1、Designation: D7438 08D7438 13Standard Practice forField Calibration and Application of Hand-Held MoistureMeters1This standard is issued under the fixed designation D7438; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 This practice applies to the measurem
3、ent of moisture content of solid wood, including solid wood products containingadditives, that is, chemicals or adhesives, by hand-held moisture meters under conditions of end-use.1.1.1 This practice includes calibration, use, and interpretation of meters for conditions that relate to wood productch
4、aracteristics, such as nonuniform grain and growth ring orientation, and to end-use process conditions, such as moisturegradients.1.1.2 Meters employing differing technologies may not provide equivalent readings under the same conditions. When thispractice has been applied, it is assumed that the re
5、ferenced meter is acceptable unless otherwise specified. Meters shall have beencalibrated by Test Methods D4444.1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate saf
6、ety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D2915 Practice for Sampling and Data-Analysis for Structural Wood and Wood-Based ProductsD4442 Test Methods for Direct Moisture Content Measurement of Wood and Wo
7、od-Base MaterialsD4444 Test Method for Laboratory Standardization and Calibration of Hand-Held Moisture MetersD4933 Guide for Moisture Conditioning of Wood and Wood-Based MaterialsD6782 Test Methods for Standardization and Calibration of In-Line Dry Lumber Moisture Meters2.2 Other ASTM Sources:ASTM
8、Standards on Precision and Bias for Various Applications, 19923. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 conductance metersconductance meters are those that measure predominantly ionic conductance between points ofapplied voltage, usually direct current. Direct-current co
9、nductance meters are have been commonly referred to as“resistance”“resistance-type” meters. Most commercial conductance meters are high-input impedance (about 1012 ), wide-range(104 to 1012 ) ohmmeters. Their scales are calibrated to read directly in moisture content (oven-dry mass basis) for a part
10、icularcalibration species and at a specific reference temperature.3.1.2 dielectriccapacitive-admittance metersdielectriccapacitive-admittance meters transmit electromagnetic wave energyinto the wood to detect the influence of moisture in the wood on these waves as an estimate of moisture content. Wa
11、ve energy ismost often in the radio frequency range; hand-held meters commonly are placed directly on the wood surface.1 This practice is under the jurisdiction of ASTM Committee D07 on Wood and is the direct responsibility of Subcommittee D07.01 on Fundamental Test Methods andProperties.Current edi
12、tion approved April 1, 2008April 1, 2013. Published May 2008May 2013. Originally approved in 2008. Last previous edition approved in 2008 as D743808.DOI: 10.1520/D7438-08.10.1520/D7438-13.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at servicea
13、stm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Beca
14、useit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 B
15、arr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14. Significance and Use4.1 Hand-held meters provide a rapid means of sampling moisture content of wood-based materials during and after processingto maintain quality assurance and compliance with standards. However, these
16、 measurements are inferential; that is, electricalparameters are measured and compared against a calibration to obtain an indirect measure of moisture content. The electricalmeasurements are influenced by actual moisture content, a number of other wood variables, environmental conditions, geometryof
17、 the measuring probe circuitry, and design of the meter. The maximum accuracy can only be obtained by an awareness of theeffect of each parameter on the meter output and correction of readings as specified by these test methods. Appendix X1 is acommentary that provides explanation of the mandatory s
18、ections and discussion of historical practices. Appendix X2 addresses theinfluence of process and wood variables.4.1.1 This practice provides for calibration and application of wood products that contain commercial characteristics and thatreflect the manufacturing environment.4.2 Most uses of hand-h
19、eld moisture meters employ correlative (predictive) relationships between the meter reading and woodareas or volumes that exceed that of the direct meter measurement (for example, larger specimens, pieces of lumber, lots). The fieldcalibration section of this practice anticipates the potential need
20、for this type of sampling. These correlative uses are examined inAppendix X3.5. Standardization5.1 GeneralStandardization shall be performed to establish the integrity of the meter and electrode under the field conditionsof use. The meter circuit shall be tested by applying the reference material in
21、 accordance with manufacturers recommendations,noting the corresponding meter response value, and comparing with the manufacturers data. Standardization shall be done beforecalibration. If alternate electrodes are to be used with a meter, standardization shall be done for all electrode types and alt
22、ernateassemblies.5.1.1 Initially, standardization should be performed before each period of use. The time interval may be extended if experienceshows that the particular meter is stable for a longer time under equivalent use conditions.5.1.2 Standardization procedures in the field will be affected b
23、y the standardization performance of the meter during evaluationunder Test Methods D4444. The report of section 5.2.3 of Test Methods D4444 provides this information.5.2 The standardization shall be carried out with the instrument, including electrodes, at the temperature of the anticipatedapplicati
24、on. This shall include the range of anticipated conditions; the reference material shall maintain its essential characteristicsover this range. The sensitivity of this standardization to temperature of the meter shall be part of the evaluation.5.2.1 If the environmental conditions change during the
25、usage period beyond those evaluated in the initial standardization, thestandardization shall be repeated.5.2.2 If the manufacturer recommends an area, a method, or a standard specimen for standardization that does not reflect theentire direct measurement area of the meter, this shall be noted as the
26、 manufacturers recommendation.5.2.3 Field standardization may be difficult to carry out under some ambient field conditions and with the electrodes to be used.One example is the use in monitoring in-kiln performance. If the measurement conditions are difficult to reproduce or are transient(for examp
27、le, in a hot dry kiln), then it shall be understood that the validity of the meter readings are dependent upon the laboratorystandardization and manufacturers recommendations.6. Calibration6.1 GeneralUnder processing conditions, laboratory calibration procedures maybe impractical, particularly becau
