ASTM D4933-99(2010)
(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-based materials to constant moisture content. The procedures apply to solid wood, wood-based 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 values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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(Reapproved 2010)
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 D4442Test Methods for Direct Moisture Content Measure-
ment of Wood and Wood-Based Materials
1.1 This guide covers standard procedures for conditioning
E104Practice for Maintaining Constant Relative Humidity
and equilibrating wood and wood-based materials to constant
by Means of Aqueous Solutions
moisture content. The procedures apply to solid wood, wood-
based fiber and particulate materials and panels, and wood 2.2 ISO Standard:
products containing adhesives. They are intended for use in ISO 554Atmospheres for Conditioning and/or Testing—
research and development activities, testing laboratories, qual- Specifications
ity control, and for all other classes of producers and users.
This guide includes background material on the importance of 3. Terminology
moisture content control, important definitions and technical
3.1 Definitions:
data, possible types of apparatus, procedures, and the impor-
3.1.1 The following terms are defined in accordance with
tance of conditioning time. Users should recognize that the
Terminology D9.
necessary degree of precision and bias varies with the inten-
3.1.2 equilibrium moisture content—a moisture content at
tions of the users. Some research and testing, for example,
whichwoodneithergainsnorlosesmoisturetothesurrounding
might require very close control of moisture content, whereas
air.
control in an industrial storage facility might not require such
3.1.2.1 Discussion—Equilibrium moisture content (EMC)
close control. This guide offers procedures that include these
generally connotes a moisture content at which a nominal
different requirements.
speciesofsolidwoodwillequilibrate.“Nominal”isusedinthe
1.2 The values stated in SI units are to be regarded as
senseofa“hypotheticalaverage”ratherthananactualspecies.
standard. No other units of measurement are included in this
At constant EMC environmental conditions, however, various
standard.
wood-base materials can reach different levels of EMC. It is
1.3 The following safety hazards caveat pertains only to the more appropriate, therefore, to refer to conditioning at speci-
procedure section, Section 6, of this guide. This standard does fied relative humidity and temperature conditions than to a
particular EMC. Recommendations for conditioning are given
not purport to address all of the safety concerns, if any,
associated with its use. It is the responsibility of the user of this in ISO 554. Nominal values for equilibrium moisture content
(EMC) are given in Appendix X1. Caution must be used in
standard to establish appropriate safety and health practices
and determine the applicability of regulatory limitations prior calculating or using these values since they represent a
compromise between variation with species, and adsorption
to use.
and desorption. Also, wood containing high levels of extrac-
2. Referenced Documents tives or chemicals may equilibrate at different moisture con-
tents. The data in Tables X1.1 and X1.2 were generated from
2.1 ASTM Standards:
the regression equation in X1.1, which is explained in more
D9Terminology Relating to Wood and Wood-Based Prod-
detail in Ref (1).
ucts
3.1.3 moisture content—the amount of water contained in
the wood, usually expressed as a percentage of the mass of the
This guide is under the jurisdiction ofASTM Committee D07 on Wood and is oven-dry wood.
thedirectresponsibilityofSubcommitteeD07.01onFundamentalTestMethodsand
3.2 Definitions of Terms Specific to This Standard:
Properties.
Current edition approved May 1, 2010. Published June 2010. Originally
approved in 1989. Last previous edition approved in 2004 as D4933–99(2004).
DOI: 10.1520/D4933-99R10.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM 4th Floor, New York, NY 10036, http://www.ansi.org.
Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. this standard.
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D4933 − 99 (2010)
3.2.1 hysteresis—the equilibrium moisture content (EMC) 5.4 Conditioning Chamber—The chamber in which speci-
that wood attains at any given relative humidity and tempera- mens are conditioned should be monitored for constant tem-
ture depends upon the direction from which the EMC is perature and humidity conditions. If aqueous solutions (satu-
approached. During desorption, the EMC will be higher rated salts, glycerin, or sulfuric acid) are to be used, follow the
(sometimes by several percent moisture content) than during procedure described in Practice E104. Commonly used satu-
adsorption. The analog of the magnetic hysteresis curve has rated salt solutions are given in Table X2.1.
beenusedtodescribethisphenomenon.Furthermore,theEMC
NOTE1—Ifsuchsolutionsareused,precautionsmustbetakentoassure
during a portion of the initial desorption from the never-dried
thatthespecimensdonotoverlydepress(orraise)theRHconditions.This
condition may be higher than those in any subsequent desorp-
can be tested by adding an equivalent dummy volume of specimens and
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—For relative humidities between 10 and
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 ((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—A decision must be made
4.1 Many physical and mechanical properties of wood and
concerning whether adsorption or desorption (or both) values
wood-basedmaterialschangeinresponsetotheenvironmental
aretobeobtained.Thismayrequirepreconditioningbeforethe
equilibrium moisture content, and any comparison of these
desired exposure. By using the relationship in the discussion
properties must take moisture content into account. A consis-
under hysteresis, an appropriate precondition MC can be
tent base for comparison among different test samples and
selected (below or above the EMC condition for adsorption or
different laboratories is necessary. Shrinkage and dimensional
desorption MC, respectively).
change in particular are dependent on moisture content, and
6.3 Specimen Preparation:
tests involving their measurement must be conducted with
6.3.1 If small specimens are used to represent larger or
good equilibrium moisture content control. Conditioning can
full-size specimens, coat the appropriate edges or ends of the
also be important in industrial settings where there are opti-
specimens, or both, to obtain moisture content distributions
mummoisturecontentlevelsformanyproductsandprocesses,
that are typical of larger specimens. Coating is necessary also
and conformance to these levels can reduce losses in quality
when using small specimens to determine the conditioning
and yield.
time requirement for larger specimens.
