ASTM D4444-92(2003)
(Test Method)Standard Test Methods for Use and Calibration of Hand-Held Moisture Meters
Standard Test Methods for Use and Calibration of Hand-Held Moisture Meters
SIGNIFICANCE AND USE
Hand-held meters provide a rapid means of sampling moisture content of wood-based materials during and after processing to maintain quality assurance and compliance with standards. However, these measurements are inferential, that is, electrical parameters are measured and compared against a calibration curve to obtain an indirect measure of moisture content. The electrical measurements are influenced by actual moisture content, a number of other wood variables, environmental conditions, geometry of the measuring probe, and design of the meter. The maximum accuracy can only be obtained by an awareness of the effect of each parameter on the meter output and correction of readings as specified by these test methods.
SCOPE
1.1 These test methods apply to the measurement of moisture content of solid wood, including veneer, and wood products containing additives, that is, chemicals or adhesives (subject to conditions in and ). They also provide guidelines for meter use and calibration by manufacturers and users as alternatives to ovendry measurements.
1.2 Conductance and dielectric meters are not necessarily equivalent in their readings under the same conditions. When these test methods are referenced, it is assumed that either type of meter is acceptable unless otherwise specified. Both types of meters are to be calibrated with respect to moisture content on an oven-dry mass basis as determined by Test Methods D 4442.
1.3 The method title indicates the procedures and uses for each type of meter:SectionMethod AConductance Meters5 to 7Method BDielectric Meters8 to 10
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:D4444–92 (Reapproved 2003)
Standard Test Methods for
Use and Calibration of Hand-Held Moisture Meters
This standard is issued under the fixed designation D4444; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3. Terminology
1.1 These test methods apply to the measurement of mois- 3.1 Definitions of Terms Specific to This Standard:
ture content of solid wood, including veneer, and wood 3.1.1 conductance meters—Conductance meters are those
products containing additives, that is, chemicals or adhesives that measure predominantly ionic conductance between points
(subject to conditions in 6.4 and 9.4). They also provide of applied voltage, usually dc. Direct-current conduct-ance
guidelines for meter use and calibration by manufacturers and meters are commonly referred to as “resistance” meters. Most
users as alternatives to ovendry measurements. commercial conductance meters are high-input impedance
12 4 12
1.2 Conductance and dielectric meters are not necessarily (about 10 V ), wide-range (10 to 10 V) ohmmeters. Their
equivalent in their readings under the same conditions. When scales are calibrated to read directly in moisture content
thesetestmethodsarereferenced,itisassumedthateithertype (oven-drymassbasis)foraparticularcalibrationspeciesandat
ofmeterisacceptableunlessotherwisespecified.Bothtypesof a specific reference temperature. Readings of conductance
meters are to be calibrated with respect to moisture content on meters are practically independent of the relative density
an oven-dry mass basis as determined by Test Methods (specific gravity) of the specimen material.
D4442. 3.1.2 dielectric meters—There are two general types of
1.3 The method title indicates the procedures and uses for dielectric meters that may be arbitrarily categorized by their
each type of meter: predominantmodeofresponse—powerlossandadmittance(or
capacitance). Both have surface contact electrodes and readout
Section
Method A Conductance Meters 5 to 7
scalesthatareusuallymarkedinarbitraryunits.Mostdielectric
Method B Dielectric Meters 8 to 10
meters operate in the r-f frequency range, generally between 1
1.4 This standard does not purport to address all of the
and 10 MHz. Admittance meters respond primarily to capaci-
safety concerns, if any, associated with its use. It is the tance (dielectric constant) of the material being measured.
responsibility of the user of this standard to establish appro-
Power loss meters react primarily to resistance of the material.
priate safety and health practices and determine the applica- Readings of dielectric meters are significantly affected by the
bility of regulatory limitations prior to use.
relative density (specific gravity) of the specimen material.
2. Referenced Documents 4. Significance and Use
2.1 ASTM Standards:
4.1 Hand-held meters provide a rapid means of sampling
D4442 TestMethodsforDirectMoistureContentMeasure- moisture content of wood-based materials during and after
ment of Wood and Wood-Base Materials
processing to maintain quality assurance and compliance with
D4933 Guide for Moisture Conditioning of Wood and standards. However, these measurements are inferential, that
Wood-Base Materials
is, electrical parameters are measured and compared against a
calibration curve to obtain an indirect measure of moisture
content. The electrical measurements are influenced by actual
These test methods are under the jurisdiction of ASTM Committee D07 on
moisture content, a number of other wood variables, environ-
Wood and are the direct responsibility of Subcommittee D07.01 on Fundamental
mental conditions, geometry of the measuring probe, and
Test Methods and Properties.
