Structurally sound and beautiful in appearance, wood is the ideal raw material. Woodworkers know that moisture content is the single most important factor affecting the quality of their project. To ensure quality, from the mill and dry kiln to the finished project, moisture meters help minimize defects such as shrinkage, cracks, and splits. Years of experience, high-tech tools, controls and premium grade lumber cannot make a difference if moisture content goes unchecked.
Are There Different Types Of Moisture Meters?
There are two types of moisture meters commonly used in lumber and woodworking applications.
Pin-Type Moisture Meters:
A pin-type uses two pins that penetrate into the wood at the users desired depth and depending on which moisture meter you purchase. Pin-type meters are the only instruments that indicate the moisture conditions inside a board or a piece of wood. Using insulated pins, only the uncoated tips are exposed to the wood fiber, providing more accurate readings of moisture content at various levels of penetration. Using a pin type meter with insulated pins is still the most effective method to determine moisture gradient, which is the difference between shell and core moisture content.
Pin-Type Features and Specifications:
- Gives precise readings and is a good indicator for evenly or unevenly dried wood.
- Leaves two pin holes.
- Indicates highest moisture content in area between the pins.
- Can measure differences between core and surface moisture by using insulated pins which measure only at the depth they are driven to.
- Not affected by surface texture or shape, as long as both pins can be inserted.
Pinless Type Moisture Meters:
Pinless moisture meters read moisture closest to the source of the magnetic field, in this case, at the surface. For quickly scanning finished product, a pin-less meter is a convenient way to identify a problem area. However, a pinless meter cannot differentiate between shell and core moisture content and will not detect a moisture gradient. Also, readings provided by pinless meters are affected by surface moisture.
Pinless Type Features and Specifications:
- Scans large areas quickly and is a good indicator for water pockets and higher moisture levels across the board.
- Leaves no pin holes.
- Indicates average moisture in 3-dimensional field.
- Cannot show differences between surface and core moisture.
- Requires a smooth surface and a flat measuring area.
- Select Ligno-Scanner with the appropriate measuring depth.
- Woodcraft offers: Measuring depths of 1/64″, 1/4″, 3/4″, 1 1/4″ and Ligno-Scanner SD with dual measuring depth 1/4″ and 3/4.
How Does A Moisture Meter Work?
Using the principle of electrical resistance, pin-type meters use the board as an element in a circuit by driving two pins or electrodes into it. This method works because moisture conducts electricity well and dry wood is an effective insulator. Most pinless meters use the capacitance method, which uses the relationship between the moisture content and the dielectric properties of the wood.
Capacitance is the ability of a body to store an electrical charge. Dielectric is in an electrical insulator. Both the resistance and the dielectric properties of wood change in direct proportion to its moisture content, within a specific range.
At What Moisture Content Level Do I Minimize Defects?
Acceptable moisture content depends on the final use of the wood. Wood for fine furniture should be dried to 6 to 8 percent in most areas of the country, with very little variation among pieces and between the shell and core.
Appropriate moisture content is also climate-driven. To determine the attainable moisture content – or equilibrium moisture content (EMC) – in your part of the country, hang small, thin samples of a wood species in your shop or plant and taking daily readings. When the samples’ moisture content remains constant, they have reached the equilibrium level. This level will change from season to season, but you’ll know the range within which to work. The equilibrium moisture content (EMC) is defined as the moisture content at which the wood is neither gaining or losing moisture; this however, is a dynamic equilibrium and changes with relative humidity and temperature. Similar to how you put your flooring in the house for a time before installing to allow it to acclimate to the environment. That’s the idea.
U.S. Forest Products Laboratory EMC Values:
We obtained the basic information to create the calculator and table from the U.S. Forest Products Laboratory. The table below provides EMC values for an equitable representative range of atmospheric conditions that wood is likely to be a participant within. Values in this table are applicable to wood of any species for most practical purposes. This is a link to the United States Department of Agriculture Forest Service report that contains the formula found on this page. http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf.
If you know the temperature and humidity, you can find the equilibrium moisture content. You also measure your temperature and humidity in your shop over a period of time by using a chart available from the US. Forest Products Laboratory thus identifying the EMC or moisture content that you should be looking for.
When working with different wood species, “specific gravity” (SG) is an important concept to understand but is not always one that is clearly understood. Specific gravity can apply to any substance but for the purposes of this blog, specific gravity will be examined with reference to various types of wood.
Moisture meters are the tools used to measure moisture content in lumber, but a meter that does not allow for species’ corrections, or user-programming of the correct species is not providing accurate information. Consider that the specific gravity for balsa is 0.17 while the specific gravity for mahogany is 1.10. That’s a significant difference!
Some meters present specific gravity information through species correction tables – printed charts that let you manually look up and adjust the reading your meter gives. Others, like Wagner’s line of pinless meters, allow for user programming the specific gravity for a range of softwood and hardwood lumbers, allowing you to select the appropriate species for instant readings. Simply put, specific gravity is a scale that indicates a substance’s density.
A specific gravity table is available at the Department of Agriculture Forest Service website, http://www.nrs.fs.fed.us/pubs/rn/rn_nrs38.pdf, or you can use the Wood Identification & Use book by Terry Porter, Woodcraft item #147956, which has specific gravity listed for many species.
AF = 8.77 + (0.25 * MM) – (15.86 * SG) – (0.62 * SG * MM) in which:
AF = Species Adjustment
MM = Meter Reading
SG = Specific Gravity
To calculate specific gravity, use the following formula:
- Weight/volume divided by specific gravity of water
- Weight lbs/volume in cubic feet divided by 62.34/cu.ft.
You may also find weight/volume information in other wood books such as The Wood Bible by Nick Gibbs, soon to be available in soft cover at your local Woodcraft store and online.
Let’s review our line up of moisture meters, both with the video and information listed below,
General Tools Model MM1E, Woodcraft item #413429
- Pin type meter
- LED bar graph display
- 10-LED display
- 7-35% measurement range, with separate high and low scales
- Protective cap doubles as calibration checker
- Full battery and low batter indications
- 1 year warranty
General Tools Model MMD4E, Woodcraft item #844319
- Pin style
- Large LCD display
- 5-50% measurement range. 1.5-33% other materials and is selectable
- Audible sounds
- Hold switch to hold/freeze the LCD reading
- 1 year warranty
Lignomat Mini-Ligno E/D, Woodcraft item #150259
- Pin style
- LED display
- 2 setting for wood species correction
- 6-45% measurement range
- 2 pair of pins the extras are stored in the battery compartment
- Internal calibration and automatic adjustment
- Fits in your shirt pocket, but comes with a protective pouch with a belt clip
- 2 year warranty
General Tools Model MMD900, Woodcraft item #153774
- Two measurement modes, pin and pinless
- Two sets of test pins. One set on top of meter and one set on end of 46in remove cable
- Pinless measurement depth of 2 inch below surface
- Check with butternut
- 1 year warranty
Lignomat Mini-Ligno DX/C, Woodcraft item #153775
- Pin type
- 5-65% measurement range depending on species setting
- 40 settings for wood species corrections
- Measuring range low enough to check hardwood floors
- Bright digital display that is readable in dark spaces
- Reference scale for non-wood materials
- Connection for external cables that are available accessories
- 2 year warranty
Lignomat Ligno-Scanner SD, Woodcraft item #150260
- Pinless style
- Dual depth scan ¼” and ¾”
- 5-45% measuring range
- 71 settings for species correction
- Reference scale for other materials
- 2 year warranty
