ASTM D7438-20

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of the standard as published by ASTM is to be considered the official document.
Designation: D7438 − 13 D7438 − 20
Standard Practice for
Field Calibration and Application of Hand-Held Moisture
Meters
This standard is issued under the fixed designation D7438; 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
1.1 This practice applies to the measurement of moisture content of solid wood, including solid wood products containing
additives, that is, chemicals or adhesives, by hand-held moisture meters under conditions of end-use.
1.1.1 This practice includes calibration, use, and interpretation of meters for conditions that relate to wood product characteristics,
such as nonuniform grain and growth ring orientation, and to end-use process conditions, such as moisture gradients.
1.1.2 Meters employing differing technologies maywill not necessarily provide equivalent readings under the same conditions.
When this practice has been applied, it is assumed that the referenced meter is acceptable unless otherwise specified. Meters shall
have been calibrated by Test Methods D4444.
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 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:
D9 Terminology Relating to Wood and Wood-Based Products
D2915 Practice for Sampling and Data-Analysis for Structural Wood and Wood-Based Products
D4442 Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials
D4444 Test Method for Laboratory Standardization and Calibration of Hand-Held Moisture Meters
D4933 Guide for Moisture Conditioning of Wood and Wood-Based Materials
D6782 Test Methods for Standardization and Calibration of In-Line Dry Lumber Moisture Meters
2.2 Other ASTM Sources:
ASTM Standards on Precision and Bias for Various Applications, 1992
This practice is under the jurisdiction of ASTM Committee D07 on Wood and is the direct responsibility of Subcommittee D07.01 on Fundamental Test Methods and
Properties.
Current edition approved April 1, 2013Oct. 15, 2020. Published May 2013November 2020. Originally approved in 2008. Last previous edition approved in 20082013 as
D7438D7438 – 13.–08. DOI: 10.1520/D7438-13.10.1520/D7438-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7438 − 20
3. Terminology
3.1 Definitions:
3.1.1 For definitions of general terms used in this standard related to wood and wood-based products, refer to Terminology D9.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 conductance meters—conductancemoisture meters are those that measure predominantly ionic conductance between points
of applied voltage, usually direct current. Direct-current conductance meters have been commonly referred to as “resistance-type”
12 4 12
meters. Most commercial conductance meters are high-input impedance (about 10 Ω), wide-range (10 to 10 Ω) ohmmeters.
Their scales are calibrated to read directly in moisture content (oven-dry mass basis) for a particular calibration species and at a
specific reference temperature.
3.2.1.1 Discussion—
Direct-current conductance meters have been commonly referred to as “resistance-type” meters. Most commercial conductance
12 4 12
meters are high-input impedance (about 10 Ω), wide-range (10 to 10 Ω) ohmmeters. Their scales are calibrated to read directly
in moisture content (oven-dry mass basis) for a particular calibration species and at a specific reference temperature.
3.2.2 capacitive-admittance meters—capacitive-admittancemoisture meters that transmit electromagnetic wave energy into the
wood to detect the influence of moisture in the wood on these waves as an estimate of moisture content. Wave energy is most often
in the radio frequency range; hand-held meters commonly are placed directly on the wood surface.
3.2.2.1 Discussion—
Wave energy is most often in the radio frequency range; hand-held meters commonly are placed directly on the wood surface.
4. Significance and Use
4.1 Hand-held meters provide a rapid means of sampling moisture content of wood-based materials during and after processing
to maintain quality assurance and compliance with standards. However, these measurements are inferential; that is, electrical
parameters are measured and compared against a calibration to obtain an indirect measure of moisture content. The electrical
measurements are influenced by actual moisture content, a number of other wood variables, environmental conditions, geometry
of the measuring probe circuitry, and design of the meter. The maximum accuracy can only be obtained by an awareness of the
effect of each parameter on the meter output and correction of readings as specified by these test methods. Appendix X1 is a
commentary that provides explanation of the mandatory sections and discussion of historical practices. Appendix X2 addresses the
influence of process and wood variables.
4.1.1 This practice provides for calibration and application of wood products that contain commercial characteristics and that
reflect the manufacturing environment.
4.2 Most uses of hand-held moisture meters employ correlative (predictive) relationships between the meter reading and wood
areas or volumes that exceed that of the direct meter measurement (for example, larger specimens, pieces of lumber, lots). The field
calibration section of this practice anticipates the potential need for this type of sampling. These correlative uses are examined in
Appendix X3.