Thesetestmethodsreplace,inpart,TestMethodsD2016(AnnualBookofASTM design of the meter. The maximum accuracy can only be
Standards, Vol 04.09).
obtainedbyanawarenessoftheeffectofeachparameteronthe
Current edition approved Feb. 15, 1992. Published April 1992. Originally
meter output and correction of readings as specified by these
published as D4444–84. Last previous edition D4444–84.
test methods.
Annual Book of ASTM Standards, Vol 04.10.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D4444–92 (2003)
METHOD A—CONDUCTANCE METERS corresponding moisture content for each specimen in the
sample by linear regression analysis. The equation for the
5. Standardization and Calibration
regression line (Y=a+bX) shall be used to establish the
5.1 Periodicstandardizationshallbeperformedonthemeter
correctionfactor(Y−X)formeterscalereadings(Y)of7to21
to test the integrity of the meter and electrode. Laboratory
inclusive.
calibration procedures are intended to provide reference data
5.1.2.4 The following wood sample information shall be
under controlled conditions that include the wood and ambient
recorded: moisture content, size (dimensions in each plane),
variables. Field calibration tests on species shall be performed
species, sapwood/heartwood percentage, relative density,
only with a meter that has been standardized and properly
growth rate (rings/25 mm), and earlywood/latewood percent-
compensated for temperature and pin configuration. Initially,
age. For other materials, the appropriate wood sample infor-
standardizationshouldbeperformedbeforeeachperiodofuse.
mation shall be recorded together with adequate data to
Thetimeintervalmaybeextendedifexperienceshowsthatthe
identify the product and its constituents. The following meter
particularmeterisstableforalongertimeunderequivalentuse
information shall be recorded: manufacturer and model, refer-
conditions.
ence temperature, applied voltage, and electrode type and
5.1.1 Standardization—The meter circuit shall be tested by
configuration.
connecting external resistors to the electrode pins, noting the
5.2 Field Calibration—Under processing conditions, labo-
corresponding MC (moisture content) value, and comparing
ratorycalibrationprocedureisimpractical,particularlybecause
with manufacturer’s data. At least two, and preferably three
ofmoisturegradients.Theprocedurein5.1.2shouldbeapplied
pointsshallbeusedtostandardizethemeter.Themanufacturer
to develop a meaningful relationship between meter reading
shallindicate(inthemanual,onthemeterormeterscale,oron
and actual MC. All field calibrations must be referenced to
the supplied resistance standard) the meter model, wood
oven-dryteststodetermineprecisionandbias.Standardization
species,andnumberofpinsforwhichtheresistancesarevalid.
procedures (5.1.1) must be followed to assure valid field
5.1.2 Laboratory Calibration—This procedure is designed
calibration at the specific field conditions during testing.
for full-scale calibration of the meter. If only a limited portion
Special care must be taken to minimize errors caused by the
of the scale requires calibration, the number of EMC (equilib-
influence of wood temperature on readings. Specimen size for
rium moisture content) levels can be reduced to as low as two.
field testing may be full size or sections thereof.
In any case, the calibration should not be extrapolated below
the lowest value. Extrapolation above 21% EMC to the fiber
6. Conductance Meter Use
saturation point is permissible, provided a value near 21% is
6.1 Readings:
obtained. Material other than solid wood shall be prepared and
6.1.1 Range—The range of moisture contents that can be
tested in a manner that is consistent with the following
detected by these meters is from a minimum of 6 or 7% MC
calibration procedures. Specimen size and shape may be
to a maximum of 25 to 27% MC (nominal value of the fiber
altered to permit testing of product-sized specimens.
saturation point). Meter scales extend above this limit only to
5.1.2.1 Test Sample Preparation—Aminimum of 75 green,
permit temperature corrections of moisture contents up to the
flat-sawnspecimens20mmthickby75mm(min)wideby100
fiber saturation point, and do not imply reliability of readings
mm along the grain shall be used for a given species.
above the fiber saturation point.
Specimens must be free of visible irregularities such as knots,
decay, reaction wood, and resin concentrations (Note 1). The NOTE 2—Oneuseofthetemperaturecorrectionisfor“hotmetering”of
kiln-driedlumberduringwhichreadingsaretakentodetermineiftheload
specimens shall be divided into 5 groups of 15 each and
has reached the desired endpoint MC. However, such readings are subject
conditioned at 25 6 1°C and selected relative humidities to
to considerable error because of “edge-readings,” assumptions of wood
each of five EMC levels between 7 and 21% (see Guide
temperature, unknown moisture gradients, and temperature effects on the
D4933). Each group will then be moisture meter tested in
meter circuitry. A further use of this correction is for moisture measure-
accordance with 5.2.2, and moisture contents determined by a
ment of dry lumber that is exposed to below-freezing temperatures. As
direct method (Test Methods D4442).Alternatively, 15 speci- with hot lumber, considerable errors are possible due to assumptions of
wood temperature, unknown moisture gradients, and temperature effects
mensmaybeequilibrated(followingadesorptionpath)ateach
on meter circuitry.
of the 5 EMC conditions.
