An excerpt from Drying Oak Lumber
by Eugene M. Wengert
Department of Forestry
University of Wisconsin
Madison, WI
Part 2 (of 3) from Section 3:
Interaction of Wood and Water
Moisture Content Measurement With Electric Meters
All electrical moisture meters for wood measure an electrical property of the wood and then convert the value of this property to MC by a previously established relationship. There are four basic types of electric moisture meters classified by the property that they measure:
a) resistance,
b) capacitance,
c) capacitance admittance, and
d) electrical fringe field.
Only the resistance type meter is in widespread use for oak lumber; rarely, a noncontact capacitance admittance meter will be used in-line to monitor the incoming MC of every piece of dried lumber at a furniture or cabinet plant rough mill. More widespread use of this or other in-line meters is warranted, as over three-fourths of the manufacturing problems in the furniture or cabinet plant are related to improper MC of the lumber.
Characteristics of the Electrical Resistance Meter (12)
The electrical resistance meter is an ohmmeter, measuring the resistance between two needles that are driven into the wood. The following are important characteristics and operating procedures of the pin- or needle-type electrical resistance moisture meter.
MC range. - The working range is 7 to 25% MC, with most measured MC values being within 1% MC of the true MC. Below 7% MC, the resistance is too high to measure easily; above 25% MC, the resistance change with changing MC is too small to be accurately measured, and also is affected by other variables, in addition to MC. In other words, above 30% MC, meter readings are subject to large variations (10% MC or more) from the true MC. The meter itself is usually quite accurate in measuring resistance; the inaccuracy or uncertainty comes from the variation of the resistance of wood as a function of MC.
Species. - The original, standard calibration is based on Douglas-fir; this calibration agrees well with oak (Table 8) and most other North American hardwoods. With the advent of the microprocessor, moisture meters now have several different species calibrations built-in.
Temperature. - There is a very large effect of temperature. For the highest accuracy, use the correction table that comes with the meter. A rule-of-thumb is to subtract 1% MC from the meter reading for every 20o F that the wood is above 70o F. Conversely, add 1% MC for every 20o F below 70o F. Again, the microprocessor has resulted in built-in temperature calibrations in some meters.
What temperature should be used when measuring hot lumber in a kiln? For wetter lumber, use a temperature between the dry-bulb and the wet-bulb temperatures; and for drier lumber, use the dry-bulb temperature. However, experience with oak lumber in the kiln has shown some uncertainty in the converted MC values. Of course, the meter can be used confidently only for MCs below 30% MC.
Grain angle. - The needles must be parallel to grain at MCs above 15% MC.
MC gradient. - To estimate the average MC when there is a moisture gradient in the lumber when the shell is drier than the core, the needles must be driven one-fourth of the lumber thickness for rough lumber and one-fifth for planed lumber.
Preservatives and glue. - The effect of preservatives and glue lines are usually insignificant, especially below 15% MC, but this should be verified for the chemicals being used by running an oven-dry test to be certain.
Surface moisture. - Liquid moisture on the surface of the lumber can be wicked down the probe and thereby give an incorrect (too high) reading. Do not use the meter when there is surface moisture present.
Condensation on meter and/or probe. - If the meter or probe is brought from a cold into a warm environment and the meter is colder than the dew point temperature of the warm air, then moisture will condense on the meter and probe. The condensation may give an extremely high reading or may just give a reading of 10% MC. Low MCs cannot be measured until the moisture is evaporated; it may take many hours for this moisture to evaporate. An operating guideline is "Don't take the meter into a hot kiln unless the meter has been thoroughly warmed." Excessive heating, above 120o F, can damage the meter and shorten battery life.
Static electricity. - In a very dry environment (under 30% RH especially) or when very dry lumber is planed, a static charge can develop on the lumber. This static will result in erroneous readings by the electric meter. Often the meter will exhibit erratic behavior of the MC readout. The meter may also begin to indicate a MC value before the needles even touch the lumber. In extreme cases, it may be necessary to take the MC readings on a grounded metal table to dissipate the static charge. The meter operator should not wear static prone clothes, such as a wool sweater.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison
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