Moisture MetersAccurate or inaccurate?

Jerill Vance Woodworks, LLC

Terms & Definitions• MC – moisture content• MCG – moisture content gradient• EMC – equilibrium moisture content• FSP – fiber saturation point• OD – oven dry• RH – relative humidity• DP – dew point• Temperature C/F – Degree in Celsius/Fahrenheit• Wet/Dry bulb temperature• Depression Temperature• Air dry & Artificially assisted air dry• Kiln dry (steam, solar and dehumidification)• Recommended MC• Acclimate• Ricked/stickered wood• Kiln dried vs Air dried vs Shipping dried

Why be concerned with moisture content in wood?

• Wood is hydroscopic – it is constantly giving off moisture or taking on moisture. The only exception is when it is at equilibrium moisture content (EMC).

• Wood expands and contracts as the MC changes – thus creating a concern for the woodworker.

• EMC changes as the wood changes location• Wood storage • Moisture effects the properties of glue and finish• Moisture reactors with hardware• Finishes and coatings do not stop the exchange of moisture – just slow it

down.• Moisture is necessary for the growth of mold and fungus

3 stages of moisture

• Solid• Liquid• Vapor

How moisture is stored in wood

• Free moisture– Above FSP– Not attached to cell structure

• Bound moisture– Below FSP– Attached to cell structure

Moisture Content

• MC in wood is expressed as a ratio (percentage) of the moisture in the wood to the weight of the dry wood.

• Dead green logs can exceed 100% MC• Saw Logs are usually 60% MC• Fiber Saturation Point is 28% to 32%• Oven dry is 0% MC• Recommended MCG for this area is 2%

Permissible Moisture Content For West Virginia

12% to 15% Thoroughly air dried wood9% to 12% Artificially assisted air dried wood6% to 8% Kiln dried wood20% to 25% Shipping dried20% Dry rot safety line18% to 20% Construction lumber9% to 15% Outdoor furniture11% to 14% Indoor furniture (unconditioned home)6% to 8% Indoor furniture (conditioned home)8% to 11% Indoor furniture (public building)

Methods to Measure MC in Wood• Moisture Meter

– Pin type– Pinless type

• Oven dry method– Kiln operation

• Inline moisture meter– Production – measure each board

• Distillation method• Gas/Pressure method• Paper (similar to litmus test)• Other

Types of Moisture Meters• Pin type moisture meters (resistance) • Measures relationship between moisture content and direct current conductance

– Length of pins – ¼ to 1/5 wood thickness for average MC – 2 pins vs 4 pins vs veneer electrode – Spacing of pins– Insulated pins– Range of meter– Parallel to grain– End grain– Calibration

• Pinless/Flat plate type moisture meters (dielectric) • The power-loss type, which uses the relationship between moisture content and the dielectric loss factor of the

wood• The capacitance type, which uses the relationship between moisture content and the dielectric constant of the

wood • A third type, usually referred to as the capacitive-admittance type, is essentially a combination of capacitance and

power-loss types.– Rough wood vs smooth surface – Average depth of ¾”

Electrical Resistance

The electrical resistance is the property of a piece of material that impedes the flow of electric current through the material. This requires that an electromotive force or electric potential difference exists across the material to cause a current to flow. In most cases, the magnitudes of resistance, electromotive force, and current are related by a simple proportion known as Ohm’s law.

Capacitance-type moisture meters use the relationshipbetween moisture content and dielectric constant. Thewood specimen is penetrated by the electric fieldassociated with the capacitor of the frequency determiningcircuit of an oscillator when the electrodeof the meter contacts the wood. The frequency of theoscillator is changed according to the effect of thespecimen on the capacitance of this capacitor or, inother words, according to the dielectric constant of thespecimen. A frequency discriminator generates a signal,read on a meter, proportional to the changes infrequency. Using the relation between dielectricconstant and moisture, the meter can be calibrated toread moisture content. Because of technical problemsand high cost, this type of meter is not at present beingmanufactured commercially.

Capacitance – type moisture meter

Power-loss type meters use the relation between moisture content and loss factor. The wood specimen is penetrated by the electric field radiating from an electrode that is coupled to a low-power oscillator inthe meter. Power absorbed by the specimen loads the oscillator and reduces its amplitude of oscillation, which is in turn indicated by the meter dial. Since the loss factor depends on moisture content, the meter dial can be related to percent moisture.