5. Standardization
5.1 General—Standardization shall be performed to establish the integrity of the meter and electrode under the field conditions
of use. The meter circuit shall be tested by applying either the reference material external reference check (calibration block or
resistance points) or the internal standardization check (if incorporated into the meter), in accordance with manufacturer’s
recommendations, noting the corresponding meter response value, and comparing with the manufacturer’s data. Standardization
shall be done before calibration. If alternate electrodes are to be used with a meter, standardization shall be done for all electrode
types and alternate assemblies.
5.1.1 Initially, standardization shouldshall be performed before each period of use. The time interval may be extended It is possible
to extend the time interval if experience shows that the particular meter is stable for a longer time under equivalent use conditions.
5.1.2 Standardization procedures in the field will be affected by the standardization performance of the meter during evaluation
under Test Methods D4444. The report of section 5.2.3 of Test Methods D4444 provides this information.
5.2 The standardization shall be carried out with the instrument, including electrodes, at the temperature of the anticipated
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application. This shall include the range of anticipated conditions; the reference material shall maintain its essential characteristics
over this range. The sensitivity of this standardization to temperature of the meter shall be part of the evaluation.
5.2.1 If the environmental conditions change during the usage period beyond those evaluated in the initial standardization, the
standardization shall be repeated.
5.2.2 If the manufacturer recommends an area, a method, or a standard specimen for standardization that does not reflect the entire
direct measurement area of the meter, this shall be noted as the manufacturer’s recommendation.
5.2.3 Field standardization may be is difficult to carry out under some ambient field conditions and with the electrodes to be used.
One example is the use in monitoring in-kiln performance. If the measurement conditions are difficult to reproduce or are transient
(for example, in a hot dry kiln), then it shall be understood that the validity of the meter readings are dependent upon the laboratory
standardization and manufacturer’s recommendations.
6. Calibration
6.1 General—Under processing conditions, it is possible that laboratory calibration procedures maybe will be impractical,
particularly because of moisture and temperature gradients, nonstandard temperatures, unverified species within commercial
species groups, non straight-grain non-straight-grain wood, and common production variables such as mixtures of heartwood and
sapwood. Further, it is possible that these process variables maywill change or invalidate some of the calibration results obtained
under laboratory conditions in Test Methods D4444.
6.2 Methods—The principles and procedures of calibration in Test Methods D4444 shall be applied to the degree possible and
relevant to develop a meaningful relationship between meter readings and actual moisture content (MC).
6.2.1 All field calibrations shall be referenced to direct MC measurements (Test Methods D4442).
6.2.2 Field calibration shall be carried out with meters that have been laboratory standardized and calibrated for appropriate wood
variables, such as species and temperature using Test Methods D4444, and subsequently field standardized.
6.3 Field Variables—The calibration mayIt is acceptable for the calibration to be based on end-use environmental and product and
process conditions that are more restrictedrestrictive than those evaluated by Test Methods D4444. In addition, it is possible that
the process conditions maywill produce interactions that must be considered in the calibration.
6.3.1 Special care must be taken to minimize errors caused by the influence of unintended wood variables, such as density and
temperature (uncorrected) on readings. Specimen size for field testing mayshall be selected to represent the appropriate geometry
of the target sample. Field meter readings are conditional upon both the prior standardization and calibration process, the influence
of wood variables in the field test, and application information supplied by the meter manufacturer.
6.4 Calibration Steps—The field calibration shall be conducted on specimens and in conditions that are representative of the
process and are carefully documented. See Appendix X2 for discussion of process variables and wood characteristics.
6.4.1 Sample Selection—The number of wood specimens used for the calibration shall be selected following the concepts of
Practice D2915, considering the variables to be represented and the desired precision of the calibration. For example, if the sample
is to represent grain patterns, moisture gradients, etc. found in a lumber grade, these variables shall be considered in setting
sampling criteria. (See also Test Methods D4444.)
6.4.2 Sample Preparation—While the sample may be intended to include If it is the intent that the sample includes process
variables such as moisture gradients, temperature, etc., the measurement and subsequent preservation of these variables prior to
and during meter measurement shall be considered part of the sampling process. See Test Methods D4444 for discussion of other
relevant issues.
