ASTM C1789-24

Standard Test Method for Calibration of Hand-Held Moisture Meters on Gypsum Panels

SIGNIFICANCE AND USE
5.1 This Standard Test Method is intended for use in calibrating hand-held meters to accurately read from approximately 30 % to 90 % ERH. Moisture content is related to the ERH or Aw of a material.  
5.2 Hand-held meters provide a rapid means of sampling the moisture content of gypsum boards and panels during manufacture and for field inspection during and after building construction. However, these measurements are inferential, that is, electrical parameters are measured and compared against a calibration curve to obtain an indirect measure of moisture content. The electrical measurements are influenced by the actual moisture content, a number of other gypsum board and panel variables, environmental conditions, the geometry of the measuring probe, and the 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.  
5.3 Electrical conductance and dielectric meters are not necessarily equivalent in their readings under the same conditions. When this test method is referenced, the type of meter that is being used must be reported with the relevant ranges for precision and bias as specified in this standard.  
5.4 Both types of meters are to be calibrated with respect to ERH as described in this standard.
SCOPE
1.1 This test method applies to the calibration of hand-held moisture meters for gypsum board, glass faced gypsum panels and fiber-reinforced gypsum panels by means of electrical conductance and dielectric meters. The test uses wetted test specimens which are dried down in at least five (5) steps to determine the moisture content based on the weight loss in comparison to the dry weight. The test also supplies the ERH values for each of the drying steps.  
1.2 This test method has not been evaluated for the influence of paint or wall covering materials on the indicated moisture content of a gypsum board or panel substrate.  
1.3 The values stated in SI (metric) are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.  
1.4 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Mar-2024
• ICS: 91.100.10 - Cement. Gypsum. Lime. Mortar
• Technical Committee: C11 - Gypsum and Related Building Materials and Systems
• Drafting Committee: C11.01 - Specifications and Test Methods for Gypsum Products

Relations

• Replaces: ASTM C1789-14(2018) - Standard Test Method for Calibration of Hand-Held Moisture Meters on Gypsum Panels
• Effective Date: 01-Apr-2024

Overview

ASTM C1789-24 is the internationally recognized standard test method for calibrating hand-held moisture meters used on gypsum panels, including gypsum board, glass-faced gypsum panels, and fiber-reinforced gypsum panels. Developed by ASTM Committee C11, this standard ensures that both electrical conductance and dielectric meters provide reliable, traceable readings of moisture content, which is critical during manufacture, on-site installation, and post-construction inspections. It directly supports quality assurance and compliance in construction, mitigation of moisture-related damage, and accurate field assessment.

Key Topics

• Calibration Process: The standard describes a laboratory procedure to calibrate hand-held moisture meters by correlating electrical measurements with the equilibrium relative humidity (ERH) and moisture content determined by weight loss after stepwise drying of wetted specimens.

• Scope of Application: Applicable to various gypsum-based products, including standard gypsum board, glass mat gypsum sheathing, and fiber-reinforced gypsum panels. The calibration covers ERH ranges from approximately 30% to 90%, ensuring accuracy in the critical moisture content spectrum encountered in building environments.

• Measurement Principles:

  • Conductance Meters: Use direct current (DC) to measure ionic conductance via pin electrodes; readings can be influenced by the panel’s composition, environmental factors, electrode geometry, and temperature.
  • Dielectric Meters: Use admittance or power-loss principles, generally with flat-surface electrodes and no penetration; readings are most accurate when matched to specific board types and thicknesses.

• Influence Factors: Recognizes that results are affected by material composition, environmental conditions (temperature, humidity), meter design, electrode configuration, and the presence of moisture gradients.

• Reporting Requirements: All meter calibrations must specify the instrument type, reference temperature, applied voltage, electrode type/configuration, and applicable calibration curve coefficients.

Applications

• Manufacturing Quality Control: Ensures hand-held moisture meters can be reliably used for in-process sampling of gypsum boards and panels, helping manufacturers maintain moisture specifications as per ASTM standards.

• Field Inspections: Supports contractors, inspectors, and facility managers in accurately measuring moisture in installed gypsum panels, which is essential for preventing water damage, mold growth, and warranty claims.

• Restoration and Remediation: Enables restoration professionals to verify drying progress and residual moisture levels with calibrated meters after water intrusion events.

• Calibration Labs and Meter Manufacturers: Provides standardized procedures for meter calibration, which improves consistency and traceability across the supply chain.

• Comparative Testing: Facilitates meaningful comparisons between different types of hand-held moisture meters by ensuring calibration curves are developed under controlled, repeatable conditions.

Related Standards

• ASTM C473 – Test Methods for Physical Testing of Gypsum Panel Products
• ASTM C1177/C1177M – Specification for Glass Mat Gypsum Substrate for Use as Sheathing
• ASTM C1178/C1178M – Specification for Coated Glass Mat Water-Resistant Gypsum Backing Panel
• ASTM C1278/C1278M – Specification for Fiber-Reinforced Gypsum Panel
• ASTM C1396/C1396M – Specification for Gypsum Board
• ASTM D4442 – Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials
• ASTM D4444 – Test Method for Laboratory Standardization and Calibration of Hand-Held Moisture Meters
• ASHRAE Fundamentals-Psychrometrics – Provides key humidity and air property data referenced in specimen preparation.

