1、Designation: D 4444 92 (Reapproved 2003)Standard Test Methods forUse and Calibration of Hand-Held Moisture Meters1This standard is issued under the fixed designation D 4444; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript 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. Scope1.1 These test methods apply to the m
3、easurement of mois-ture content of solid wood, including veneer, and woodproducts containing additives, that is, chemicals or adhesives(subject to conditions in 6.4 and 9.4). They also provideguidelines for meter use and calibration by manufacturers andusers as alternatives to ovendry measurements.1
4、.2 Conductance and dielectric meters are not necessarilyequivalent in their readings under the same conditions. Whenthese test methods are referenced, it is assumed that either typeof meter is acceptable unless otherwise specified. Both types ofmeters are to be calibrated with respect to moisture co
5、ntent onan oven-dry mass basis as determined by Test MethodsD 4442.1.3 The method title indicates the procedures and uses foreach type of meter:SectionMethod A Conductance Meters 5 to 7Method B Dielectric Meters 8 to 101.4 This standard does not purport to address all of thesafety concerns, if any,
6、associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D 4442 Test Methods for Direct Moisture Content Measur
7、e-ment of Wood and Wood-Base Materials2D 4933 Guide for Moisture Conditioning of Wood andWood-Base Materials23. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 conductance metersConductance meters are thosethat measure predominantly ionic conductance between pointsof applied volt
8、age, usually dc. Direct-current conduct-ancemeters are commonly referred to as “resistance” meters. Mostcommercial conductance meters are high-input impedance(about 1012V ), wide-range (104to 1012V) ohmmeters. Theirscales are calibrated to read directly in moisture content(oven-dry mass basis) for a
9、 particular calibration species and ata specific reference temperature. Readings of conductancemeters are practically independent of the relative density(specific gravity) of the specimen material.3.1.2 dielectric metersThere are two general types ofdielectric meters that may be arbitrarily categori
10、zed by theirpredominant mode of responsepower loss and admittance (orcapacitance). Both have surface contact electrodes and readoutscales that are usually marked in arbitrary units. Most dielectricmeters operate in the r-f frequency range, generally between 1and 10 MHz. Admittance meters respond pri
11、marily to capaci-tance (dielectric constant) of the material being measured.Power loss meters react primarily to resistance of the material.Readings of dielectric meters are significantly affected by therelative density (specific gravity) of the specimen material.4. Significance and Use4.1 Hand-held
12、 meters provide a rapid means of samplingmoisture content of wood-based materials during and afterprocessing to maintain quality assurance and compliance withstandards. However, these measurements are inferential, thatis, electrical parameters are measured and compared against acalibration curve to
13、obtain an indirect measure of moisturecontent. The electrical measurements are influenced by actualmoisture content, a number of other wood variables, environ-mental conditions, geometry of the measuring probe, anddesign of the meter. The maximum accuracy can only beobtained by an awareness of the e
14、ffect of each parameter on themeter output and correction of readings as specified by thesetest methods.1These test methods are under the jurisdiction of ASTM Committee D07 onWood and are the direct responsibility of Subcommittee D07.01 on FundamentalTest Methods and Properties.These test methods re
15、place, in part, Test Methods D 2016 (Annual Book of ASTMStandards, Vol 04.09).Current edition approved Feb. 15, 1992. Published April 1992. Originallypublished as D 4444 84. Last previous edition D 4444 84.2Annual Book of ASTM Standards, Vol 04.10.1Copyright ASTM International, 100 Barr Harbor Drive
16、, PO Box C700, West Conshohocken, PA 19428-2959, United States.METHOD ACONDUCTANCE METERS5. Standardization and Calibration5.1 Periodic standardization shall be performed on the meterto test the integrity of the meter and electrode. Laboratorycalibration procedures are intended to provide reference
17、dataunder controlled conditions that include the wood and ambientvariables. Field calibration tests on species shall be performedonly with a meter that has been standardized and properlycompensated for temperature and pin configuration. Initially,standardization should be performed before each perio
18、d of use.The time interval may be extended if experience shows that theparticular meter is stable for a longer time under equivalent useconditions.5.1.1 StandardizationThe meter circuit shall be tested byconnecting external resistors to the electrode pins, noting thecorresponding MC (moisture conten