6.4.3 Testing—Field calibration shall be based on the relationship of the meter readings to Test Methods D4442 moisture
measurement values. Because process conditions may be transient (for example, When process variables such as temperature and
moisture gradients, or both), both, are transient, calibration that reflects these variables requires special treatment of specimens
(such as subdividing specimens) or additional equipment (such as temperature probes). Care shall be taken to not distort the
original specimen condition with these additional steps.
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6.4.4 Determination of Corrections—To establish a correction that reflects the influence of the measured variables, the principles
of Test Methods D4444, section 6.2.4, shall be followed.
6.5 Report—Useful application of field test calibration is conditional upon the relevance of the test sample. Consequently, accurate
reporting of the wood and process variables (see 6.3 and 6.4) is critical. The report shall follow the practice of Test Methods D4444,
section 6.2.5.
6.5.1 Field samples often contain uncertainties with respect to exact species or species mixtures, temperature at the point of
electrode measurement, in-exact moisture gradients, and other specimen variables. Where these non-uniformities and uncertainties
cannot be measured or corrected, their presence shall be noted in the report and quantified where possible.
APPENDIXES
(Nonmandatory Information)
X1. COMMENTARY
INTRODUCTION
The purpose of this appendix is to supply auxiliary information on the basis for and practice of this
practice. It is organized with paragraphs that correspond by section number to those in the mandatory
text; text paragraphs needing no explanation are not listed. This concept permits changes at any time
in order to keep the practice current and to improve its usefulness.
This is a practice standard; thus, it describes and standardizes, to the degree possible, the calibration
and measurement practices that occur outside the environment of the testing laboratory.
TABLE X1.1
Section Comments
1.1 The principal concepts of this practice, as first incorporated in Practice D2016 in 1965 and then in subsequent editions of this practice, addressed
only meters based on the change of wood conductance or dielectric properties with moisture content. Specific electrode configurations were
anticipated, based on early commercial use. Meters were classified as “resistance-type” and “dielectric-type”; no provisions were made other elec-
trode configurations or measurement technologies. Meters are now classified as “conductance” rather than “resistance-type,” and “capacitive-
admittance” rather than “dielectric-type” to better reflect current understanding of the underlying physics of their function. The current practice
makes no distinction between meter measurement technologies for standardization and calibration requirements. Provision for unique characteris-
tics of measurement technologies is accommodated in Appendix X1 – Appendix X3. The use of “field” to describe calibrations and measurement
issues denotes conditions that cannot be controlled as in a laboratory, yet the conditions are very commonly the environment in which the meters
are used.
1.1.1 This practice targets use outside of the laboratory where controlled conditions are not usually possible. In addition, most commercial wood prod-
ucts are not “clear” and straight-grain and are heterogenous in other characteristics. Sampling is necessarily tied to commercial product descrip-
tions.
1.1.2 Requiring calibration under Test Methods D4444 ensures prior technical evaluation of a meter, with an associated report describing performance
under controlled conditions. Although the intended use may not adhere to these same conditions, the performance in the laboratory establishes
the minimum performance criteria for field use as well as reference points on sensitivity to variables such as species, density, temperature, etc.
3.1 This practice is designed to apply to meters using technologies other than the two technologies included in the section. Conductance and
capacitive-admittance meters are included because they provide the generic descriptions of principal, current commercial meters. Individual char-
acteristics of commercial meters are not intended to be covered in these generic descriptions. As other meter technologies are developed, more
generic descriptions should be added to this section.
3.2 This practice is designed to apply to meters using technologies other than the two technologies included in the section. Conductance and
capacitive-admittance meters are included because they provide the generic descriptions of principal, current commercial meters. Individual char-
acteristics of commercial meters are not intended to be covered in these generic descriptions. As other meter technologies are developed, more
generic descriptions should be added to this section.
4.1.1 Much of the content of this practice was incorporated in previous versions and drafts of Test Methods D4444, and some earlier in Practice D2016.
The mixing of test methods and practices in one standard is not desirable; thus, this practice attempts to capture the critical elements of the many
and varied commercial applications of hand-held meters while Test Methods D4444 concentrates on the base-line laboratory test methods. Spe-
cific issues of meter technology in use are covered in more detail in Appendix X2.
5.1.1 Meter features and the conditions under which the meters are used can vary widely. The intent is not to say that the meter needs to be standard-
ized before it is used with each piece of lumber, although that might be the case in some applications. As with any piece of equipment, the user
should build on the manufacturer’s recommendations and augment as necessary to suit the end use but only after understanding how the equip-
ment operates for their application. This then becomes part of the user’s quality procedures.