Keywords: ASTM C1789-24, gypsum board moisture meter calibration, electrical conductance meter, dielectric meter, gypsum panel testing, equilibrium relative humidity (ERH), hand-held moisture meters, moisture content, calibration procedure, construction standards.

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Ensuring field and laboratory accuracy in gypsum moisture measurement, ASTM C1789-24 provides the construction and building materials industry with a standardized, traceable approach to instrument calibration, directly supporting performance, compliance, and material integrity throughout the lifecycle of gypsum-based panels.

Standards Content (Sample)

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: C1789 − 24
Standard Test Method for
Calibration of Hand-Held Moisture Meters on Gypsum
Panels
This standard is issued under the fixed designation C1789; 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* C1177/C1177M Specification for Glass Mat Gypsum Sub-
strate for Use as Sheathing
1.1 This test method applies to the calibration of hand-held
C1178/C1178M Specification for Coated Glass Mat Water-
moisture meters for gypsum board, glass faced gypsum panels
Resistant Gypsum Backing Panel
and fiber-reinforced gypsum panels by means of electrical
C1278/C1278M Specification for Fiber-Reinforced Gypsum
conductance and dielectric meters. The test uses wetted test
Panel
specimens which are dried down in at least five (5) steps to
C1396/C1396M Specification for Gypsum Board
determine the moisture content based on the weight loss in
D4442 Test Methods for Direct Moisture Content Measure-
comparison to the dry weight. The test also supplies the ERH
ment of Wood and Wood-Based Materials
values for each of the drying steps.
D4444 Test Method for Laboratory Standardization and
1.2 This test method has not been evaluated for the influ-
Calibration of Hand-Held Moisture Meters
ence of paint or wall covering materials on the indicated
2.2 ASHRAE Standard:
moisture content of a gypsum board or panel substrate.
2009 ASHRAE Handbook – Fundamentals, Chapter 1 –
1.3 The values stated in SI (metric) are to be regarded as
Psychrometrics, American Society of Heating, Refrigerat-
standard. The values given in parentheses are mathematical
ing and Air-conditioning Engineers
conversions to inch-pound units that are provided for informa-
tion only and are not considered standard.
3. Terminology
1.4 This standard does not purport to address all of the
3.1 Definitions of Terms Specific to This Standard:
safety concerns, if any, associated with its use. It is the
3.1.1 absolute humidity, d , n—the ratio of the mass of water
v
responsibility of the user of this standard to establish appro-
vapor to the total volume of the moist air sample.
priate safety, health, and environmental practices and deter-
3.1.2 admittance, n—inverse of impedance, a measure of
mine the applicability of regulatory limitations prior to use.
how easily an electric current can flow through a material.
1.5 This international standard was developed in accor-
3.1.3 conductance meters, n—conductance meters are those
dance with internationally recognized principles on standard-
that measure predominantly ionic conductance between points
ization established in the Decision on Principles for the
of applied voltage, usually dc.
Development of International Standards, Guides and Recom-
3.1.3.1 Discussion—Conductance meters generally have
mendations issued by the World Trade Organization Technical
pins that penetrate into the material being measured. Direct-
Barriers to Trade (TBT) Committee.
current conductance meters are commonly referred to as
2. Referenced Documents “resistance” meters. Most commercial conductance meters are
12 4
2 high-input impedance (about 10 Ω), wide-range (10 to
2.1 ASTM Standards:
10 Ω) ohmmeters. Their scales are generally calibrated to
C473 Test Methods for Physical Testing of Gypsum Panel
read directly in moisture content (oven-dry mass basis) for a
Products
particular calibration material and at a specific reference
temperature.
This test method is under the jurisdiction of ASTM Committee C11 on Gypsum
3.1.4 dew-point temperature, t , n—the temperature at
d
and Related Building Materials and Systems and is the direct responsibility of
which a sample of moist air being cooled at constant pressure
Subcommittee C11.01 on Specifications and Test Methods for Gypsum Products.
Current edition approved April 1, 2024. Published April 2024. Originally
and moisture content reaches 100 % relative humidity.
approved in 2013. Last previous edition approved in 2018 as C1789 – 14 (2018).
DOI: 10.1520/C1789-24.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Available from American Society of Heating, Refrigerating, and Air-
Standards volume information, refer to the standard’s Document Summary page on Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle, NE, Atlanta, GA
the ASTM website. 30329, http://www.ashrae.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1789 − 24
3.1.4.1 Discussion—The dew-point temperature is the tem- 4. Summary of Test Method
perature at which water condensation begins to occur on a
4.1 These test methods provide a method for calibrating the
cooled surface in contact with moist air.
scale on conductance and dielectric meters for various types of
3.1.5 dielectric meters, n—meters that measure primarily by gypsum boards and panels for use in field measurement of
admittance or power-loss. moisture content during storage, construction and use in
building assemblies.
3.1.5.1 Discussion—Dielectric meters generally do not have
pins that penetrate into the material being measured. There are
