ASTM D4933-99(2004)
(Guide)Standard Guide for Moisture Conditioning of Wood and Wood-Base Materials
Standard Guide for Moisture Conditioning of Wood and Wood-Base Materials
SIGNIFICANCE AND USE
Many physical and mechanical properties of wood and wood-based materials change in response to the environmental equilibrium moisture content, and any comparison of these properties must take moisture content into account. A consistent base for comparison among different test samples and different laboratories is necessary. Shrinkage and dimensional change in particular are dependent on moisture content, and tests involving their measurement must be conducted with good equilibrium moisture content control. Conditioning can also be important in industrial settings where there are optimum moisture content levels for many products and processes, and conformance to these levels can reduce losses in quality and yield.
SCOPE
1.1 This guide covers standard procedures for conditioning and equilibrating wood and wood-base materials to constant moisture content. The procedures apply to solid wood, wood-base fiber and particulate materials and panels, and wood products containing adhesives. They are intended for use in research and development activities, testing laboratories, quality control, and for all other classes of producers and users. This guide includes background material on the importance of moisture content control, important definitions and technical data, possible types of apparatus, procedures, and the importance of conditioning time. Users should recognize that the necessary degree of precision and bias varies with the intentions of the users. Some research and testing, for example, might require very close control of moisture content, whereas control in an industrial storage facility might not require such close control. This guide offers procedures that include these different requirements.
1.2 The following safety hazards caveat pertains only to the procedure section, Section 6, of this guide. 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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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D4933–99 (Reapproved2004)
Standard Guide for
Moisture Conditioning of Wood and Wood-Based Materials
This standard is issued under the fixed designation D4933; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 ISO Standard:
ISO 554 Atmospheres for Conditioning and/or Testing—
1.1 This guide covers standard procedures for conditioning
Specifications
and equilibrating wood and wood-based materials to constant
moisture content. The procedures apply to solid wood, wood-
3. Terminology
based fiber and particulate materials and panels, and wood
3.1 Definitions—The following terms are defined in accor-
products containing adhesives. They are intended for use in
dance with Terminology D9.
research and development activities, testing laboratories, qual-
3.1.1 equilibrium moisture content—a moisture content at
ity control, and for all other classes of producers and users.
whichwoodneithergainsnorlosesmoisturetothesurrounding
This guide includes background material on the importance of
air.
moisture content control, important definitions and technical
3.1.1.1 Discussion—Equilibrium moisture content (EMC)
data, possible types of apparatus, procedures, and the impor-
generally connotes a moisture content at which a nominal
tance of conditioning time. Users should recognize that the
speciesofsolidwoodwillequilibrate.“Nominal”isusedinthe
necessary degree of precision and bias varies with the inten-
senseofa“hypotheticalaverage”ratherthananactualspecies.
tions of the users. Some research and testing, for example,
At constant EMC environmental conditions, however, various
might require very close control of moisture content, whereas
wood-base materials can reach different levels of EMC. It is
control in an industrial storage facility might not require such
more appropriate, therefore, to refer to conditioning at speci-
close control. This guide offers procedures that include these
fied relative humidity and temperature conditions than to a
different requirements.
particular EMC. Recommendations for conditioning are given
1.2 The following safety hazards caveat pertains only to the
in ISO554. Nominal values for equilibrium moisture content
procedure section, Section 6, of this guide. This standard does
(EMC) are given in Appendix X1. Caution must be used in
not purport to address all of the safety concerns, if any,
calculating or using these values since they represent a
associated with its use. It is the responsibility of the user of this
compromise between variation with species, and adsorption
standard to establish appropriate safety and health practices
and desorption. Also, wood containing high levels of extrac-
and determine the applicability of regulatory limitations prior
tives or chemicals may equilibrate at different moisture con-
to use.
tents. The data in Tables X1.1 and X1.2 were generated from
2. Referenced Documents the regression equation in X1.2, which is explained in more
2 detail in (1).
2.1 ASTM Standards:
3.1.2 moisture content—the amount of water contained in
D9 Terminology Relating to Wood and Wood-Based Prod-
the wood, usually expressed as a percentage of the mass of the
ucts
oven-dry wood.
D4442 Test Methods for Direct Moisture Content Measure-
3.2 Definitions of Terms Specific to This Standard:
ment of Wood and Wood-Base Materials
3.2.1 hysteresis—the equilibrium moisture content (EMC)
E104 Practice for Maintaining Constant Relative Humidity
that wood attains at any given relative humidity and tempera-
by Means of Aqueous Solutions
ture depends upon the direction from which the EMC is
approached. During desorption, the EMC will be higher
This guide is under the jurisdiction ofASTM Committee D07 on Wood and is
(sometimes by several percent moisture content) than during
thedirectresponsibilityofSubcommitteeD07.01onFundamentalTestMethodsand
adsorption. The analog of the magnetic hysteresis curve has
Properties.