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TABLE X1.1 Continued
Section Comments
4.2 Correlative methods of data analysis are critical to many meter uses; however, they vary widely and are difficult to characterize as true calibration
of a meter. Consequently, Appendix X3 addresses both calibration of a meter and predictive uses as separate topics of standard practice.
5.2.3 The standardization of a meter under severe environmental conditions can be a serious operating issue. If the manufacturer’s recommendations
(for example, standard specimens or standard methods for standardization) cannot be followed, the correct operation of the meter is in question.
6.1 The essence of this practice is that many uses of meters demand performance on products and under conditions that are not covered by
laboratory calibration (Test Methods D4444). Field calibration is the only recourse. It is emphasized that field-type calibrations often will apply very
narrowly to the conditions of that calibration and do not extend to other uses or conditions.
6.2.2 Although the calibrations carried out in the field may be the only ones of commercial interest because they reflect the actual operating conditions,
the meter must have the basis of laboratory calibration to ensure satisfactory operation under stable conditions and knowledge of response to
variables important in the field.
6.3 Common wood variables encountered when conducting a calibration on commercial samples include non-uniform grain and growth ring
orientation, moisture and density gradients within the measurement zone, mineral streaks, and “wet wood” (bacterially infected pockets). In many
cases, the size and shape of the wood specimens is different from that used in laboratory calibration because the goal of the field calibration may
be to calibrate the meter assembly for a particular application in which influence of specimen size and shape is an important element. These fac-
tors pose a challenge to field calibration because they produce results conditional upon all the incorporated factors; the resultant calibration may
not extend to other uses. The results also have a component of sample “error” that may not be clearly identified (see Appendix X3). A common
example of a field calibration with a narrow scope is a calibration of a meter used with deep members (for example, 200 mm) dried in a commer-
cial kiln. A gradient is expected and accommodated in the calibration; however, this calibration may not be valid if the depth of the specimens in
the next kiln charge is different from the original or if the kiln schedule is changed to accommodate the change in size.
6.5 Because field calibrations can have a very narrow scope, the report must clarify the variables evaluated, the variables not controlled, the “grade”
description, etc. These provide boundaries within which the results may apply.
X2. METER USE
X2.1 General
X2.1.1 Measurement of moisture content in end-use applications requires consideration of meter technology, wood characteristics
and environmental influences. Sampling and related analysis are additional essential elements. Appendix X2 incorporates historical
observations on end-use applications.
X2.1.2 Meter Technology—Different meter technologies require differing operating procedures in end-use. Some operating
variables, however, have a generally common influence on most meter/electrode assemblies. The following sections include both
these generally common influences and those that are technology specific.
X2.2 Common Operating Variables
X2.2.1 Among the more common operating variables that can be addressed in a generic fashion are environmental factor of
temperature, the wood variable of species, the sensitivity to chemicals, and the issues of sampling.
X2.2.2 Temperature Influence and Corrections:
X2.2.2.1 Temperature Effect on Meter—Meter circuits can be temperature sensitive, therefore, frequent zero or span adjustments,
or both, may be necessary during use. The manufacturer should indicate the optimum range of temperature for operation of the
meter without loss of accuracy due to temperature. It is recommended that whenever possible, the meter be equilibrated with the
measurement environment before readings are taken. The intent is that in no case temperature or humidity alter the operating
characteristics of a meter (that has been equilibrated and adjusted) to the degree that the accuracy is impaired.
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X2.2.2.2 Temperature Correction—Temperature corrections are obtainable from manufacturer’s data, published data, or using
built-in adjustments in the meter. Temperature corrections require special care to obtain the wood (not air) temperature, and may
be unreliable to correct some species. A reference temperature of 2°C shall be standard for zero correction. Clearly indicate the
reference temperature at some point on the meter. Always make temperature correction before species correction.
X2.2.3 Species Influence and Corrections:
X2.2.3.1 Species Correction—Only use manufacturer’s data developed in accordance with acceptable calibration procedures for
the particular meter. The data should be accompanied with documentation on whether the corrections are for either the dial
calibration species, a specific species, or for a species market group. Where appropriate correction data are not available, calibrate
the meter in accordance with Section 6.