4.2 The calibration is based on the MC of the test specimen.
two (2) general types of dielectric meters that may be arbi-
The corresponding ERH is determined by use of a calibrated
trarily categorized by their predominant mode of response –
direct read relative humidity meter.
admittance (or capacitance) and power-loss. Both have surface
4.3 ERH is essentially equivalent to water activity, A ,
w
contact electrodes and readout scales that are usually marked in
which is a measure of the amount of moisture in a material that
arbitrary units. Most dielectric meters operate in the r-f
is available to impact the performance characteristics of that
frequency range, generally between 1 MHz and 10 MHz.
material.
Admittance meters respond primarily to the capacitance (di-
electric constant) of the material being measured. Power-loss 4.4 Due to the various core and/or facing additives that are
meters react primarily to the resistance of the material. Read-
used to modify the moisture absorption characteristics, strength
ings of dielectric meters are significantly affected by the and/or other properties for specific applications, a separate
relative density (specific gravity) of the specimen material. calibration is required for each type of gypsum board or panel
product to be measured.
3.1.6 equilibrium moisture content, EMC, n—the moisture
content of a material that is in thermodynamic equilibrium with 4.5 The test method has the following steps:
the surrounding air at a given temperature and relative humid-
4.5.1 Measure the dry weights of the test specimens.
ity.
4.5.2 Determine the time step for the drying intervals that
will provide sufficient data points to develop a calibration
3.1.7 equilibrium relative humidity, ERH—the relative hu-
curve.
midity of the air in a sealed chamber that is in thermodynamic
4.5.3 Saturate the samples with water.
equilibrium with a sample of material in that chamber.
4.5.4 Dry the samples in steps, recording after each interval
3.1.8 humidity ratio, W, n—the ratio of the mass of water
the moisture content by weight of each sample and the
vapor to the mass of dry air contained in a sample of moist air.
temperature, relative humidity (ERH), and absolute humidity
3.1.9 moisture content, MC, n—the ratio of the mass of
of the atmosphere in moisture equilibrium with each sample.
water in a material to the oven-dry mass of the sample
expressed as a decimal fraction or percentage.
5. Significance and Use
3.1.9.1 Discussion—Oven-dry refers to the removal by heat-
5.1 This Standard Test Method is intended for use in
ing of all adsorbed and free water in the interstitial pores of the
calibrating hand-held meters to accurately read from approxi-
material. Crystalline water such as contained in gypsum
mately 30 % to 90 % ERH. Moisture content is related to the
molecules is not included.
ERH or A of a material.
w
3.1.10 relative humidity, ϕ, n—the ratio of the amount of
5.2 Hand-held meters provide a rapid means of sampling the
water vapor in air to the amount of water vapor in saturated air
moisture content of gypsum boards and panels during manu-
at the same temperature and pressure.
facture and for field inspection during and after building
3.1.10.1 Discussion—Equivalent to the ratio of the partial
construction. However, these measurements are inferential,
pressure of water vapor in the air to the saturated vapor
that is, electrical parameters are measured and compared
pressure at the same temperature and pressure.
against a calibration curve to obtain an indirect measure of
moisture content. The electrical measurements are influenced
3.1.11 test uncertainty ratio, TUR, n—comparison between
by the actual moisture content, a number of other gypsum
the accuracy of the Unit Under Test (UUT) and the estimated
board and panel variables, environmental conditions, the ge-
calibration uncertainty stated with a confidence level of 95 %
ometry of the measuring probe, and the design of the meter.
(K=2).
The maximum accuracy can only be obtained by an awareness
3.1.12 water activity, A , n—the ratio of the water vapor
w
of the effect of each parameter on the meter output and
pressure in a material to the vapor pressure of pure water at the
correction of readings as specified by these test methods.
same temperature.
5.3 Electrical conductance and dielectric meters are not
3.1.12.1 Discussion—Water activity is an intrinsic property
necessarily equivalent in their readings under the same condi-
derived from fundamental principles of thermodynamics and
tions. When this test method is referenced, the type of meter
physical chemistry. It is a measure of the energy status of the
that is being used must be reported with the relevant ranges for
water in a system. Commonly used for food preservation
precision and bias as specified in this standard.
analyses, it can be interpreted here as the amount of water in a
porous material that is available to impact the performance 5.4 Both types of meters are to be calibrated with respect to
characteristics of the material or to support mold growth. ERH as described in this standard.
C1789 − 24
6. Apparatus holders to keep the specimens separated sufficiently to allow
air flow between the specimens.
6.1 Laboratory equipment for moisture content determina-
8.1.2 Remove and weigh each test specimen at one-hour
tion by direct method:
intervals.
6.1.1 Forced Air Oven—Vented electric furnace capable of
8.1.3 The test specimen is deemed to be dry when three (3)
maintaining a steady-state temperature of 45 °C 6 3 °C
consecutive weighings show no change in weight within
(113 °F 6 5 °F).
60.10 % of the dried sample weight.
6.1.2 Environmental Chamber—Chamber capable of main-
