1、Designation: D4933 16Standard Guide forMoisture Conditioning of Wood and Wood-Based Materials1This standard is issued under the fixed designation D4933; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A n
2、umber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers standard procedures for conditioningand equilibrating wood and wood-based materials to constantmoisture content. The p
3、rocedures apply to solid wood, wood-based fiber and particulate materials and panels, and woodproducts containing adhesives. They are intended for use inresearch and development activities, testing laboratories, qual-ity control, and for all other classes of producers and users.This guide includes b
4、ackground material on the importance ofmoisture content control, important definitions and technicaldata, possible types of apparatus, procedures, and the impor-tance of conditioning time. Users should recognize that thenecessary degree of precision and bias varies with the inten-tions of the users.
5、 Some research and testing, for example,might require very close control of moisture content, whereascontrol in an industrial storage facility might not require suchclose control. This guide offers procedures that include thesedifferent requirements.1.2 The values stated in SI units are to be regard
6、ed asstandard. No other units of measurement are included in thisstandard.1.3 The following safety hazards caveat pertains only to theprocedure section, Section 6, of this guide. This standard doesnot purport to address all of the safety concerns, if any,associated with its use. It is the responsibi
7、lity of the user of thisstandard to establish appropriate safety and health practicesand determine the applicability of regulatory limitations priorto use.2. Referenced Documents2.1 ASTM Standards:2D9 Terminology Relating to Wood and Wood-Based Prod-uctsD4442 Test Methods for Direct Moisture Content
8、 Measure-ment of Wood and Wood-Based MaterialsE104 Practice for Maintaining Constant Relative Humidityby Means of Aqueous Solutions2.2 ISO Standard:ISO 554 Atmospheres for Conditioning and/or TestingSpecifications33. Terminology3.1 Definitions:3.1.1 The following terms are defined in accordance with
9、Terminology D9.3.1.2 equilibrium moisture contenta moisture content atwhich wood neither gains nor loses moisture to the surroundingair.3.1.2.1 DiscussionEquilibrium moisture content (EMC)generally connotes a moisture content at which a nominalspecies of solid wood will equilibrate.“ Nominal” is use
10、d in thesense of a “hypothetical average” rather than an actual species.At constant EMC environmental conditions, however, variouswood-base materials can reach different levels of EMC. It ismore appropriate, therefore, to refer to conditioning at speci-fied relative humidity (RH) and temperature con
11、ditions than toa particular EMC. Recommendations for conditioning aregiven in ISO 554. Nominal values for equilibrium moisturecontent (EMC) are given in Appendix X1. Caution must beused in calculating or using these values since they represent acompromise between variation with species, and adsorpti
12、onand desorption. Also, wood containing high levels of extrac-tives or chemicals may equilibrate at different moisture con-tents. The data in Tables X1.1 and X1.2 were generated fromthe regression equation in X1.1, which is explained in moredetail in Ref (1).43.1.3 moisture contentthe amount of wate
13、r contained inthe wood, usually expressed as a percentage of the mass of theoven-dry wood.3.2 Definitions of Terms Specific to This Standard:1This guide is under the jurisdiction of ASTM Committee D07 on Wood and isthe direct responsibility of Subcommittee D07.01 on Fundamental Test Methods andPrope
14、rties.Current edition approved Aug. 1, 2016. Published September 2016. Originallyapproved in 1989. Last previous edition approved in 2010 as D4933 99 (2010).DOI: 10.1520/D4933-16.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org.