19、t) value, and comparingwith manufacturers data. At least two, and preferably threepoints shall be used to standardize the meter. The manufacturershall indicate (in the manual, on the meter or meter scale, or onthe supplied resistance standard) the meter model, woodspecies, and number of pins for whi
20、ch the resistances are valid.5.1.2 Laboratory CalibrationThis procedure is designedfor full-scale calibration of the meter. If only a limited portionof the scale requires calibration, the number of EMC (equilib-rium moisture content) levels can be reduced to as low as two.In any case, the calibratio
21、n should not be extrapolated belowthe lowest value. Extrapolation above 21 % EMC to the fibersaturation point is permissible, provided a value near 21 % isobtained. Material other than solid wood shall be prepared andtested in a manner that is consistent with the followingcalibration procedures. Spe
22、cimen size and shape may bealtered to permit testing of product-sized specimens.5.1.2.1 Test Sample PreparationA minimum of 75 green,flat-sawn specimens 20 mm thick by 75 mm (min) wide by 100mm along the grain shall be used for a given species.Specimens must be free of visible irregularities such as
23、 knots,decay, reaction wood, and resin concentrations (Note 1). Thespecimens shall be divided into 5 groups of 15 each andconditioned at 25 6 1C and selected relative humidities toeach of five EMC levels between 7 and 21 % (see GuideD 4933). Each group will then be moisture meter tested inaccordance
24、 with 5.2.2, and moisture contents determined by adirect method (Test Methods D 4442). Alternatively, 15 speci-mens may be equilibrated (following a desorption path) at eachof the 5 EMC conditions.NOTE 1Ideally, samples shall be chosen to be entirely sapwood orheartwood, or two separate groups of ea
25、ch, but not mixed in the samespecimens. In the event that sapwood/heartwood mixing is unavoidable,testing and test results shall be modified to report the effect of mixing onthe results.5.1.2.2 Moisture Meter TestingThe equalized specimensare numbered, weighed, and moisture meter tested at theircent
26、ers using an electrode in accordance with 6.5. The pins areto be aligned so that the current flow is parallel to the grain.Meter scale readings are to be taken and recorded immediatelyafter the electrode pins are inserted.5.1.2.3 Species Correction Factor DeterminationThemoisture meter scale reading
27、 must be regressed against thecorresponding moisture content for each specimen in thesample by linear regression analysis. The equation for theregression line (Y=a+bX) shall be used to establish thecorrection factor (YX) for meter scale readings (Y) of7to21inclusive.5.1.2.4 The following wood sample
28、 information shall berecorded: moisture content, size (dimensions in each plane),species, sapwood/heartwood percentage, relative density,growth rate (rings/25 mm), and earlywood/latewood percent-age. For other materials, the appropriate wood sample infor-mation shall be recorded together with adequa
29、te data toidentify the product and its constituents. The following meterinformation shall be recorded: manufacturer and model, refer-ence temperature, applied voltage, and electrode type andconfiguration.5.2 Field CalibrationUnder processing conditions, labo-ratory calibration procedure is impractic
30、al, particularly becauseof moisture gradients. The procedure in 5.1.2 should be appliedto develop a meaningful relationship between meter readingand actual MC. All field calibrations must be referenced tooven-dry tests to determine precision and bias. Standardizationprocedures (5.1.1) must be follow
31、ed to assure valid fieldcalibration at the specific field conditions during testing.Special care must be taken to minimize errors caused by theinfluence of wood temperature on readings. Specimen size forfield testing may be full size or sections thereof.6. Conductance Meter Use6.1 Readings:6.1.1 Ran
32、geThe range of moisture contents that can bedetected by these meters is from a minimum of 6 or 7 % MCto a maximum of 25 to 27 % MC (nominal value of the fibersaturation point). Meter scales extend above this limit only topermit temperature corrections of moisture contents up to thefiber saturation p
33、oint, and do not imply reliability of readingsabove the fiber saturation point.NOTE 2One use of the temperature correction is for “hot metering” ofkiln-dried lumber during which readings are taken to determine if the loadhas reached the desired endpoint MC. However, such readings are subjectto consi
34、derable error because of “edge-readings,” assumptions of woodtemperature, unknown moisture gradients, and temperature effects on themeter circuitry. A further use of this correction is for moisture measure-ment of dry lumber that is exposed to below-freezing temperatures. Aswith hot lumber, consider