8.1.4 Record the dry weight of each specimen.
taining a controlled temperature of 20 °C 6 2 ºC (68 °F 6
4 ºF) and relative humidity within the range 30 % to 90 % 6
8.2 Determine the humidity ratio of the trapped environ-
5 %. ment that is in moisture equilibrium with each specimen.
6.1.3 Electronic Balance or Scale—Electronic scale capable
NOTE 2—Humidity ratio is used as the temperature is likely to vary
of weighing each test specimen to within 60.10 g
during the course of the test and relative humidity will vary with
(60.0035 oz).
temperature at constant moisture content. Humidity ratio and dew-point
temperature do not vary with temperature at constant moisture content and
6.1.4 Relative Humidity Test Meter—The meter shall be
can be calculated from the temperature and relative humidity values
capable of reading between 30 % and 90 % relative humidity.
measured by direct read instruments.
The calibrated test uncertainty ratio (TUR) of this meter shall
8.2.1 Place the specimen in a zip sealed plastic bag to
not be less than 4:1 over the range of measure cited.
contain it in a trapped atmosphere.
6.1.4.1 Meter shall have removable temperature/humidity
8.2.2 Insert a temperature/relative humidity probe through
probes that can be inserted into sealed plastic bags.
the wall of the bag and seal tightly.
6.1.5 Humidity Box—Insulated box made of materials im-
pervious to water vapor such as plastic or sheet metal, sealed
NOTE 3—Making a small slit in the side of the bag to stretch around the
with a gasketed lid. Open trays of clean, distilled water are
relative humidity probe and sealing around the probe, if necessary, has
positioned in the box so as to keep the atmosphere within the proven to provide a reliable method.
box saturated with water vapor. Open mesh shelving or racks
8.2.3 Record the temperature and relative humidity within
are used to support samples above the water.
the bag at one-hour intervals.
6.1.6 Zip Sealed Plastic Bags—Commercially available
8.2.3.1 Calculate the humidity ratio using a psychrometric
plastic freezer weight food storage bag with a zipper type
chart or a table of thermodynamic properties of moist air.
closure that seals and prevents water vapor transmission.
8.2.3.2 Record the dry specimen temperature, relative hu-
6.1.7 Psychrometric Chart—Graphical presentation of the
midity and humidity ratio when three (3) consecutive measure-
thermodynamic properties of moist air.
ments show no change in humidity ratio as calculated from the
meter readings.
7. Laboratory Calibration
9. Saturate the Test Specimens
7.1 This procedure is designed for full-scale calibration of
the meter. A minimum of 45 calibration specimens shall be
9.1 As the moisture level for physical damage is an ERH of
fabricated with a target of ten (10) calibration steps ranging
80 % (A of 0.8) the test specimens must be saturated above
w
from ERH of 30 % to 90 %.
this point as a starting point for calibration. The target
NOTE 1—30 % relative humidity represents a practical lower limit on
saturation level is 95 % relative humidity at 20 °C (68 °F).
moisture content found in buildings and the accuracy of readings above
90 % relative humidity is problematic. The calibration should not be
9.2 Place specimens in a water vapor saturated atmosphere
extrapolated below the lowest value tested or above the highest value
in an environmental chamber or humidity cabinet with relative
tested. Material of the type to be calibrated shall be prepared and tested in
humidity equal to or greater than 95 % at 20 °C (68 °F).
a manner that is consistent with the following calibration procedures.
9.2.1 Document the environmental chamber conditions us-
7.2 Specimens shall be free of visible irregularities.
ing a calibrated relative humidity sensor.
7.3 Select a minimum of 45 specimens, each measuring
9.3 Maintain specimens in the water saturated atmosphere
100 mm wide by 200 mm long (4 in. by 8 in.), for each given
until they reach moisture equilibrium with the atmosphere.
sample of gypsum board or panel.
9.3.1 Maintain specimens in chamber or cabinet until rela-
7.3.1 The specimens shall be divided into a minimum of
tive humidity stabilizes at a reading of 95 % or greater.
three (3) groups of 15 specimens each.
9.3.2 Remove each specimen and weigh at eight (8) hour
7.3.2 Each specimen shall be assigned a group designation
intervals.
and a specimen number (for example, A-1, A-2, A-3, B-1, B-2,
9.3.2.1 Determine the weight of water in the specimen by
B-3, and so forth) and labeled with a pencil or waterproof ink.
subtracting the dry weight of the sample as determined
according to Section 8 above.
8. Determine Dry Specimen Weights and Equilibrium
9.3.2.2 Calculate the specimen moisture content by dividing
Humidity Ratios
the weight of water in the sample by the dry weight of the
sample and multiplying by 100.
8.1 Determine the dry weight of each specimen.
8.1.1 Place the test specimens into forced air oven set at 9.3.3 The test specimens are deemed to be saturated when
45 °C (113 °F). Arrange the specimens so that heated air three (3) consecutive weighings show no change in moisture
circulates freely around all sides of the specimens. Use racks or content within 60.10 %.
C1789 − 24
9.3.4 The length of time required to saturate the specimens 10.1.1 The range of moisture content that is of interest may
can be reduced by soaking each specimen in a saturated vary with different types of gypsum board or panel products,
gypsum solution long enough to visibly saturate the facers of
by composition of a product by a specific manufacturer in a
the panel. Soak for no more than one (1) hour. specific geographical location.
