ASTM D4933-1999(2004) Standard Guide for Moisture Conditioning of Wood and Wood-Base Materials《木材和木基材料湿度的调节的标准指南》.pdf

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1、Designation: D 4933 99 (Reapproved 2004)Standard Guide forMoisture Conditioning of Wood and Wood-Based Materials1This standard is issued under the fixed designation D 4933; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o

2、f 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.1. Scope1.1 This guide covers standard procedures for conditioningand equilibrating wood and wood-based materials to constantmo

3、isture content. The procedures 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.

4、This guide includes background 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 inte

5、n-tions of the users. 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 following safety haza

6、rds 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 responsibility of the user of thisstandard to establish appropriate safety and health practicesand determine the

7、applicability of regulatory limitations priorto use.2. Referenced Documents2.1 ASTM Standards:2D 9 Terminology Relating to WoodD 4442 Test Methods for Direct Moisture Content Measure-ment of Wood and Wood-Base MaterialsE 104 Practice for Maintaining Constant Relative Humidityby Means of Aqueous Solu

8、tions2.2 ISO Standard:ISO 554 Atmospheres for Conditioning and/or TestingSpecifications33. Terminology3.1 DefinitionsThe following terms are defined in accor-dance with Terminology D 9.3.1.1 equilibrium moisture contenta moisture content atwhich wood neither gains nor loses moisture to the surroundi

9、ngair.3.1.1.1 DiscussionEquilibrium moisture content (EMC)generally connotes a moisture content at which a nominalspecies of solid wood will equilibrate.“ Nominal” is used in thesense of a “hypothetical average” rather than an actual species.At constant EMC environmental conditions, however, various

10、wood-base materials can reach different levels of EMC. It ismore appropriate, therefore, to refer to conditioning at speci-fied relative humidity and temperature conditions than to aparticular EMC. Recommendations for conditioning are givenin ISO 554. Nominal values for equilibrium moisture content(

11、EMC) are given in Appendix X1. Caution must be used incalculating or using these values since they represent acompromise between variation with species, and adsorptionand desorption. Also, wood containing high levels of extrac-tives or chemicals may equilibrate at different moisture con-tents. The d

12、ata in Tables X1.1 and X1.2 were generated fromthe regression equation in X1.2, which is explained in moredetail in (1).43.1.2 moisture contentthe amount of water contained inthe wood, usually expressed as a percentage of the mass of theoven-dry wood.3.2 Definitions of Terms Specific to This Standar

13、d:3.2.1 hysteresisthe equilibrium moisture content (EMC)that wood attains at any given relative humidity and tempera-ture depends upon the direction from which the EMC isapproached. During desorption, the EMC will be higher(sometimes by several percent moisture content) than duringadsorption. The an

14、alog of the magnetic hysteresis curve has1This guide is under the jurisdiction of ASTM Committee D07 on Wood and isthe direct responsibility of Subcommittee D07.01 on Fundamental Test Methods andProperties.Current edition approved Oct. 10, 1999. Published December 1999. Originallypublished as D 4933

15、 89. Last previous edition D 4933 91 (1997)e1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from

16、 the American National Standards Institute, 25 W. 43rd St., 4thFloor, New York, NY 10036.4The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

17、been used to describe this phenomenon. Furthermore, the EMCduring a portion of the initial desorption from 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

18、hysteresisratio (absorption MC/desorption MC) is approximately 0.85.3.2.2 time constantthe time required for 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

19、 it varies withtime, t,asekt(Ref (2).3.2.2.1 DiscussionWhen applying the concept of timeconstant to moisture conditioning, 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

20、start of exposure to thepoint of MC that is 63.2 % of the change between initial andfinal values. This applies in 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 mat

21、erials change in response to the environmentalequilibrium moisture content, and any comparison of theseproperties must take moisture content into account. A consis-tent base for comparison among different test samples anddifferent laboratories is necessary. Shrinkage and dimensionalchange in particu

22、lar are dependent on moisture content, andtests involving their measurement must be conducted withgood equilibrium 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 th

23、ese levels can reduce losses in qualityand yield.5. Apparatus5.1 Hygrometers, PsychrometersThe accuracy of hy-grometers and psychrometers should be within the range ofrequired RH control, which depends on the desired level ofEMC control.5.2 ThermometersThermometers to measure air tempera-ture should

24、 be capable of measuring temperature withinone-half of the temperature control requirement (see Section8). Thermometers used in psychrometers for determiningrelative humidity (see 5.1) must have an accuracy which isconsistent with the required sensitivity. This sensitivity can bedetermined from anal

25、yzing the tables which convert measuredtemperatures to relative humidities.5.3 Weighing DeviceA balance is required to weigh speci-mens with an accuracy that will allow measurement of theEMC within the desired limits (see Test Methods D 4442).5.4 Conditioning ChamberThe chamber in which speci-mens a

26、re conditioned should be monitored for constant tem-perature and humidity conditions. If aqueous solutions (satu-rated salts, glycerin, or sulfuric acid) are to be used, follow theprocedure described in Practice E 104. Commonly used satu-rated salt solutions are given in Table X2.1.NOTE 1If such sol

27、utions are used, precautions must be taken to assurethat the specimens do not overly depress (or raise) the RH conditions. 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.