35、able errors are possible due to assumptions ofwood temperature, unknown moisture gradients, and temperature effectson meter circuitry.6.1.2 Moisture Content ReadingsConductance moisturemeters can be used to determine “point” moisture contentdirectly or average moisture content indirectly. Take all r
36、ead-ings with the pins aligned so that the current flow is parallel tothe grain. Average moisture content can be obtained throughthe thickness by integrating moisture content versus thickness.Under the following conditions it can also be inferred from asingle point measure.6.1.2.1 Single Point Avera
37、ge MC ReadingWood of rect-angular cross section tends to develop a parabolic gradientduring drying (assuming that the maximum moisture content isbelow FSP (fiber saturation point). From the geometry of aparabola, the point of average MC lies between one fourth andD 4444 92 (2003)2one fifth of the to
38、tal thickness. Therefore, if the pins are drivento this point, an approximation can be obtained for average MCof the cross section. Using the same principle, a circular crosssection has its average MC at one sixth to one seventh of thediameter.NOTE 3The above generalizations do not pertain if lumber
39、 has beendried in conditions that induce steep moisture gradients (such as in dryingabove 100C) or if the lumber is known or thought to contain wet pocketsor streaks. This can be examined by driving pins to mid-thickness.6.1.3 Moisture GradientsUnless the moisture distributionand measuring technique
40、s are well understood, readings can beeasily misinterpreted. Four special problems should be consid-ered:6.1.3.1 Noninsulated electrodes (see 6.5.1).6.1.3.2 Nonparabolic gradients (see Note 3).6.1.3.3 Surface Moisture on ElectrodeSurface films ofmoisture, particularly from condensation on the electr
41、ode(insulated pin holder) may cause larger errors. Keep electrodesclean, and store and use under noncondensing conditions.6.1.3.4 High Surface MC on SampleHigh surface MC ofthe material from condensation, wetting, and high relativehumidity can cause excessively high readings if noninsulatedpins are
42、used.6.1.4 DriftDirect current conductance meters may showappreciable drift toward lower MC when readings are taken atthe upper portion of the MC range. If such drift occurs, take thereading as soon as possible after the pins are driven in andvoltage applied.6.2 Temperature Corrections:6.2.1 Tempera
43、ture Effect on MeterMeter circuits can betemperature-sensitive, therefore, frequent zero or span adjust-ments, or both, may be necessary during use. The manufacturershall indicate the optimum range of temperature for operationof the meter without loss of accuracy due to temperature. It isrecommended
44、 that whenever possible, the meter be equili-brated with the measurement environment before readings aretaken. In no case shall temperature or humidity alter theoperating characteristics of a meter (that has been equilibratedand adjusted) to the degree that the accuracy is impaired.6.2.2 Temperature
45、 CorrectionMake temperature correc-tions. These are obtainable from manufacturers data, pub-lished data, or using built-in adjustments in the meter. Tem-perature corrections require special care to obtain the wood(not air) temperature, and may be unreliable to correct somespecies. A reference temper
46、ature of 25C shall be standard forzero correction. Clearly indicate the reference temperature atsome point on the meter. Always make temperature correctionbefore species correction.6.3 Species Corrections:6.3.1 Species CorrectionOnly use manufacturers data forthe particular meter for either the dial
47、 calibration species orcorrections for other species or species groups (Note 4) if thedata have been developed in accordance with acceptablecalibration procedures (5.2) (Note 5). Where correction dataare not available, calibrate the meter in accordance withprocedure 5.2.NOTE 4Species groups (such as
48、 Hem-Fir and Spruce-Pine-Fir) maycontain species which cannot be visually separated at the point of moisturemeasurement, or where such separation is impractical.NOTE 5For some species, or species groups, property variationsrelated to site or genetics may introduce discrepancies in the correction. In
49、this case, a special calibration should be made, with emphasis ondocumenting the wood properties.6.3.2 Heartwood/SapwoodSome species have substan-tial differences in meter readings for heartwood and sapwoodportions having the same actual moisture contents. In fieldmeasurements where these zones cannot be visually separatedor where separate heartwood/sapwood measurements are im-practical, make some judgment for the correct calibration.6.4 Corrections For Additives:6.4.1 ChemicalsWood products which have been treatedwith preservatives, fire retardants
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