10.1.2 The range of interest for standard drywall is typically
9.4 After the specimens are saturated determine the absolute
between 0.3 % and 8.5 % moisture content by weight. This
humidity of the trapped environment that is in moisture
typically corresponds to an ERH of about 30 % to 95 % at
equilibrium with each specimen. Humidity ratio (or absolute
20 °C (88 °F). Typically, hand-held moisture meters are able to
humidity) is used for this purpose as relative humidity varies
measure moisture in gypsum board or panels within this range.
with temperature, and temperature is likely to vary during the
Moisture levels above and below this range are outside of the
course of the test. Humidity ratio, absolute humidity and dew
measuring capabilities of most meters.
point temperature do not vary with air temperature at constant
moisture contents and can be calculated from the temperature
10.2 Drying may be accomplished by use of a convection
and relative humidity that are measured by direct read instru-
oven, or by air drying by allowing circulation of room air
ments.
around samples.
9.4.1 Place the specimen in the contained atmosphere inside
10.2.1 Convection Oven—Dry samples in accordance with
a zip sealed plastic bag. Insert a temperature/relative humidity
the procedure set forth in Section 8.
probe through the wall of the bag and seal tightly.
10.2.2 Air Drying—Dry in room with samples in a rack
NOTE 4—Making a slit in the side of the bag to stretch around a relative
permitting air circulation on all sides. Arrange so that there are
humidity probe and sealing around the probe, if necessary, has proven to
no strong air currents across samples that could promote
provide a reliable method.
uneven drying.
9.4.2 Record the relative humidity and temperature within
10.3 The time step and moisture content by weight will need
the bag at one-hour intervals and calculate absolute humidity,
to be adjusted for the characteristics of the specific type of
humidity ratio or dew point.
gypsum board or panel for which the meter is being calibrated.
9.4.3 Record the temperature and humidity when three
For example, the following values have been found typical for
consecutive measurements show no change in absolute humid-
⁄2 in. thick gypsum wallboard.
ity as calculated from the meter readings (this will be approxi-
mately within 62.0 % relative humidity and 61.0 °C (1.8 °F)). 10.3.1 Drying by oven typically requires:
10.3.1.1 30-min time step between 16 % and 2 % moisture
9.5 Measure moisture content with the meter to be cali-
content by weight.
brated.
10.3.1.2 15-min time step below 2 % moisture content by
9.5.1 Take three (3) measurements on each specimen. One
weight.
within 25 mm (1 in.) from each end, and one from the
approximate center of the specimen. Record each measurement
10.3.2 Air drying typically requires one (1) hour time steps
and report the average of the three (3) measurements.
at all moisture contents.
9.5.1.1 For pin type meters stab the pins through the plastic
bag taking care to avoid any marking printed on the surface of
11. Dry Specimens in Steps
the bag.
11.1 Dry specimens in steps of duration as determined
9.5.1.2 For pinless type meters remove the specimen from
above. At each step measure temperature, relative humidity,
the bag to take the measurements. Support the specimen on a
and moisture
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C1789 − 14 (Reapproved 2018) C1789 − 24
Standard Test Method for
Calibration of Hand-Held Moisture Meters on Gypsum
Panels
This standard is issued under the fixed designation C1789; 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 Scope*
1.1 This test method applies to the calibration of handheldhand-held moisture meters for gypsum board, glass faced gypsum panels
and fiber-reinforced gypsum panels by means of electrical conductance and dielectric meters. The test uses wetted test specimens
which are dried down in at least 5 five (5) steps to determine the moisture content based on the weight loss in comparison to the
dry weight. The test also supplies the ERH values for each of the drying steps.
1.2 This test method has not been evaluated for the influence of paint or wall covering materials on the indicated moisture content
of a gypsum board or panel substrate.
1.3 The values stated in SI (metric) are to be regarded as standard. The values given in parentheses are mathematical conversions
to inch-pound units that are provided for information only and are not considered standard.
1.4 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 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:
C473 Test Methods for Physical Testing of Gypsum Panel Products
C1177/C1177M Specification for Glass Mat Gypsum Substrate for Use as Sheathing
C1178/C1178M Specification for Coated Glass Mat Water-Resistant Gypsum Backing Panel
C1278/C1278M Specification for Fiber-Reinforced Gypsum Panel
C1396/C1396M Specification for Gypsum Board
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
This test method is under the jurisdiction of ASTM Committee C11 on Gypsum and Related Building Materials and Systems and is the direct responsibility of
Subcommittee C11.01 on Specifications and Test Methods for Gypsum Products.
Current edition approved Oct. 1, 2018April 1, 2024. Published October 2018April 2024. Originally approved in 2013. Last previous edition approved in 20142018 as
C1789 – 14. DOI: 10.1520/C1789-14R18.14 (2018). DOI: 10.1520/C1789-24.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1789 − 24
2.2 ASHRAE Standard:
2009 ASHRAE Handbook – Fundamentals, Chapter 1 – Psychrometrics, American Society of Heating, Refrigerating and