Power loss type moisture meter

“The accuracy of an electric moisture meter in good condition is never limited by the ability of the meter to respond precisely to the fundamental electrical property of wood on which its calibration is based, nor by the precision to which the dial can be read. The accuracy of a meter is limited by the influence of factors other than moisture content on the readings of the meter, insofar as these factors are unknown or not properly taken into account.”

Issues of MC by moisture meter• Moisture Content of wood (FSP) vs range of meter • Moist conditions – humidity/rain/ice/snow• Insulated vs Uninsulated vs length of pins• Temperature of wood (2% per 10 degrees C)• Moisture Gradient• Number and location of samples• Grain angle• Parallel vs perpendicular vs end (pins inserted)• Type of cut of log (plain, quarter, rift)• Limbs vs Boule vs Buttress vs Root Stock • Species of wood • Thickness of wood (thick vs standard vs veneer)• Chemical treatment• Chemical composition (mineral stains, salts, etc)• Sticker stain• Density of grain• Water pockets/pitch pockets/pitch veins• Adhesives & Finishes (plywood)

•Shape of Wood (Round vs Flat vs Irregular surfaces)•Defects/Abnormalities of board (knots, crotches, burls, quilted, birds eye, grain patterns, etc)•Cambium layer vs heart wood vs pith•Bast & Medullary Rays (weakest)•Growth of the tree (wind, climate, hillside, soil, etc)•Tension vs Compression wood•Internal sapwood•Ring galls (ingrown bark)•Spalted/fungus/mold, etc•Insect damage/pith flecks•Case hardening/Honeycombing/Collapse

•Maintenance/Calibration of instrument•Quality of Instrument•Margin of error•Electrical interference/noise•Knowledge of operator•Operator error

WV Charleston 13.7 13.0 12.1 11.4 12.5 13.3 14.1 14.3 14.0 13.6 13.0 13.5

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Equilibrium moisture content for outside conditions

EMC values were determined from the average of 30 or more years of relative humidity and temperature data available from the National Climatic Data Center of the National Oceanic and Atmospheric Administration.

Title: Wood Handbook, Chapter 13: Drying and Control of Moisture ContentPublication: General Technical Report FPL-GTR-190. Madison, WI: U.S.

Department of Agriculture, Forest Service, Forest Products Laboratory: 13-1 - 13-20. Chapter 13.

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Temperature corrections for reading of resistance-type moisture meters, based on combined data from several investigators. Find meter reading on vertical left margin, follow horizontally to vertical line corresponding to the temperature of the wood, interpolate truemoisture from family of curves. Example: if meter indicated 18 percent on wood at 120° F., true moisture content would be 14 percent. This chart is based on a calibration temperatureof 70° F. For other calibration temperatures near 70° F., adequate corrections can be obtained simply by shifting the temperature scale so that the true calibration temperature coincides with 70" on the percent scale. For example, for meters calibrated at 80° F., add 10°to each point on the temperature scale (shift the scale 10" toward the left), and use the chart

How to read graph on previous slide

Approximate temperature corrections for readings of power-loss-type moisture meters; data taken using a Moisture Register model L.

Approximate temperature corrections for capacitive admittance meter; data taken using a “Sentry” hand meter with calibration setting of 15 or less.

Approximate temperature corrections for capacitive admittance meter; data taken using a “Sentry” hand meter with calibration setting of 20 or greater.

Locate the point whose coordinates are theobserved scale reading and the specimentemperature, and trace back parallel to thecurves to the calibration temperature of themeter (usually 80 °F). The vertical coordinatehere is the corrected scale reading, which isthen converted to moisture content using theusual species conversion tables. Solid lines arefor the meter itself at room temperature; dottedlines are for the meter at the same temperatureas the specimens.

How to read the 3 previous graphs

References

www.ces.ncsu.edu/nreos/wood/wpn/methods_moisture.htm

www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

US Forestry ServiceNorth Carolina State UniversityForestry Products LaboratoryWood Handbook, Wood as an Engineering MaterialEncyclopedia of Furniture Making