5. Apparatus
6.3.2 Stacking—Stackwithspacerssothatadjacentsurfaces
are separated.
5.1 Hygrometers,Psychrometers—Theaccuracyofhygrom-
etersandpsychrometersshouldbewithintherangeofrequired
6.4 Equilibrium Determination—The rate of moisture con-
RH control, which depends on the desired level of EMC
tentchangesduringconditioningisapproximatelyexponential,
control.
that is, rapid changes early in conditioning are followed by a
gradual decrease in rate of change. As equilibrium is
5.2 Thermometers—Thermometers to measure air tempera-
approached, the mass change becomes very slow. One of the
ture should be capable of measuring temperature within
greaterpotentialsforerrorinconditioningtestsisinterpretation
one-half of the temperature control requirement (see Section
of slow mass changes as equilibrium. There are several
8). Thermometers used in psychrometers for determining
approaches to endpoint determination, all of which require
relative humidity (see 5.1) must have an accuracy which is
some judgment.
consistent with the required sensitivity. This sensitivity can be
determined from analyzing the tables which convert measured
NOTE 3—If one knew the exact final EMC that samples would attain, it
temperatures to relative humidities. would be easy to determine the endpoint. Because of variability in the
EMC-relative humidity relationship and the lack of initial dry mass data
5.3 Weighing Device—Abalanceisrequiredtoweighspeci-
that often occurs, this approach is seldom exact. Knowledge of approxi-
mens with an accuracy that will allow measurement of the
mate final EMC, however, can still be a useful guideline. A specified
EMC within the desired limits (see Test Methods D4442). percentage change in mass over some specified time period could also be
D4933 − 99 (2010)
used in endpoint determination. Such changes, however, are only relative,
the conditioning test. Unless some other method can establish
and there is no real basis for establishing exact percentages. Individual
amoreexactendpoint,thereversalofdirectionofmasschange
experiences with repetitive conditioning tests may, however, lead to more
can be used for endpoint determination. A minimum of three
useful guidelines.
reversals is recommended.
6.4.1 Periodic Weighings—Weigh the specimens periodi-
callytoestablisharecordofmasschangesothatjudgmentson 7. Calculation
equilibrium can be made. A general guideline is: frequent
7.1 CalculatemoisturecontentasdescribedinTestMethods
weighings early in conditioning (perhaps once or twice a day),
D4442.
followedbyagradualincreaseintimebetweenweighings,and
ending with periods possibly up to several weeks.Ageometric 8. Report
progression in time is recommended. The trend is clearer in a
8.1 Reportthemethodofrelativehumiditycontrol,thelevel
plot of specimen mass versus logarithm of time. A significant
of EMC control specified, temperature, initial and final mois-
change in linearity connotes an approach to equilibrium.
ture contents, a summary of the results of the periodic
6.4.1.1 The plotted data can be analyzed for the time to
weighings, a statement of how endpoint was determined, and
equilibrium; equilibrium is usually assumed to occur in 4 or 5
whether the value of MC is for adsorption or desorption.
time constants. Although actual equilibrium mass is usually
9. Precision and Bias
greaterthancalculated,itwillnotcauseappreciableerrorinthe
time constant. In any case, the time constant can be recalcu-
9.1 The precision of measurements will depend on the
lated to adjust the prediction. The relationship between time
desired precision of resulting moisture content which depends
constant and the proximity to the final value is:
largely on the requirements of the user. Industrial quality
Time Constant Percentage of Change
control,forexample,usuallywillnotrequireasprecisecontrol
of EMC as a scientific test.
1 63.2
NOTE 5—The major controllable variable that influences EMC is
relative humidity. Thus, a user specifying that EMC should be controlled
within certain limits is also, in effect, specifying the RH should be
controlled within certain limits. Furthermore, the effect of RH control on
NOTE 4—The following examples demonstrate the calculation of time
EMC control is not constant with levels of RH.At high RH levels, much
constantforspecimenseitherincreasingordecreasingtowardequilibrium:
closer control of RH is required for a given level of EMC control than at
lower levels. Similarly, temperature has an effect on EMC, and tempera-
(a) Initial MC: 6 %; EMC: 18 % (assumed to be the final value). The MC value
at one time constant is the initial value (6 %) plus 0.632 of the difference ture variations, even at constant RH, cause EMC to vary.The temperature
between initial and final values: MCtc = MCi + 0.632 (MCf − MCi)=6+0.632 effect, however, is much smaller than the effect of RH. Figs. X1.1 and
(18 − 6) = 13.6 %. The MC at two time constants is 16.4 %, etc.
X1.2 (3) give the degree of RH control necessary to control EMC of solid
wood and composites within four different levels (60.25, 60.50, 61.0,
(b) Initial MC: 18 %; EMC: 6 % (reverse of conditions in (a)):
and6 2.0% MC). For example, to control EMC of solid wood within
MCtc = MCi + 0.632 (MCf − MCi) = 18 + 0.632 (6 − 18) = 10.4 %. The MC at two
61% moisture content at 30% RH and 27°C, it is necessary to control
time constants is 7.6 %, etc. Either mass or moisture content can be used in
within 66% RH (Fig. X1.1). Fig. X1.3 gives the degree of temperature
the above relat
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