ASTM D4933-16(2021)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D4933 − 16 (Reapproved 2021)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.1 This guide covers standard procedures for conditioning
and equilibrating wood and wood-based materials to constant
2. Referenced Documents
moisture content. The procedures apply to solid wood, wood-
2.1 ASTM Standards:
based fiber and particulate materials and panels, and wood
D9 Terminology Relating to Wood and Wood-Based Prod-
products containing adhesives. They are intended for use in
ucts
research and development activities, testing laboratories, qual-
D4442 Test Methods for Direct Moisture Content Measure-
ity control, and for all other classes of producers and users.
ment of Wood and Wood-Based Materials
This guide includes background material on the importance of
E104 Practice for Maintaining Constant Relative Humidity
moisture content control, important definitions and technical
by Means of Aqueous Solutions
data, possible types of apparatus, procedures, and the impor-
2.2 ISO Standard:
tance of conditioning time. Users should recognize that the
ISO 554 Atmospheres for Conditioning and/or Testing—
necessary degree of precision and bias varies with the inten-
Specifications
tions of the users. Some research and testing, for example,
might require very close control of moisture content, whereas
3. Terminology
control in an industrial storage facility might not require such
3.1 Definitions:
close control. This guide offers procedures that include these
3.1.1 The following terms are defined in accordance with
different requirements.
Terminology D9.
1.2 The values stated in SI units are to be regarded as
3.1.2 equilibrium moisture content, n—moisture content at
standard. The values of temperature in degrees Fahrenheit
which wood neither gains nor loses moisture to the surrounding
given in Table X1.2 are mathematical conversions that are
air.
provided for information only and are not considered standard.
3.1.2.1 Discussion—Equilibrium moisture content (EMC)
1.3 The following safety hazards caveat pertains only to the
generally connotes a moisture content at which a nominal
procedure section, Section 6, of this guide. This standard does
species of solid wood will equilibrate.“ Nominal” is used in the
not purport to address all of the safety concerns, if any,
sense of a “hypothetical average” rather than an actual species.
associated with its use. It is the responsibility of the user of this
At constant EMC environmental conditions, however, various
standard to establish appropriate safety, health, and environ-
wood-base materials can reach different levels of EMC. It is
mental practices and determine the applicability of regulatory
more appropriate, therefore, to refer to conditioning at speci-
limitations prior to use.
fied relative humidity (RH) and temperature conditions than to
1.4 This international standard was developed in accor-
a particular EMC. Recommendations for conditioning are
dance with internationally recognized principles on standard-
given in ISO 554. Nominal values for equilibrium moisture
ization established in the Decision on Principles for the
content (EMC) are given in Appendix X1. Caution must be
Development of International Standards, Guides and Recom-
used in calculating or using these values since they represent a
1 2
This guide is under the jurisdiction of ASTM Committee D07 on Wood and is For referenced ASTM standards, visit the ASTM website, www.astm.org, or
the direct responsibility of Subcommittee D07.01 on Fundamental Test Methods and contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Properties. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Oct. 15, 2021. Published November 2021. Originally the ASTM website.
approved in 1989. Last previous edition approved in 2016 as D4933 – 16. DOI: Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
10.1520/D4933-16R21. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4933 − 16 (2021)
compromise between variation with species, and adsorption 5. Apparatus
and desorption. Also, wood containing high levels of extrac-
5.1 Hygrometers, Psychrometers—The accuracy of hygrom-
tives or chemicals may equilibrate at different moisture con-
eters and psychrometers should be within the range of required
tents. The data in Tables X1.1 and X1.2 were generated from
RH control, which depends on the desired level of EMC
the regression equation in X1.1, which is explained in more
control.
detail in Ref (1).
5.2 Thermometers—Thermometers to measure air tempera-
3.1.3 moisture content, n—amount of water contained in the
ture should be capable of measuring temperature within
wood, usually expressed as a percentage of the mass of the
one-half of the temperature control requirement (see Section
oven-dry wood.
8). Thermometers used in psychrometers for determining
3.2 Definitions of Terms Specific to This Standard:
relative humidity (see 5.1) must have an accuracy which is
consistent with the required sensitivity. This sensitivity can be
3.2.1 hysteresis, n—dependence of the equilibrium moisture
determined from analyzing the tables which convert measured
content (EMC) that wood attains at any given relative humidity
temperatures to relative humidities.
and temperature upon the direction from which the EMC is
approached.
5.3 Weighing Device—A balance is required to weigh speci-
3.2.1.1 Discussion—During desorption, the EMC will be
mens with an accuracy that will allow measurement of the
higher (sometimes by several percent moisture content) than
EMC within the desired limits (see Test Methods D4442).
during adsorption. The analog of the magnetic hysteresis curve
5.4 Conditioning Chamber—The chamber in which speci-
has been used to describe this phenomenon. Furthermore, the
mens are conditioned should be monitored for constant tem-
EMC during a portion of the initial desorption from the
perature and humidity conditions. If aqueous solutions (satu-
never-dried condition may be higher than those in any subse-
rated salts, glycerin, or sulfuric acid) are to be used, follow the
quent desorption cycle. For relative humidities between 10 and
procedure described in Practice E104. Commonly used satu-
85 % and within a broad range of temperatures, the hysteresis
rated salt solutions are given in Table X2.1.
ratio (absorption EMC/desorption EMC) is approximately
0.85.