28、6. Procedure6.1 SpecimensWeigh an appropriate number of specimensperiodically to determine when equilibrium is reached. 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 asta

29、tistical analysis. The specimens should be uniformly distrib-uted throughout the conditioning chamber. Considerationshould 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

30、wood specimens, initially at equilibrium at50 % RH and 20C, and exposed to 90 % RH at 20C, is 60 days. As a ruleof thumb, 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 i

31、n about 15 days.6.2 Specimen Moisture ContentA decision must be madeconcerning whether adsorption or desorption (or both) valuesare to be obtained. This may require preconditioning before thedesired exposure. By using the relationship in the discussionunder hysteresis, an appropriate precondition MC

32、 can beselected (below or above the EMC condition for adsorption ordesorption MC, respectively).6.3 Specimen Preparation: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 distributionst

33、hat are typical of larger specimens. Coating is necessary alsowhen using small specimens to determine the conditioningtime 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 durin

34、g conditioning is approximately exponential,that is, rapid changes early in conditioning are followed by agradual decrease in rate of change. As equilibrium is ap-proached, the mass change becomes very slow. One of thegreater potentials for error in conditioning tests is interpretationof slow mass c

35、hanges as equilibrium. There are severalapproaches to endpoint determination, all of which requiresome judgment.NOTE 3If 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

36、initial dry mass datathat often occurs, this approach is seldom exact. Knowledge of approxi-mate final EMC, however, can still be a useful guideline. A specifiedpercentage change in mass over some specified time period could also beused in endpoint determination. Such changes, however, are only rela

37、tive,and there is no real basis for establishing exact percentages. Individualexperiences with repetitive conditioning 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

38、be made. A general guideline is: frequentD 4933 99 (2004)2weighings early in conditioning (perhaps once or twice a day),followed 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 i

39、n aplot of specimen mass versus logarithm of time. A significantchange in linearity connotes an approach to equilibrium.6.4.1.1 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 usuallygre

40、ater than calculated, it will not cause appreciable error in thetime constant. In any case, the time constant can be recalcu-lated to adjust the prediction. The relationship between timeconstant and the proximity to the final value is:Time Constant Percentage of Change1 63.2286395498599NOTE 4The fol

41、lowing examples demonstrate the calculation of timeconstant for specimens either increasing or decreasing toward equilibrium:(a) Initial MC: 6 %; EMC: 18 % (assumed to be the final value). TheMC value at one time constant is the initial value (6 %) plus 0.632of the difference between initial and fin

42、al 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: 18 %; EMC: 6 % (reverse of conditions in (a):MCtc = MCi + 0.632 (MCf MCi) = 18 + 0.632 (6 18) = 10.4 %The MC at two time constants is 7.6 %, etc.Either mass or moisture content c

43、an be used in the above relation-ships.6.4.2 Endpoint FluctuationsIn practice, relative humiditycontrol is not exact, and regular or irregular fluctuations occurover time. Since the fluctuations are usually small relative tothe total change that a conditioning specimen will experience,a steady incre

44、ase or decrease in mass will occur during most ofthe conditioning period. As the specimen approaches veryclose to equilibrium, the fluctuations in relative humidity beginto affect the periodic weighings. The direction of mass changemay begin to change randomly, which is a reliable sign thatequilibri

45、um has been reached within the practical limitations ofthe conditioning test. Unless some other method can establisha more 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 c

46、ontent as described in Test MethodsD 4442.8. Report8.1 Report the method of relative humidity control, the levelof EMC control 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 v

47、alue of MC is for adsorption or desorption.9. Precision and Bias9.1 The precision of measurements will depend on thedesired precision of resulting moisture content which dependslargely on the requirements of the user. Industrial qualitycontrol, for example, usually will not require as precise contro

48、lof EMC as a scientific test.NOTE 5The major controllable variable that influences EMC isrelative humidity. Thus, a user specifying that EMC should be controlledwithin certain limits is also, in effect, specifying the RH should becontrolled within certain limits. Furthermore, the effect of RH contro

49、l 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) give the degree of RH control necessary to control EMC of solidwood and composites within four different levels (60.25, 60.50, 61.0,and6 2.0 % MC). For example, to control EMC of