X2.2.3.2 Other Species Correction Considerations—There are numerous species-related effects that may result in different meter
readings for the same actual moisture content. These include wood property variations related to site or genetics and, for some
species, differences in between the heartwood and sapwood portions. Quantifying these effects to a precision sufficient to justify
a separate correction, such as for temperature, may be difficult. Similarly, species market groups (such as Hem-Fir and
Spruce-Pine-Fir) may contain species that cannot be visually separated at the point of moisture measurement or where such
separation is impractical. In field measurements where these influences cannot be practically separated, make some judgment in
defining the population that encompass these sources of variation and calibrate the meter in accordance with Section 6.
X2.2.4 Chemical Additives and Adhesive Influence and Corrections:
X2.2.4.1 Chemicals—Wood products which have been treated with preservatives, fire retardants, or dimensional stabilization
agents may give abnormal readings (usually high). Of these chemicals, creosote and pentachlorophenol solutions appear to have
insignificant effects. However, salt solutions may cause abnormally high readings that should be considered qualitative or
semiquantitative at best. Conductance meters having insulated pins can be used to measure MC of materials that have been
surface-treated with chemicals provided that confirmation is made of the accuracy through direct MC determination (for example,
Test Methods D4442).
(1) Specific Treatments—CCA-C treatment has been reported to be less conductive than salt treatments, reducing the error of
readings of treated southern pine to about 2 % MC in the range of 12 to 24 % MC. New preservative treatments were introduced
in 2004, replacing some traditional treatments; no data on the effect of these treatments on moisture meter readings is available.
X2.2.4.2 Adhesives—Adhesives may cause abnormally high readings in reconstituted wood products. Before any particular meter
is used in moisture sensing of any particular product containing adhesives, its calibration must be demonstrated on that product.
Re-calibration must be carried out following any change in processing conditions. The calibrations must be consistent with these
test methods.
X2.2.5 Sampling:
X2.2.5.1 Sampling Plan—The goals of sampling (for example, estimating mean MC or MC variability, wet spot identification,
measurement of within piece gradients, etc.) and the intended use of the information (for example, kiln performance, specification
adherence, effect of wood variables, etc.) directly influence a sampling plan. If the moisture and wood characteristics, or both, vary
significantly within a piece, more than one location on a piece will need to be sampled. The size of the lot and the number of
sampling locations both influence the confidence with which conclusions may be drawn from the resulting data.
(1) Sampling Location—Selection of the location for sampling should consider wood characteristics and possible moisture
gradients. For example, in selecting the location on a piece of lumber, take the readings at least 500 mm from the end and in the
James, W. L., “Effects of Wood Preservatives on Electric Moisture Meter Readings,” U.S. Forest Service Research Note, FPL-0.06, 1965.
Richards, M. J., “Effect of CCA-C Wood Preservative on Moisture Content Readings by Electronic-Type Moisture Meter,” Forest Products Journal, Vol 40, No. 2, pp.
29–33, 1990.
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center of the face. Readings should be taken in areas that are reasonably straight grain and free of characteristics such as knots
that affect the moisture level and influence the reading, or both.
X2.2.5.2 Lot Size—The number of readings per sample or per lot should be selected for consistency with the desired accuracy.
Practice D2915 provides guidance on selection of lot size.
X2.3 Technology-Specific Considerations for End-Use
X2.3.1 The majority of hand-held meters employ either conductance or capacitive-admittance operating technologies. The
following sections contain observations on operating characteristics of, and recommendations for, these technologies in end-use.
X2.3.2 Use of Conductance Meters:
X2.3.2.1 Electrode Sensing Region—Conductance moisture meters respond to the moisture content between the electrodes. They
can be used to determine “point” moisture content directly, if insulated pins are used, or average moisture content indirectly. All
readings should be taken with the pins aligned so that the current flow is parallel to the grain. Average moisture content can be
obtained through the thickness by integrating moisture content versus thickness. Under the following conditions it can also be
inferred from a single point measure.
(1) Obtaining an Average MC Reading—Wood of rectangular cross section tends to develop a parabolic gradient during drying
(assuming that the maximum moisture content is below fiber saturation point (FSP). From the geometry of a parabola, the point
of average MC lies between one fourth and one fifth of the total thickness. Therefore, if the pins are driven to this point, an
approximation can be obtained for average MC of the cross section. Using the same principle, a circular cross section has its
average MC at one sixth to one seventh of the diameter. The parabolic generalization generally does not pertain if lumber has been
dried in conditions that induce steep moisture
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