry (or "oven dry") and is usually expressed as a percentage. The "green" wood of a freshly felled tree may have a moisture content anywhere in the range 30% to over 200%, depending on the species.
Almost all of this water must be removed from the wood before it is fit to be used. The desired moisture content depends upon the intended use, and also upon the annual average relative humidity at the place where the wood is to be used.
Wood is a cellulose material which behaves somewhat like a sponge, so that even wood which has been "kiln dried" down to 7% (a typical value for furniture use) may in fact later reabsorb water from the atmosphere. Actually all wood is constantly gaining or losing water to or from the environment--the moisture content of wood changes as the relative humidity changes. Coats of varnish or paint may greatly slow down this process, but can generally not stop it completely.
As the moisture content of improperly dried or stored wood changes, so does the wood expand or contract, potentially producing all manner of disastrous defects (and hence wood warping nightmares). With a wood moisture meter from Electrophysics, you will have the means to measure and hence control the moisture content of your valuable wood.
No more nightmares!
Why Wood Warps
This tree cross-section shows the distortion of flat, square, and round wood as affected by the direction of the annual growth rings. The clear space surrounding each wood section is the shrinkage in drying from green to oven dry condition.
Moisture Content & Wood
OD = Oven Dry, KD = Typical Kiln Dried, AD = Air Dried.
Bound Water is contained within the wood cells; Free Water exists in the spaces in between the wood cells. At the Fiber Saturation Point (FSP) the wood attains its maximum possible amount of bound water.
Relative Humidity & Wood EMC
Equilibrium Moisture Content (EMC) of wood is the moisture content which will eventually be attained by any piece of wood, when stored indefinitely at a particular relative humidity. For furniture, cabinets, or works of art which will be used indoors, the wood you work with should be dried down to the EMC corresponding to the annual average relative humidity at that location. For most woodworkers across most of North America, 6% to 8% is a good wood moisture content to aim for.