28、se ofmoisture and temperature gradients, nonstandard temperatures, unverified species within commercial species groups, nonstraight-grain wood, and common production variables such as mixtures of heartwood and sapwood. Further, these processvariables may change or invalidate some of the calibration
29、results obtained under laboratory conditions in Test Methods D4444.6.2 MethodsThe principles and procedures of calibration in Test Methods D4444 shall be applied to the degree possible andrelevant to develop a meaningful relationship between meter readings and actual moisture content (MC).6.2.1 All
30、field calibrations shall be referenced to direct MC measurements (Test Methods D4442).6.2.2 Field calibration shall be carried out with meters that have been laboratory standardized and calibrated for appropriatewood variables, such as species and temperature using Test Methods D4444, and subsequent
31、ly field standardized.6.3 Field VariablesThe calibration may be based on end-use environmental and product and process conditions that are morerestricted than those evaluated by Test Methods D4444. In addition, the process conditions may produce interactions that must beconsidered in the calibration
32、6.3.1 Special care must be taken to minimize errors caused by the influence of unintended wood variables, such as density andtemperature (uncorrected) on readings. Specimen size for field testing may be selected to represent the appropriate geometry ofthe target sample. Field meter readings are con
33、ditional upon both the prior standardization and calibration process, the influenceof wood variables in the field test, and application information supplied by the meter manufacturer.6.4 Calibration StepsThe field calibration shall be conducted on specimens and in conditions that are representative
34、of theprocess and are carefully documented. See Appendix X2 for discussion of process variables and wood characteristics.D7438 1326.4.1 Sample SelectionThe number of wood specimens used for the calibration shall be selected following the concepts ofPractice D2915, considering the variables to be rep
35、resented and the desired precision of the calibration. For example, if the sampleis to represent grain patterns, moisture gradients, etc. found in a lumber grade, these variables shall be considered in settingsampling criteria. (See also Test Methods D4444.)6.4.2 Sample PreparationWhile the sample m
36、ay be intended to include process variables such as moisture gradients,temperature, etc., the measurement and subsequent preservation of these variables prior to and during meter measurement shall beconsidered part of the sampling process. See Test Methods D4444 for discussion of other relevant issu
37、es.6.4.3 TestingField calibration shall be based on the relationship of the meter readings to Test Methods D4442 moisturemeasurement values. Because process conditions may be transient (for example, temperature and moisture gradients, or both),calibration that reflects these variables requires speci
38、al treatment of specimens (such as subdividing specimens) or additionalequipment (such as temperature probes). Care shall be taken to not distort the original specimen condition with these additionalsteps.6.4.4 Determination of CorrectionsTo establish a correction that reflects the influence of the
39、measured variables, theprinciples of Test Methods D4444, section 6.2.4, shall be followed.6.5 ReportUseful application of field test calibration is conditional upon the relevance of the test sample. Consequently,accurate reporting of the wood and process variables (see 6.3 and 6.4) is critical. The
40、report shall follow the practice of TestMethods D4444, section 6.2.5.6.5.1 Field samples often contain uncertainties with respect to exact species or species mixtures, temperature at the point ofelectrode measurement, in-exact moisture gradients, and other specimen variables. Where these non-uniform
41、ities and uncertaintiescannot be measured or corrected, their presence shall be noted in the report and quantified where possible.APPENDIXES(Nonmandatory Information)X1. COMMENTARYINTRODUCTIONThe purpose of this appendix is to supply auxiliary information on the basis for and practice of thispractic
42、e. It is organized with paragraphs that correspond by section number to those in the mandatorytext; text paragraphs needing no explanation are not listed. This concept permits changes at any timein order to keep the practice current and to improve its usefulness.This is a practice standard; thus, it
43、 describes and standardizes, to the degree possible, the calibrationand measurement practices that occur outside the environment of the testing laboratory.TABLE X1.1Section Comments1.1 The principal concepts of this practice, as first incorporated in Practice D2016 and then in subsequent editions of
44、 this practice, addressed onlymeters based on the change of wood conductance or dielectric properties with moisture content. Specific electrode configurations wereanticipated, based on early commercial use. Conductance and dielectric meters were separated in the practice; no provisions were made oth
45、erelectrode configurations or measurement technologies. The current practice makes no distinction between meter measurement technologies forstandardization and calibration requirements. Provision for unique characteristics of measurement technologies is accommodated in AppendixX1-Appendix X3. The us
46、e of “field” to describe calibrations and measurement issues denotes conditions that cannot be controlled as in alaboratory, yet the conditions are very commonly the environment in which the meters are used.1.1 The principal concepts of this practice, as first incorporated in Practice D2016 in 1965
47、and then in subsequent editions of this practice, addressedonly meters based on the change of wood conductance or dielectric properties with moisture content. Specific electrode configurations wereanticipated, based on early commercial use. Meters were classified as “resistance-type” and “dielectric
48、type”; no provisions were made other elec-trode configurations or measurement technologies. Meters are now classified as “conductance” rather than “resistance-type,” and “capacitive-admittance” rather than “dielectric-type” to better reflect current understanding of the underlying physics of their
49、function. The current practicemakes no distinction between meter measurement technologies for standardization and calibration requirements. Provision for unique characteris-tics of measurement technologies is accommodated in Appendix X1-Appendix X3. The use of “field” to describe calibrations and measurementissues denotes conditions that cannot be controlled as in a laboratory, yet the conditions are very commonly the environment in which the metersare used.1.1.1 This practice targets use outside of the laboratory where controlled conditions are not usually possib
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1