Air-conditioning Engineers
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 absolute humidity, d , n—the ratio of the mass of water vapor to the total volume of the moist air sample.
v
3.1.2 admittance, n—inverse of impedance, a measure of how easily an electric current can flow through a material.
3.1.3 conductance meters, n—conductance meters are those that measure predominantly ionic conductance between points of
applied voltage, usually dc.
3.1.3.1 Discussion—
Conductance meters generally have pins that penetrate into the material being measured. Direct-current conductance meters are
commonly referred to as "resistance"“resistance” meters. Most commercial conductance meters are high-input impedance (about
12 4 12
10 Ω), wide-range (10 to 10 Ω) ohmmeters. Their scales are generally calibrated to read directly in moisture content (oven-dry
mass basis) for a particular calibration material and at a specific reference temperature.
3.1.4 dew-point temperature, t , n—the temperature at which a sample of moist air being cooled at constant pressure and moisture
d
content reaches 100 % relative humidity.
3.1.4.1 Discussion—
The dew-point temperature is the temperature at which water condensation begins to occur on a cooled surface in contact with
moist air.
3.1.5 dielectric meters, n—meters that measure primarily by admittance or power loss.power-loss.
3.1.5.1 Discussion—
Dielectric meters generally do not have pins that penetrate into the material being measured. There are two (2) general types of
dielectric meters that may be arbitrarily categorized by their predominant mode of response – admittance (or capacitance) and
power loss. power-loss. Both have surface contact electrodes and readout scales that are usually marked in arbitrary units. Most
dielectric meters operate in the r-f frequency range, generally between 11 MHz and 10 MHz. Admittance meters respond primarily
to the capacitance (dielectric constant) of the material being measured. Power loss Power-loss meters react primarily to the
resistance of the material. Readings of dielectric meters are significantly affected by the relative density (specific gravity) of the
specimen material.
3.1.6 equilibrium moisture content, EMC, n—the moisture content of a material that is in thermodynamic equilibrium with the
surrounding air at a given temperature and relative humidity.
3.1.7 equilibrium relative humidity, ERH—the relative humidity of the air in a sealed chamber that is in thermodynamic
equilibrium with a sample of material in that chamber.
3.1.8 humidity ratio, W, n—the ratio of the mass of water vapor to the mass of dry air contained in a sample of moist air.
3.1.9 moisture content, MC, n—the ratio of the mass of water in a material to the oven-dry mass of the sample expressed as a
decimal fraction or percentage.
3.1.9.1 Discussion—
Oven-dry refers to the removal by heating of all adsomcrbedadsorbed and free water in the interstitial pores of the material.
Crystalline water such as contained in gypsum molecules is not included.
3.1.10 relative humidity, ϕ, n—the ratio of the amount of water vapor in air to the amount of water vapor in saturated air at the
same temperature and pressure.
Available from American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle, NE, Atlanta, GA 30329,
http://www.ashrae.org.
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3.1.10.1 Discussion—
Equivalent to the ratio of the partial pressure of water vapor in the air to the saturated vapor pressure at the same temperature and
pressure.
3.1.11 test uncertainty ratio, TUR, n—comparison between the accuracy of the Unit Under Test (UUT) and the estimated
calibration uncertainty stated with a confidence level of 95 % (K=2).
3.1.12 water activity, A , n—the ratio of the water vapor pressure in a material to the vapor pressure of pure water at the same
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temperature.
3.1.12.1 Discussion—
Water activity is an intrinsic property derived from fundamental principles of thermodynamics and physical chemistry. It is a
measure of the energy status of the water in a system. Commonly used for food preservation analyses, it can be interpreted here
as the amount of water in a porous material that is available to impact the performance characteristics of the material or to support
mold growth.
4. Summary of Test Method
4.1 These test methods provide a method for calibrating the scale on conductance and dielectric meters for various types of
gypsum boards and panels for use in field measurement of moisture content during storage, construction and use in building
assemblies.
4.2 The calibration is based on the MC of the test specimen. The corresponding ERH is determined by use of a calibrated direct
read relative humidity meter.
4.3 ERH is essentially equivalent to water activity Awactivity, A , which is a measure of the amount of moisture in a material that
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is available to impact the performance characteristics of that material.
4.4 Due to the various core and/or facing additives that are used to modify the moisture absorption characteristics, strength and/or
other properties for specific applications, a separate calibration is required for each type of gypsum board or panel product to be
measured.
4.5 The test method has the following steps:
4.5.1 Measure the dry weights of the test specimens.
4.5.2 Determine the time step for the drying intervals that will provide sufficient data points to develop a calibration curve.
4.5.3 Saturate the samples with water.
4.5.4 Dry the samples in steps, recording after each interval the moisture content by weight of each sample and the temperature,