6.1.2 Moisture Content Readings—Conductance moisture
NOTE 1—Ideally, samples shall be chosen to be entirely sapwood or
meters can be used to determine “point” moisture content
heartwood, or two separate groups of each, but not mixed in the same
directly or average moisture content indirectly. Take all read-
specimens. In the event that sapwood/heartwood mixing is unavoidable,
testing and test results shall be modified to report the effect of mixing on
ings with the pins aligned so that the current flow is parallel to
the results.
the grain. Average moisture content can be obtained through
5.1.2.2 Moisture Meter Testing—The equalized specimens the thickness by integrating moisture content versus thickness.
are numbered, weighed, and moisture meter tested at their Under the following conditions it can also be inferred from a
centers using an electrode in accordance with 6.5.The pins are single point measure.
to be aligned so that the current flow is parallel to the grain. 6.1.2.1 Single Point Average MC Reading—Wood of rect-
Meter scale readings are to be taken and recorded immediately angular cross section tends to develop a parabolic gradient
after the electrode pins are inserted. duringdrying(assumingthatthemaximummoisturecontentis
5.1.2.3 Species Correction Factor Determination—The below FSP (fiber saturation point). From the geometry of a
moisture meter scale reading must be regressed against the parabola, the point of average MC lies between one fourth and
D4444–92 (2003)
measurement, or where such separation is impractical.
onefifthofthetotalthickness.Therefore,ifthepinsaredriven
NOTE 5—For some species, or species groups, property variations
tothispoint,anapproximationcanbeobtainedforaverageMC
relatedtositeorgeneticsmayintroducediscrepanciesinthecorrection.In
of the cross section. Using the same principle, a circular cross
this case, a special calibration should be made, with emphasis on
section has its average MC at one sixth to one seventh of the
documenting the wood properties.
diameter.
6.3.2 Heartwood/Sapwood——Some species have substan-
NOTE 3—The above generalizations do not pertain if lumber has been
tial differences in meter readings for heartwood and sapwood
dried in conditions that induce steep moisture gradients (such as in drying
portions having the same actual moisture contents. In field
above100°C)orifthelumberisknownorthoughttocontainwetpockets
measurements where these zones cannot be visually separated
or streaks. This can be examined by driving pins to mid-thickness.
or where separate heartwood/sapwood measurements are im-
6.1.3 Moisture Gradients—Unless the moisture distribution
practical, make some judgment for the correct calibration.
andmeasuringtechniquesarewellunderstood,readingscanbe
6.4 Corrections For Additives:
easily misinterpreted. Four special problems should be consid-
6.4.1 Chemicals—Wood products which have been treated
ered:
with preservatives, fire retardants, or dimensional stabilization
6.1.3.1 Noninsulated electrodes (see 6.5.1).
agents may give abnormal readings (usually high). Of these
6.1.3.2 Nonparabolic gradients (see Note 3).
chemicals, creosote and pentachlorophenol solutions appear to
6.1.3.3 Surface Moisture on Electrode—Surface films of
have insignificant effects. However, salt solutions may cause
moisture, particularly from condensation on the electrode
abnormallyhighreadings,thatshouldbeconsideredqualitative
(insulated pin holder) may cause larger errors. Keep electrodes
or semiquantitative at best. Conductance meters having insu-
clean, and store and use under noncondensing conditions.
lated pins can be used to measure MC of materials that have
6.1.3.4 High Surface MC on Sample—High surface MC of
beensurface-treatedwithchemicalsprovidedthatconfirmation
the material from condensation, wetting, and high relative
ismadeoftheaccuracythroughdirect MCdetermination(Test
humidity can cause excessively high readings if noninsulated
Methods D4442).
pins are used.
NOTE 6—CCA-C treatment has been reported to be less conductive
6.1.4 Drift—Direct current conductance meters may show
thansalttreatments,reducingtheerrorofreadingsoftreatedsouthernpine
appreciable drift toward lower MC when readings are taken at
to about 2% MC in the range of 12 to 24% MC.
theupperportionoftheMCrange.Ifsuchdriftoccurs,takethe
6.4.2 Adhesives—Adhesives may cause abnormally high
reading as soon as possible after the pins are driven in and
readings in reconstituted wood products. Before any particular
voltage applied.
meter is used in moisture sensing of any particular product
6.2 Temperature Corrections:
containing adhesives, its calibration must be demonstrated on
6.2.1 Temperature Effect on Meter—Meter circuits can be
that product. Recalibration must be carried out following any
temperature-sensitive, therefore, frequent zero or span adjust-
change in processing conditions. The calibrations must be
ments,orboth,maybenecessaryduringuse.Themanufacturer
consistent with these test methods.
shall indicate the optimum range of temperature for operation
6.5 Electrodes:
of the meter without loss of accuracy due to temperature. It is
6.5.1 Preferred elec
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