Conductance-Type (Resistance-Type) Wood Moisture Meters
Importance of Wood Moisture Meters
Wood moisture meters are widely used instruments to rapidly determine moisture content (MC). These meters are invaluable because the traditional methods to determine MC are slow and destructive to the sample. Wood MC is important for maximum utilization because it effects numerous important wood properties (e.g., strength, energy, acoustics, decay resistance, and dimensional stability). Wood MC also influences the electrical conductance of wood which is the basis of moisture meters.
Effect of Moisture Content
The direct current conductance (resistance) of wood varies greatly with MC below the fiber saturation point (generally 30% for most species). As the MC decreases from 30% to 0%, the conductance decreases by a factor of over 10 million. In this MC range a roughly linear relationship exists between the logarithm of conductance and the logarithm of MC. At MC levels over 30%, the electrical conductance correlates very poorly with MC.
Effect of Temperature
The electrical conductance of wood increases as the temperature of the wood increases. This is opposite to the effect of temperature on resistance in metals and suggests that in wood the mechanism of conduction is by charge carriers whose number or mobility is increased by thermal activity. Thus, the conduction of current by wood is likely to be at least in part ionic. For MC levels above approximately 10%, the conductance of wood is roughly doubled for each increase in temperature of 10 °C. Therefore, the MC reading of a conductance-type meter needs to be corrected for temperature based on the information provided by the meter manufacturer.
Effect of Grain Angle
The conductance of wood parallel to the grain is about double that of conductance perpendicular to the grain. Ratios of conductance perpendicular to the grain, in relation to the longitudinal value, are about 0.055 for radial and 0.50 for tangential. I suggest using the average of values taken parallel and perpendicular to the grain.
Effect of Pin Depth
The MC of wood using a conductance-type meter is measured at the depth of the pins. Some of the meters have ¼ in. pins so only a surface reading can be taken. The outer “shell” of a board will gain and lose moisture much faster than the inner “core.” Therefore, I prefer a meter with long pins and a slide hammer that allows the pins to be driven to the core of the board. The downsize to this approach is that the lumber is damaged and will have two small holes due to the pins.
For more information, please read, “CALIBRATION OF MOISTURE METERS
FOR SOUTHERN HARDWOODS.”
Meet the Author
Dr. Todd Shupe is the President of Wood Science Consulting, LLC. He is a well-recognized expert on wood forensics, wood preservation, wood decay and degradation, and wood species identification. He has a broad background in new product development, quality management, and marketing and sales in both the public and private sectors. For more information please visit DrToddShupe.com.
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