Newly-cut logs can have a moisture content (MC) of 80% or more, depending on species. Since wood shrinks, and can also split, twist or otherwise change shape as it dries, most wood is dried before being used. This is most often done using a kiln, but may use the air drying method, which is much slower. Depending on your location, the minimum moisture content that can be generally obtained in air drying is about 12 to 15 percent. Most air-dried material is usually closer to 20 percent moisture content when used.

In-kiln drying is usually monitored by some type of moisture meter. Moisture meters are used to measure the amount of water in the wood so that the woodworker can determine if it is suitable for the intended purpose. Building inspectors and many more, carpenters, hobbyists, and other woodworkers often are required to have moisture meters. Wood flooring installers, for example, have to verify that the MC of the wood matches the relative humidity in the air of the building. If this step is skipped, a vast array of problems may present itself: cracking, cupping, crowning, buckling, sunken joints, and cracked finishes.

The problems caused by varying degrees of moisture content in wood go beyond simple shrinkage in the dimensions of wood parts. Problems with distortions in the shape of the wood, such as twisting, warping and cupping, occur because of the difference in the degree of dimensional change in wood cells tangentially (perpendicular to the grain and parallel to the growth rings) versus radially (perpendicular to the growth rings).

A moisture meter gives a reading of the approximate moisture content of wood. The reading helps in determining whether the wood is suitably dry for its intended purpose. The moisture content reading can also assist in planning a project design that will accommodate future changes in dimension caused by changes in relative humidity. The amount of overall shrinkage timber will undergo in the drying process varies from wood species to wood species. The difference between radial and tangential shrinkage also varies from species to species. Woods with a low ratio of tangential to radial shrinkage, such as teak and mahogany, are less prone to distortion due to changes in moisture content than woods with a high ratio, such as pine and certain species of hardwood. Species with both low overall shrinkage and a low tangential/radial shrinkage ratio are more stable and will react better to changes in moisture content.

All but basic moisture meters have the ability to select the species or species group being tested. Once selected the meter will factor in different parameters for that timber and more accurately provide a moisture content reading. Many meters allow for selecting a species group. The meter’s instructions will have a table indicating which species are in each group. Most meters are manufactured for non-Australian markets and will not have many or any Australian species listed in the group table; so you will need to do a little research to find an equivalent species. In practice, depending on how precise you need to be, selecting an equivalent species based on your experience works well, or selecting between “hardwood” and “softwood” groups may be accurate enough.

In many circumstances, the precise MC of the timber is less important than the relative MC level at different points in time or throughout a process; or ultimately the Equilibrium Moisture Content (EMC) measurement is the key consideration.

For wood that is to be used in making furniture, for wood floors, in construction or for any building project, the ideal state is one of equilibrium moisture content (EMC). EMC means that the wood is in balance with the relative humidity it surrounding environment, and is therefore neither gaining or losing in moisture content. In reality, however, it is extremely rare for an environment to maintain a constant fixed relative humidity, and some degree of dimensional change along with seasonal changes in relative humidity is to be expected.

This video by James Wright does a good job of explaining measuring and EMC. See explanation from 2min 40 sec.

He describes the difference in obtaining wood in equilibrium with the current environment and how a sample smaller control piece of timber (often a small off-cut) can assist in determining when the slab or board is at equilibrium; by having the same MC as the control off-cut.

Pin Type – For typical woodworking operations, two basic types of moisture meters are available. Depending in the brand, pin-type meters measure the electrical resistance, termed resistivity, or its reciprocal, conductance, of the wood substrate. Water freely conducts electricity; consequently, increasing water content correlates to increased conductance. Pin-type meters incorporate two pin electrodes which are driven into the wood fibers and directly measure electrical resistance or conductivity. Each brand of moisture meter utilises proprietary calculations and displays the final moisture content as a percentage.

Pinless – The second type of moisture meter relies on the dielectric properties of wood. Many pinless moisture meters incorporates two pads which serve as rubber electrodes that transmit and receive a signal when pressed into the wood substrate. The pad type moisture meter is non-invasive in nature and requires only surface contact with the wood to obtain a reading. The non-invasive meter creates a low-frequency electrical wave between the two pads and measures the electrical properties of the wood, similar to the invasive pin-type meter. Some pinless moisture meters will have a spherical or other metal sensor to measure in a similar way to the pad meters, but measure at a greater depth within the wood.

Moisture meters may also be utilised by building-industry professionals and water damage restoration consultants to ascertain precise moisture content of a wide range of materials found in the built environment, such as gypsum board drywall or interior finish plaster. Locating high moisture within buildings is essential in locating leaks that may not be visible to the eye. This ability to locate and measure moisture anomalies can help manage and control indoor air quality as in relates to mould growth.

Pinless causes no damage tot he material under test, can test deeper into the material, and is faster and easier to take one or many measurements.

Primarily consider the type and location of the sensor. Pinless meters with a protruding sensor can be helpful for testing boards in a kiln drying or air-drying stack. However spherical ball type sensor will detect in all directions it is making contact so be aware you may be measuring the board below or above the sensor. The Testo 616 meter sensor protrudes out front and is ideal for testing boards in a stack, but only detects underneath the curved metal sensor plate, so you cannot test two boards by mistake.

Pinless meters detect into the material up to the specified depth. If the measurement depth is deeper than the material is thick it will also measure the material underneath. So consider required measurement depth, review the meter’s specification, and be aware when using a deep sensing meter that you may be sensing a substrate, board underneath, or even you hand if you are holding the board. So a air-gap between material being tested may be important, like there is in a stickered stack of timber. Measuring s substrate may be desirable in some cases, so a meter with excellent measurement depth may be appropriate for your needs.

As discussed above, it may be important or less so to specify the timber species being tested. Select a meter that supports the settings you will need. For example, the Excech MO53 is an excellent but can only select between “Timber” and “Building Materials”; where as the Testo 616 can select between “Softwood” group, “Hardwood” group, and eight types of building materials. The Extech MO280 and Flir MR60 has up to eight timber groups to choose from, some of which do actually list Australian species.