relative humidity (ERH), and absolute humidity of the atmosphere in moisture equilibrium with each sample.
5. Significance and Use
5.1 This Standard Test Method is intended for use in calibrating hand-held meters to accurately read from approximately 3030 %
to 90 % ERH. Moisture content is related to the ERH or waterA activity of a material.
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5.2 Hand-held meters provide a rapid means of sampling the moisture content of gypsum boards and panels during manufacture
and for field inspection during and after building construction. However, these measurements are inferential, that is, electrical
parameters are measured and compared against a calibration curve to obtain an indirect measure of moisture content. The electrical
measurements are influenced by the actual moisture content, a number of other gypsum board and panel variables, environmental
conditions, the geometry of the measuring probe, and the 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.
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5.3 Electrical conductance and dielectric meters are not necessarily equivalent in their readings under the same conditions. When
this test method is referenced, the type of meter that is being used must be reported with the relevant ranges for precision and bias
as specified in this standard.
5.4 Both types of meters are to be calibrated with respect to ERH as described in this standard.
6. Apparatus
6.1 Laboratory equipment for moisture content determination by direct method:
6.1.1 Forced Air Oven—Vented electric furnace capable of maintaining a steady-state temperature of 45 6 3°C (113 6 5°F).45 °C
6 3 °C (113 °F 6 5 °F).
6.1.2 Environmental Chamber—Chamber capable of maintaining a controlled temperature of 20 6 2ºC (68 6 4ºF)20 °C 6 2 ºC
(68 °F 6 4 ºF) and relative humidity within the range 30 to 90 6 5 %.30 % to 90 % 6 5 %.
6.1.3 Electronic Balance or Scale—Electronic scale capable of weighing each test specimen to within 60.10 g (60.0035 oz).
6.1.4 Relative Humidity Test Meter—The meter shall be capable of reading between 3030 % and 90 % relative humidity. The
calibrated test uncertainty ratio (TUR) of this meter shall not be less than 4:1 over the range of measure cited.
6.1.4.1 Meter shall have removable temperature/humidity probes that can be sealedinserted into sealed plastic bags.
6.1.5 Humidity Box—Insulated box made of materials impervious to water vapor such as plastic or sheet metal, sealed with a
gasketed lid. Open trays of clean, distilled water are positioned in the box so as to keep the atmosphere within the box saturated
with water vapor. Open mesh shelving or racks are used to support samples above the water.
6.1.6 Zip Sealed Plastic Bags—Commercially available plastic freezer weight plastic food storage bag with a zipper type closure
that seals and prevents water vapor transmission.
6.1.7 Psychrometric Chart—Graphical presentation of the thermodynamic properties of moist air.
7. Laboratory Calibration
7.1 This procedure is designed for full-scale calibration of the meter. A minimum of 45 calibration specimens shall be fabricated
with a target of ten (10) calibration steps ranging from ERH of 3030 % to 90 %.
NOTE 1—30 % relative humidity represents a practical lower limit on moisture content found in buildings and the accuracy of readings above 90 % relative
humidity is problematic. The calibration should not be extrapolated below the lowest value tested or above the highest value tested. Material of the type
to be calibrated shall be prepared and tested in a manner that is consistent with the following calibration procedures.
7.2 Specimens shall be free of visible irregularities.
7.3 Select a minimum of 45 specimens, each measuring 100 mm wide by 200 mm long (4(4 in. by 8 in.), for each given sample
of gypsum board or panel.
7.3.1 The specimens shall be divided into a minimum of three (3) groups of 15 specimens each.
7.3.2 Each specimen shall be assigned a group designation and a specimen number (for example, A-1, A-2, A-3, B-1, B-2, B-3,
and so forth) and labeled with a pencil or waterproof ink.
8. Determine Dry Specimen Weights and Equilibrium Humidity Ratios
8.1 Determine the dry weight of each specimen.
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8.1.1 Place the test specimens into forced air oven set at 45°C (113°F).45 °C (113 °F). Arrange the specimens so that heated air
circulates freely around all sides of the specimens. Use racks or holders to keep the specimens separated sufficiently to allow air
flow between the specimens.
8.1.2 Remove and weigh each test specimen at one hour one-hour intervals.
8.1.3 The test specimen is deemed to be dry when three (3) consecutive weighings show no change in weight within 60.10 %
of the dried sample weight.
8.1.4 Record the dry weight of each specimen.
8.2 Determine the humidity ratio of the trapped environment that is in moisture equilibrium with each specimen.
NOTE 2—Humidity ratio is used as the temperature is likely to vary during the course of the test and relative humidity will vary with temperature at
constant moisture content. Humidity ratio and dew-point temperature do not vary with temperature at constant moisture content and can be calculated
from the temperature and relative humidity values measured by direct read instruments.
8.2.1 Place the specimen in a zip sealed plastic bag to contain it in a trapped atmosphere.
8.2.2 Insert a temperature/relative humidity probe through the wall of the bag and seal tightly.
NOTE 3—Making a small slit in the side of the bag to stretch around the relative humidity probe and sealing around the probe, if necessary, has proven
