ASTM D4933-99

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Designation: D 4933 – 99
Standard Guide for
Moisture Conditioning of Wood and Wood-Based Materials
This standard is issued under the fixed designation D 4933; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope ISO 554 Atmospheres for Conditioning and/or Testing—
Specifications
1.1 This guide covers standard procedures for conditioning
and equilibrating wood and wood-based materials to constant
3. Terminology
moisture content. The procedures apply to solid wood, wood-
3.1 Definitions—The following terms are defined in accor-
based fiber and particulate materials and panels, and wood
dance with Terminology D 9.
products containing adhesives. They are intended for use in
3.1.1 equilibrium moisture content—a moisture content at
research and development activities, testing laboratories, qual-
which wood neither gains nor loses moisture to the surrounding
ity control, and for all other classes of producers and users.
air.
This guide includes background material on the importance of
3.1.1.1 Discussion—Equilibrium moisture content (EMC)
moisture content control, important definitions and technical
generally connotes a moisture content at which a nominal
data, possible types of apparatus, procedures, and the impor-
species of solid wood will equilibrate.“ Nominal” is used in the
tance of conditioning time. Users should recognize that the
sense of a “hypothetical average” rather than an actual species.
necessary degree of precision and bias varies with the inten-
At constant EMC environmental conditions, however, various
tions of the users. Some research and testing, for example,
wood-base materials can reach different levels of EMC. It is
might require very close control of moisture content, whereas
more appropriate, therefore, to refer to conditioning at speci-
control in an industrial storage facility might not require such
fied relative humidity and temperature conditions than to a
close control. This guide offers procedures that include these
particular EMC. Recommendations for conditioning are given
different requirements.
in ISO 554. Nominal values for equilibrium moisture content
1.2 The following safety hazards caveat pertains only to the
(EMC) are given in Appendix X1. Caution must be used in
procedure section, Section 6, of this guide. This standard does
calculating or using these values since they represent a
not purport to address all of the safety concerns, if any,
compromise between variation with species, and adsorption
associated with its use. It is the responsibility of the user of this
and desorption. Also, wood containing high levels of extrac-
standard to establish appropriate safety and health practices
tives or chemicals may equilibrate at different moisture con-
and determine the applicability of regulatory limitations prior
tents. The data in Tables X1.1 and X1.2 were generated from
to use.
the regression equation in X1.2, which is explained in more
2. Referenced Documents detail in (1).
3.1.2 moisture content—the amount of water contained in
2.1 ASTM Standards:
the wood, usually expressed as a percentage of the mass of the
D 9 Terminology Relating to Wood
oven-dry wood.
D 4442 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)
E 104 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
2.2 ISO Standard:
approached. During desorption, the EMC will be higher
(sometimes by several percent moisture content) than during
adsorption. The analog of the magnetic hysteresis curve has
been used to describe this phenomenon. Furthermore, the EMC
This guide is under the jurisdiction of ASTM Committee D-7 on Wood and is
the direct responsibility of Subcommittee D07.01 on Fundamental Test Methods and
Properties.
Current edition approved Oct. 10, 1999. Published December 1999. Originally Available from the American National Standards Institute, 1430 Broadway,
published as D 4933 – 89. Last previous edition D 4933 – 91 (1997)e1. New York, NY 10018.
2 5
Annual Book of ASTM Standards, Vol 04.10. The boldface numbers in parentheses refer to the list of references at the end of
Annual Book of ASTM Standards, Vol 11.03. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D4933–99
that the specimens do not overly depress (or raise) the RH conditions. 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—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—Weigh an appropriate number of specimens
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)).
statistical analysis. The specimens should be uniformly distrib-
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-mm wide end-coated solid wood specimens, initially at equilibrium at
point of MC that is 63.2 % of the change between initial and
50 % RH and 20°C, and exposed to 90 % RH at 20°C, is 60 days. As a rule
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-based materials change in response to the environmental
are to be obtained. This may require preconditioning before the
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
mum moisture content levels for many products and processes,
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.
6.3.2 Stacking—Stack with spacers so that adjacent surfaces
5. Apparatus
are separated.
6.4 Equilibrium Determination—The rate of moisture con-
5.1 Hygrometers, Psychrometers—The accuracy of hy-
tent changes during conditioning is approximately exponential,
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-
ture should be capable of measuring temperature within greater potentials for error in conditioning tests is interpretation
of slow mass changes as equilibrium. There are several
one-half of the temperature control requirement (see Section
8). Thermometers used in psychrometers for determining approaches to endpoint determination, all of which require
some judgment.
relative humidity (see 5.1) must have an accuracy which is
consistent with the required sensitivity. This sensitivity can be
NOTE 3—If one knew the exact final EMC that samples would attain, it
determined from analyzing the tables which convert measured
would be easy to determine the endpoint. Because of variability in the
temperatures to relative humidities.
EMC-relative humidity relationship and the lack of initial dry mass data
5.3 Weighing Device—A balance is required to weigh speci- that often occurs, this approach is seldom exact. Knowledge of approxi-
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 D 4442).
used in endpoint determination. Such changes, however, are only relative,
5.4 Conditioning Chamber—The chamber in which speci-
and there is no real basis for establishing exact percentages. Individual
mens are conditioned should be monitored for constant tem-
experiences with repetitive conditioning tests may, however, lead to more
perature and humidity conditions. If aqueous solutions (satu-
useful guidelines.
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 E 104. Commonly used satu-
cally to establish a record of mass change so that judgments on
rated salt solutions are given in Table X2.1.
equilibrium can be made. A general guideline is: frequent
NOTE 1—If such solutions are used, precautions must be taken to assure weighings early in conditioning (perhaps once or twice a day),
D4933–99
followed by a gradual increase in time between weighings, and can be used for endpoint determination. A minimum of three
ending with periods possibly up to several weeks. A geometric reversals is recommended.
progression in time is recommended. The trend is clearer in a
7. Calculation
plot of specimen mass versus logarithm of time. A significant
7.1 Calculate moisture content as described in Test Methods
change in linearity connotes an approach to equilibrium.
D 4442.
6.4.1.1 The plotted data can be analyzed for the time to
equilibrium; equilibrium is usually assumed to occur in 4 or 5
8. Report
time constants. Although actual equilibrium mass is usually
8.1 Report the method of relative humidity control, the level
greater than calculated, it will not cause appreciable error in the
of EMC control specified, temperature, initial and final mois-
time constant. In any case, the time constant can be recalcu-
ture contents, a summary of the results of the periodic
lated to adjust the prediction. The relationship between time
weighings, a statement of how endpoint was determined, and
constant and the proximity to the final value is:
whether the value of MC is for adsorption or desorption.
Time Constant Percentage of Change
9. Precision and Bias
1 63.2
9.1 The precision of measurements will depend on the
desired precision of resulting moisture content which depends
largely on the requirements of the user. Industrial quality
control, for example, usually will not require as precise control
NOTE 4—The following examples demonstrate the calculation of time
of EMC as a scientific test.
constant for specimens either increasing or decreasing toward equilibrium:
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
MC value at one time constant is the initial value (6 %) plus 0.632
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)):
ture variations, even at constant RH, cause EMC to vary. The temperature
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) give the degree of RH control necessary to control EMC of solid
Either mass or moisture content can be used in the above relation-
wood and composites within four different levels (60.25, 60.50, 61.0,
ships.
and6 2.0 % MC). For example, to control EMC of solid wood within
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 sm
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