15、 For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.4The boldface numbers in parentheses refer to the list
16、of references at the end ofthis standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.1 hysteresisthe equilibrium moisture content (EMC)that wood attains at any given relative humidity and tempera-ture depends upon the directio
17、n from which the EMC isapproached. During desorption, the EMC will be higher(sometimes by several percent moisture content) than duringadsorption. The analog of the magnetic hysteresis curve hasbeen used to describe this phenomenon. Furthermore, the EMCduring a portion of the initial desorption from
18、 the never-driedcondition may be higher than those in any subsequent desorp-tion cycle.3.2.1.1 DiscussionFor relative humidities between 10 and85 % and within a broad range of temperatures, the hysteresisratio (absorption MC/desorption MC) is approximately 0.85.3.2.2 time constantthe time required f
19、or a physical quan-tity to (a) rise from 0 to 1 1 e (that is, 63.2 %) of its finalsteady value when it varies with time, t,as1 ekt, or (b) fallto 1/e (that is, 36.8 %) of its initial value when it varies withtime, t,asekt(2).3.2.2.1 DiscussionWhen applying the concept of timeconstant to moisture con
20、ditioning, the “initial value” is theinitial MC of the specimen and the“ final value” is the EMCthat would be attained with extended exposure. One timeconstant is the time period from the start of exposure to thepoint of MC that is 63.2 % of the change between initial andfinal values. This applies i
21、n adsorption or desorption. The useof the time constant in conditioning is explained in 6.4.1.1.4. Significance and Use4.1 Many physical and mechanical properties of wood andwood-based materials change in response to the environmentalequilibrium moisture content, and any comparison of thesepropertie
22、s must take moisture content into account. A consis-tent base for comparison among different test samples anddifferent laboratories is necessary. Shrinkage and dimensionalchange in particular are dependent on moisture content, andtests involving their measurement must be conducted withgood equilibri
23、um moisture content control. Conditioning canalso be important in industrial settings where there are opti-mum moisture content levels for many products and processes,and conformance to these levels can reduce losses in qualityand yield.5. Apparatus5.1 Hygrometers, PsychrometersThe accuracy of hygro
24、m-eters and psychrometers should be within the range of requiredRH control, which depends on the desired level of EMCcontrol.5.2 ThermometersThermometers to measure air tempera-ture should be capable of measuring temperature withinone-half of the temperature control requirement (see Section8). Therm
25、ometers used in psychrometers for determiningrelative humidity (see 5.1) must have an accuracy which isconsistent with the required sensitivity. This sensitivity can bedetermined from analyzing the tables which convert measuredtemperatures to relative humidities.5.3 Weighing DeviceAbalance is requir
26、ed to weigh speci-mens with an accuracy that will allow measurement of theEMC within the desired limits (see Test Methods D4442).5.4 Conditioning ChamberThe chamber in which speci-mens are conditioned should be monitored for constant tem-perature and humidity conditions. If aqueous solutions (satu-r
27、ated salts, glycerin, or sulfuric acid) are to be used, follow theprocedure described in Practice E104. Commonly used satu-rated salt solutions are given in Table X2.1.NOTE 1If such solutions are used, precautions must be taken to assurethat the specimens do not overly depress (or raise) the RH cond
28、itions. Thiscan be tested by adding an equivalent dummy volume of specimens andobserving how RH is affected. An RH sensor or simple mechanicalhygrometer can show relative effects on RH.6. Procedure6.1 SpecimensWeigh an appropriate number of specimensperiodically to determine when equilibrium is reac
29、hed. Nostrict number of specimens can be established because theintent of the test will determine how critical sampling shouldbe. A guideline would be to include enough samples for astatistical analysis. The specimens should be uniformly distrib-uted throughout the conditioning chamber. Consideratio
30、nshould also be given to selecting samples that are representa-tive of the material of interest.NOTE 2Typical conditioning time required for 20-mm thick and100-mm wide end-coated solid wood specimens, initially at equilibrium at50 % RH and 20C, and exposed to 90 % RH at 20C, is 60 days.As a ruleof t
31、humb, required conditioning time is proportional to the square of ratioof thickness. A similar specimen of 40 mm thickness, therefore, wouldequilibrate in about 240 days; a 10-mm one in about 15 days.6.2 Specimen Moisture ContentA decision must be madeconcerning whether adsorption or desorption (or
32、both) valuesare to be obtained. This may require preconditioning before thedesired exposure. By using the relationship in the discussionunder hysteresis, an appropriate precondition MC can beselected (below or above the EMC condition for adsorption ordesorption MC, respectively).6.3 Specimen Prepara