to provide a reliable seal.method.
8.2.3 Record the temperature and relative humidity within the bag at one hour one-hour intervals.
8.2.3.1 Calculate the humidity ratio using a psychrometric chart or a table of thermodynamic properties of moist air.
8.2.3.2 Record the dry specimen temperature, relative humidity and humidity ratio when three (3) consecutive measurements show
no change in humidity ratio as calculated from the meter readings.
9. Saturate the Test Specimens
9.1 As the moisture level for physical damage is an ERH of 80 % (A of 0.8) the test specimens must be saturated above this point
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as a starting point for calibration. The target saturation level is 95 % relative humidity at 20°C (68°F).20 °C (68 °F).
9.2 Place specimens in a water vapor saturated atmosphere in an environmental chamber or humidity cabinet with relative
humidity equal to or greater than 95 % at 20°C (68°F).20 °C (68 °F).
9.2.1 Document the environmental chamber conditions using a calibrated relative humidity sensor.
9.3 Maintain specimens in the water saturated atmosphere until they reach moisture equilibrium with the atmosphere.
9.3.1 Maintain specimens in chamber or cabinet until relative humidity stabilizes at a reading of 95 % or greater.
9.3.2 Remove each specimen and weigh at eight (8) hour intervals.
9.3.2.1 Determine the weight of water in the specimen by subtracting the dry weight of the sample as determined according to
Section 8 above.
9.3.2.2 Calculate the specimen moisture content by dividing the weight of water in the sample by the dry weight of the sample
and multiplying by 100.
9.3.3 The test specimens are deemed to be saturated when three (3) consecutive weighings show no change in moisture content
within 6 0.10 60.10 %.
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9.3.4 The length of time required to saturate the specimens can be reduced by soaking each specimen in a saturated gypsum
solution long enough to visibly saturate the paper faces facers of the panel. Soak for no more than one (1) hour.
9.4 After the specimens are saturated determine the absolute humidity of the trapped environment that is in moisture equilibrium
with each specimen. Humidity ratio (or absolute humidity) is used for this purpose as relative humidity varies with temperature,
and temperature is likely to vary during the course of the test. Humidity ratio, absolute humidity and dew point temperature do
not vary with air temperature at constant moisture contents and can be calculated from the temperature and relative humidity that
are measured by direct read instruments.
9.4.1 Place the specimen in the trappedcontained atmosphere inside a zip sealed plastic bag. Insert a temperature/relative humidity
probe through the wall of the bag and seal tightly.
NOTE 4—Making a slit in the side of the bag to stretch around a relative humidity probe and sealing around the probe, if necessary, has proven to provide
a reliable seal.method.
9.4.2 Record the relative humidity and temperature within the bag at one hour one-hour intervals and calculate absolute humidity,
humidity ratio or dew point.
9.4.3 Record the temperature and humidity when three consecutive measurements show no change in absolute humidity as
calculated from the meter readings (this will be approximately within 62.0 % relative humidity and 61.0°C).61.0 °C (1.8 °F)).
9.5 Measure moisture content with the meter to be calibrated.
9.5.1 Take three (3) measurements on each specimen. One within 2.5 cm25 mm (1 in.) from each end, and one from the
approximate center of the specimen. Record each measurement and report the average of the three (3) measurements.
9.5.1.1 For pin type meters stab the pins through the plastic bag taking care to avoid any marking printed on the surface of the
bag.
9.5.1.2 For pinless type meters remove the specimen from the bag to take the measurements. Support the specimen on a
low-density polystyrene foam block at least 25 mm (1 in.) thick on a wooden surface with no metal braces.
9.6 Remove the specimen from the bag and weigh. Report the weight and calculate and report the moisture content by weight.
9.7 Continue to dry the specimens stepwise in accordance with Section 11.
10. Determine Time Step for Drying
10.1 Determine the time step necessary to dry a typical specimen by increments sufficiently close together to permit development
of an accurate calibration curve for the instrument. In selecting time increments target ten (10) data points separated by equal steps
in moisture content as measured by the meter being calibrated. A minimum of five (5) data steps are necessary for sufficient
precision. Adjust time steps as necessary to provide a minimum of five (5) data points including the maximum and minimum
readings.
10.1.1 The range of moisture content that is of interest may vary with different types of gypsum board or panel products, by
composition of a product by a specific manufacturer in a specific geographical location.
10.1.2 The range of interest for standard drywall is typically between 0.30.3 % and 8.5 % moisture content by weight. This
typically corresponds to an ERH of about 3030 % to 95 % at 20°C (88°F). Typically, hand held 20 °C (88 °F). Typically, hand-held
moisture meters are able to measure moisture in gypsum board or panels within this range. Moisture levels above and below this
range are outside of the measuring capabilities of most meters.
10.2 Drying may be accomplished by use of a convection oven, or by air drying by allowing circulation of room
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