Current edition approved April 1, 2004. Published May 2004. Originally
approved in 1989. Last previous edition approved in 1999 as D4933–99. DOI:
10.1520/D4933-99R04.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from theAmerican National Standards Institute, 25 W. 43rd St., 4th
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Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. this standard.
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D4933–99 (2004)
NOTE 1—Ifsuchsolutionsareused,precautionsmustbetakentoassure
beenusedtodescribethisphenomenon.Furthermore,theEMC
thatthespecimensdonotoverlydepress(orraise)theRHconditions.This
during a portion of the initial desorption from the never-dried
can be tested by adding an equivalent dummy volume of specimens and
condition may be higher than those in any subsequent desorp-
observing how RH is affected. An RH sensor or simple mechanical
tion cycle.
hygrometer can show relative effects on RH.
3.2.1.1 Discussion—Forrelativehumiditiesbetween10and
85% and within a broad range of temperatures, the hysteresis 6. Procedure
ratio (absorption MC/desorption MC) is approximately 0.85.
6.1 Specimens—Weighanappropriatenumberofspecimens
3.2.2 time constant—the time required for a physical quan-
periodically to determine when equilibrium is reached. No
tity to (a) rise from 0 to 1−1/ e (that is, 63.2%) of its final
strict number of specimens can be established because the
−kt
steady value when it varies with time, t,as1− e , or (b) fall
intent of the test will determine how critical sampling should
to 1/e (that is, 36.8%) of its initial value when it varies with
be. A guideline would be to include enough samples for a
−kt
time, t,as e (Ref (2)).
statisticalanalysis.Thespecimensshouldbeuniformlydistrib-
3.2.2.1 Discussion—When applying the concept of time
uted throughout the conditioning chamber. Consideration
constant to moisture conditioning, the “initial value” is the
should also be given to selecting samples that are representa-
initial MC of the specimen and the“ final value” is the EMC
tive of the material of interest.
that would be attained with extended exposure. One time
NOTE 2—Typical conditioning time required for 20-mm thick and
constant is the time period from the start of exposure to the
100-mmwideend-coatedsolidwoodspecimens,initiallyatequilibriumat
point of MC that is 63.2% of the change between initial and
50%RHand20°C,andexposedto90%RHat20°C,is60days.Asarule
final values. This applies in adsorption or desorption. The use
of thumb, required conditioning time is proportional to the square of ratio
of the time constant in conditioning is explained in 6.4.1.1.
of thickness. A similar specimen of 40 mm thickness, therefore, would
equilibrate in about 240 days; a 10-mm one in about 15 days.
4. Significance and Use
6.2 Specimen Moisture Content—Adecision must be made
concerning whether adsorption or desorption (or both) values
4.1 Many physical and mechanical properties of wood and
aretobeobtained.Thismayrequirepreconditioningbeforethe
wood-basedmaterialschangeinresponsetotheenvironmental
desired exposure. By using the relationship in the discussion
equilibrium moisture content, and any comparison of these
under hysteresis, an appropriate precondition MC can be
properties must take moisture content into account. A consis-
selected (below or above the EMC condition for adsorption or
tent base for comparison among different test samples and
desorption MC, respectively).
different laboratories is necessary. Shrinkage and dimensional
6.3 Specimen Preparation:
change in particular are dependent on moisture content, and
6.3.1 If small specimens are used to represent larger or
tests involving their measurement must be conducted with
full-size specimens, coat the appropriate edges or ends of the
good equilibrium moisture content control. Conditioning can
specimens, or both, to obtain moisture content distributions
also be important in industrial settings where there are opti-
that are typical of larger specimens. Coating is necessary also
mummoisturecontentlevelsformanyproductsandprocesses,
when using small specimens to determine the conditioning
and conformance to these levels can reduce losses in quality
time requirement for larger specimens.
and yield.
6.3.2 Stacking—Stackwithspacerssothatadjacentsurfaces
are separated.
5. Apparatus
6.4 Equilibrium Determination—The rate of moisture con-
5.1 Hygrometers, Psychrometers—The accuracy of hy-
tentchangesduringconditioningisapproximatelyexponential,
grometers and psychrometers should be within the range of
that is, rapid changes early in conditioning are followed by a
required RH control, which depends on the desired level of
gradual decrease in rate of change. As equilibrium is ap-
EMC control.
proached, the mass change becomes very slow. One of the
5.2 Thermometers—Thermometers to measure air tempera-
greaterpotentialsforerrorinconditioningtestsisinterpretation
ture should be capable of measuring temperature within
of slow mass changes as equilibrium. There are several
one-half of the temperature control requirement (see Section
approaches to endpoint determination, all of which require
8). Thermometers used in psychrometers for determining
some judgment.