NOTE 1—If such solutions are used, precautions must be taken to assure
that the specimens do not overly depress (or raise) the RH conditions. This
3.2.2 time constant, n—time required for a physical quantity
can be tested by adding an equivalent dummy volume of specimens and
to (a) rise from 0 to 1 − 1 ⁄ e (that is, 63.2 %) of its final steady
observing how RH is affected. An RH sensor or simple mechanical
−kt
value when it varies with time, t, as 1 − e , or (b) fall to 1/e hygrometer can show relative effects on RH.
(that is, 36.8 %) of its initial value when it varies with time, t,
−kt
6. Procedure
as e ((Ref 2)).
3.2.2.1 Discussion—When applying the concept of time
6.1 Specimens—Weigh an appropriate number of specimens
constant to moisture conditioning, the “initial value” is the
periodically to determine when equilibrium is reached. No
initial MC of the specimen and the“ final value” is the EMC
strict number of specimens can be established because the
that would be attained with extended exposure. One time
intent of the test will determine how critical sampling should
constant is the time period from the start of exposure to the
be. A guideline would be to include enough specimens for a
point of MC that is 63.2 % of the change between initial and
statistical analysis. The specimens should be uniformly distrib-
final values. This applies in adsorption or desorption. The use
uted throughout the conditioning chamber. Consideration
of the time constant in conditioning is explained in 6.4.1.1.
should also be given to selecting specimens that are represen-
tative of the material of interest.
4. Significance and Use
NOTE 2—Typical conditioning time required for 20-mm thick and
100-mm wide end-coated solid wood specimens, initially at equilibrium at
4.1 Many physical and mechanical properties of wood and
50 % RH and 20°C, and exposed to 90 % RH at 20°C, is 60 days. As a rule
wood-based materials change in response to the environmental
of thumb, required conditioning time is proportional to the square of ratio
equilibrium moisture content, and any comparison of these
of thickness. A similar specimen of 40 mm thickness, therefore, would
properties must take moisture content into account. A consis-
equilibrate in about 240 days; a 10-mm one in about 15 days.
tent base for comparison among different test samples and
6.2 Specimen Moisture Content—A decision must be made
different laboratories is necessary. Shrinkage and dimensional
concerning whether adsorption or desorption (or both) values
change in particular are dependent on moisture content, and
are to be obtained. This may require preconditioning before the
tests involving their measurement must be conducted with
desired exposure. By using the relationship in the discussion
good equilibrium moisture content control. Conditioning can
under hysteresis, an appropriate precondition MC can be
also be important in industrial settings where there are opti-
selected (below or above the EMC condition for adsorption or
mum moisture content levels for many products and processes,
desorption MC, respectively).
and conformance to these levels can reduce losses in quality
and yield.
6.3 Specimen Preparation:
6.3.1 If small specimens are used to represent larger or
full-size specimens, coat the appropriate edges or ends of the
specimens, or both, to obtain moisture content distributions
The boldface numbers in parentheses refer to the list of references at the end of
this standard. that are typical of larger specimens. Coating is necessary also
D4933 − 16 (2021)
when using small specimens to determine the conditioning 6.4.2 Endpoint Fluctuations—In practice, relative humidity
time requirement for larger specimens. control is not exact, and regular or irregular fluctuations occur
over time. Since the fluctuations are usually small relative to
6.3.2 Stacking—Stack with spacers so that adjacent surfaces
the total change that a conditioning specimen will experience,
are separated.
a steady increase or decrease in mass will occur during most of
6.4 Equilibrium Determination—The rate of moisture con-
the conditioning period. As the specimen approaches very
tent changes during conditioning is approximately exponential,
close to equilibrium, the fluctuations in relative humidity begin
that is, rapid changes early in conditioning are followed by a
to affect the periodic weighings. The direction of mass change
gradual decrease in rate of change. As equilibrium is
may begin to change randomly, which is a reliable sign that
approached, the mass change becomes very slow. One of the
equilibrium has been reached within the practical limitations of
greater potentials for error in conditioning tests is interpretation
the conditioning test. Unless some other method can establish
of slow mass changes as equilibrium. There are several
a more exact endpoint, the reversal of direction of mass change
approaches to endpoint determination, all of which require
can be used for endpoint determination. A minimum of three
some judgment.
reversals is recommended.
NOTE 3—If one knew the exact final EMC that samples would attain, it
7. Calculation
would be easy to determine the endpoint. Because of variability in the
EMC-relative humidity relationship and the lack of initial dry mass data
7.1 Calculate moisture content as described in Test Methods
that often occurs, this approach is seldom exact. Knowledge of approxi-
D4442.
mate final EMC
...