Wood Moisture Monitoring

Moisture in wood affects many aspects about the wood. For this reason, lumber is usually dried before being sold at a lumber yard.
The amount of water within rough timber varies by species and region. It is not uncommon for newly cut logs to have raw moisture content of 80 percent or higher. Depending on the planned use of the lumber this may or may not be an issue.

Lumber used for general use has a wide acceptable range so moisture content is not a critical issue. However, for some purposes moisture content requires tighter controls.

For example, when wood flooring is installed, the moisture content of the flooring must match the relative humidity of the interior air. Mismatched moisture contents can lead to poor surface quality and buckling or crowning.

For this reason, good installers use a wood moisture meter to verify the moisture content of the flooring before installation. Materials inspectors, professional carpenters and woodworking hobbyists who want to achieve professional results also invest in a good moisture meter.

If the wood is going to be used in a situation that requires precision fitting with tight tolerances, that lumber will ideally have a moisture content that is in equilibrium with the relative humidity of the surrounding air as well. When this balance occurs, the wood will not swell by absorbing moisture from humid air, nor will it shrink or crack by losing its own moisture to drier air. This balanced condition is called equilibrium moisture content.

Equilibrium moisture content is a precise point that is rarely achieved under real world conditions. Humidity levels constantly fluctuate, and this is why doors or wooden drawers with tight tolerances sometimes stick in humid summer months. As long as the wood’s moisture content and the air’s humidity are relatively close at the time of construction, inevitable shrinkage and expansion of the wood will be minimal.

Two Types of Wood Moisture Metersr
Two types of wood moisture meters exist that can be used to determine if the moisture content of the wood is acceptable for the planned use.

Electrical Resistance or Pin Moisture Meters
Some moisture meters relate the electrical resistance of the wood to its moisture content. They are often called pin moisture meters. Small pins are inserted into the wood, and the resistance across the wood fiber between the pins is measured. The pins function as wood moisture sensor electrodes. Water conducts electricity, and the measured resistance across the wood fiber is inversely proportional to the moisture content of the wood; lower resistance means higher moisture content.

Because the electrodes must be inserted into the board, pin moisture meters mar the surface of the wood. They are also relatively slow to make measurements.

Dielectric or Pinless Moisture Metersr
Meters of higher quality use a different technology and have no pins to be inserted. They measure the dielectric of wood through entirely non-destructive surface contact.

The dielectric is a measurement of the amount of electrical energy that a material can store. As the moisture content of wood increases, the dielectric of the wood increases. Although the wood moisture sensor is placed on the surface of the wood, the dielectric is electronically measured deep within the board. This deep measurement eliminates inaccurate readings from surface moisture and provides a more accurate picture of the true moisture content of the wood.

The elimination of pinholes makes these meters highly desirable for expensive woods, and results are provided in a matter of seconds. Dielectric properties change with temperature, but a good wood moisture meter will compensate for temperature automatically.

The best moisture meters are also adjustable for wood types. Expensive tropical woods have very different moisture contents than standard hardwoods or soft pines. The differences can be critical when creating inlays or joining exotic woods to domestic varieties in furniture construction.

Although a quality woodworking moisture meter can be expensive, true craftsmen understand the value of this tool. The cost of the device is easily justified by avoiding even one buckled floor or ruined cabinet.