relative humidity (see 5.1) must have an accuracy which is
consistent with the required sensitivity. This sensitivity can be NOTE 3—IfoneknewtheexactfinalEMCthatsampleswouldattain,it
would be easy to determine the endpoint. Because of variability in the
determined from analyzing the tables which convert measured
EMC-relative humidity relationship and the lack of initial dry mass data
temperatures to relative humidities.
that often occurs, this approach is seldom exact. Knowledge of approxi-
5.3 Weighing Device—Abalanceisrequiredtoweighspeci-
mate final EMC, however, can still be a useful guideline. A specified
mens with an accuracy that will allow measurement of the
percentage change in mass over some specified time period could also be
EMC within the desired limits (see Test Methods D4442).
usedinendpointdetermination.Suchchanges,however,areonlyrelative,
5.4 Conditioning Chamber—The chamber in which speci- and there is no real basis for establishing exact percentages. Individual
experiences with repetitive conditioning tests may, however, lead to more
mens are conditioned should be monitored for constant tem-
useful guidelines.
perature and humidity conditions. If aqueous solutions (satu-
rated salts, glycerin, or sulfuric acid) are to be used, follow the 6.4.1 Periodic Weighings—Weigh the specimens periodi-
procedure described in Practice E104. Commonly used satu- callytoestablisharecordofmasschangesothatjudgmentson
rated salt solutions are given in Table X2.1. equilibrium can be made. A general guideline is: frequent
D4933–99 (2004)
weighings early in conditioning (perhaps once or twice a day), can be used for endpoint determination. A minimum of three
followedbyagradualincreaseintimebetweenweighings,and reversals is recommended.
ending with periods possibly up to several weeks.Ageometric
7. Calculation
progression in time is recommended. The trend is clearer in a
7.1 CalculatemoisturecontentasdescribedinTestMethods
plot of specimen mass versus logarithm of time. A significant
D4442.
change in linearity connotes an approach to equilibrium.
6.4.1.1 The plotted data can be analyzed for the time to
8. Report
equilibrium; equilibrium is usually assumed to occur in 4 or 5
8.1 Reportthemethodofrelativehumiditycontrol,thelevel
time constants. Although actual equilibrium mass is usually
of EMC control specified, temperature, initial and final mois-
greaterthancalculated,itwillnotcauseappreciableerrorinthe
ture contents, a summary of the results of the periodic
time constant. In any case, the time constant can be recalcu-
weighings, a statement of how endpoint was determined, and
lated to adjust the prediction. The relationship between time
whether the value of MC is for adsorption or desorption.
constant and the proximity to the final value is:
9. Precision and Bias
Time Constant Percentage of Change
9.1 The precision of measurements will depend on the
1 63.2
286 desired precision of resulting moisture content which depends
largely on the requirements of the user. Industrial quality
control,forexample,usuallywillnotrequireasprecisecontrol
of EMC as a scientific test.
NOTE 4—The following examples demonstrate the calculation of time
constantforspecimenseitherincreasingordecreasingtowardequilibrium: NOTE 5—The major controllable variable that influences EMC is
relative humidity. Thus, a user specifying that EMC should be controlled
(a) Initial MC: 6%; EMC: 18% (assumed to be the final value). The within certain limits is also, in effect, specifying the RH should be
MCvalueatonetimeconstantistheinitialvalue(6%)plus0.632 controlled within certain limits. Furthermore, the effect of RH control on
of the difference between initial and final values: EMC control is not constant with levels of RH.At high RH levels, much
MCtc=MCi+0.632 (MCf−MCi)=6+0.632 (18−6)=13.6% closer control of RH is required for a given level of EMC control than at
The MC at two time constants is 16.4%, etc. lower levels. Similarly, temperature has an effect on EMC, and tempera-
(b) Initial MC: 18%; EMC: 6% (reverse of conditions in (a)): turevariations,evenatconstantRH,causeEMCtovary.Thetemperature
MCtc=MCi+0.632 (MCf−MCi)=18+0.632 (6−18)=10.4% effect, however, is much smaller than the effect of RH. Figs. X1.1 and
The MC at two time constants is 7.6%, etc. X1.2 (3)givethedegreeofRHcontrolnecessarytocontrolEMCofsolid
wood and composites within four different levels (60.25, 60.50, 61.0,
Either mass or moisture content can be used in the above relation-
and6 2.0% MC). For example, to control EMC of solid wood within
ships.
61% moisture content at 30% RH and 27°C, it is necessary to control
6.4.2 Endpoint Fluctuations—In practice, relative humidity
within 66% RH (Fig. X1.1). Fig. X1.3 gives the degree of temperature
control is not exact, and regular or irregular fluctuations occur
control necessary to maintain EMC of solid wood and wood-base
over time. Since the fluctuations are usually small relative to
materials within6 0.25% MC at a number of relative humidities. For
the total change that a conditi
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