33、tion:6.3.1 If small specimens are used to represent larger orfull-size specimens, coat the appropriate edges or ends of thespecimens, or both, to obtain moisture content distributionsthat are typical of larger specimens. Coating is necessary alsowhen using small specimens to determine the conditioni
34、ngtime requirement for larger specimens.6.3.2 StackingStack with spacers so that adjacent surfacesare separated.6.4 Equilibrium DeterminationThe rate of moisture con-tent changes during conditioning is approximately exponential,that is, rapid changes early in conditioning are followed by agradual de
35、crease in rate of change. As equilibrium isapproached, the mass change becomes very slow. One of thegreater potentials for error in conditioning tests is interpretationof slow mass changes as equilibrium. There are severalapproaches to endpoint determination, all of which requiresome judgment.NOTE 3
36、If one knew the exact final EMC that samples would attain, itwould be easy to determine the endpoint. Because of variability in theEMC-relative humidity relationship and the lack of initial dry mass datathat often occurs, this approach is seldom exact. Knowledge of approxi-mate final EMC, however, c
37、an still be a useful guideline. A specifiedpercentage change in mass over some specified time period could also beD4933 162used in endpoint determination. Such changes, however, are only relative,and there is no real basis for establishing exact percentages. Individualexperiences with repetitive con
38、ditioning tests may, however, lead to moreuseful guidelines.6.4.1 Periodic WeighingsWeigh the specimens periodi-cally to establish a record of mass change so that judgments onequilibrium can be made. A general guideline is: frequentweighings early in conditioning (perhaps once or twice a day),follow
39、ed by a gradual increase in time between weighings, andending with periods possibly up to several weeks. A geometricprogression in time is recommended. The trend is clearer in aplot of specimen mass versus logarithm of time. A significantchange in linearity connotes an approach to equilibrium.6.4.1.
40、1 The plotted data can be analyzed for the time toequilibrium; equilibrium is usually assumed to occur in 4 or 5time constants. Although actual equilibrium mass is usuallygreater than calculated, it will not cause appreciable error in thetime constant. In any case, the time constant can be recalcu-l
41、ated to adjust the prediction. The relationship between timeconstant and the proximity to the final value is:Time Constant Percentage of Change1 63.2286395498599NOTE 4The following examples demonstrate the calculation of timeconstant for specimens either increasing or decreasing toward equilibrium:(
42、a) Initial MC: 6 %; EMC: 18 % (assumed to be the final value). The MC valueat one time constant is the initial value (6 %) plus 0.632 of the differencebetween initial and final values: MCtc = MCi + 0.632 (MCf MCi) =6+0.632(18 6) = 13.6 %. The MC at two time constants is 16.4 %, etc.(b) Initial MC: 1
43、8 %; EMC: 6 % (reverse of conditions in (a):MCtc = MCi + 0.632 (MCf MCi) = 18 + 0.632 (6 18) = 10.4 %. The MC at twotime constants is 7.6 %, etc. Either mass or moisture content can be used inthe above relationships.6.4.2 Endpoint FluctuationsIn practice, relative humiditycontrol is not exact, and r
44、egular or irregular fluctuations occurover time. Since the fluctuations are usually small relative tothe total change that a conditioning specimen will experience,a steady increase or decrease in mass will occur during most ofthe conditioning period. As the specimen approaches veryclose to equilibri
45、um, the fluctuations in relative humidity beginto affect the periodic weighings. The direction of mass changemay begin to change randomly, which is a reliable sign thatequilibrium has been reached within the practical limitations ofthe conditioning test. Unless some other method can establisha more
46、exact endpoint, the reversal of direction of mass changecan be used for endpoint determination. A minimum of threereversals is recommended.7. Calculation7.1 Calculate moisture content as described in Test MethodsD4442.8. Report8.1 Report the method of relative humidity control, the levelof EMC contr
47、ol specified, temperature, initial and final mois-ture contents, a summary of the results of the periodicweighings, a statement of how endpoint was determined, andwhether the value of MC is for adsorption or desorption.9. Precision and Bias9.1 The precision of measurements will depend on thedesired
48、precision of resulting moisture content which dependslargely on the requirements of the user. Industrial qualitycontrol, for example, usually will not require as precise controlof EMC as a scientific test.NOTE 5The major controllable variable that influences EMC isrelative humidity. Thus, a user spe
49、cifying that EMC should be controlledwithin certain limits is also, in effect, specifying the RH should becontrolled within certain limits. Furthermore, the effect of RH control onEMC control is not constant with levels of RH. At high RH levels, muchcloser control of RH is required for a given level of EMC control than atlower levels. Similarly, temperature has an effect on EMC, and tempera-ture variations, even at constant RH, cause EMC to vary. The temperatureeffect, however, is much smaller than the effect of RH. Figs. X1.1 andX1.2 (3) g
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