ASTM C177-19e1

Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus

SIGNIFICANCE AND USE
5.1 This test method covers the measurement of heat flux and associated test conditions for flat specimens. The guarded-hot-plate apparatus is generally used to measure steady-state heat flux through materials having a “low” thermal conductivity and commonly denoted as “thermal insulators.” Acceptable measurement accuracy requires a specimen geometry with a large ratio of area to thickness.  
5.2 Two specimens are selected with their thickness, areas, and densities as identical as possible, and one specimen is placed on each side of the guarded-hot-plate. The faces of the specimens opposite the guarded-hot-plate and primary guard are placed in contact with the surfaces of the cold surface assemblies.  
5.3 Steady-state heat transmission through thermal insulators is not easily measured, even at room temperature. This is due to the fact heat transmission through a specimen occurs by any or all of three separate modes of heat transfer (radiation, conduction, and convection). It is possible that any inhomogeneity or anisotropy in the specimen will require special experimental precautions to measure that flow of heat. In some cases it is possible that hours or even days will be required to achieve the thermal steady-state. No guarding system can be constructed to force the metered heat to pass only through the test area of insulation specimen being measured. It is possible that moisture content within the material will cause transient behavior. It is also possible that and physical or chemical change in the material with time or environmental condition will permanently alter the specimen.  
5.4 Application of this test method on different test insulations requires that the designer make choices in the design selection of materials of construction and measurement and control systems. Thus it is possible that there will be different designs for the guarded-hot-plate apparatus when used at ambient versus cryogenic or high temperatures. Test thickness, temperature range,...
SCOPE
1.1 This test method establishes the criteria for the laboratory measurement of the steady-state heat flux through flat, homogeneous specimen(s) when their surfaces are in contact with solid, parallel boundaries held at constant temperatures using the guarded-hot-plate apparatus.  
1.2 The test apparatus designed for this purpose is known as a guarded-hot-plate apparatus and is a primary (or absolute) method. This test method is comparable, but not identical, to ISO 8302.  
1.3 This test method sets forth the general design requirements necessary to construct and operate a satisfactory guarded-hot-plate apparatus. It covers a wide variety of apparatus constructions, test conditions, and operating conditions. Detailed designs conforming to this test method are not given but must be developed within the constraints of the general requirements. Examples of analysis tools, concepts and procedures used in the design, construction, calibration and operation of a guarded-hot-plate apparatus are given in Refs (1-41).2  
1.4 This test method encompasses both the single-sided and the double-sided modes of measurement. Both distributed and line source guarded heating plate designs are permitted. The user should consult the standard practices on the single-sided mode of operation, Practice C1044, and on the line source apparatus, Practice C1043, for further details on these heater designs.  
1.5 The guarded-hot-plate apparatus can be operated with either vertical or horizontal heat flow. The user is cautioned however, since the test results from the two orientations may be different if convective heat flow occurs within the specimens.  
1.6 Although no definitive upper limit can be given for the magnitude of specimen conductance that is measurable on a guarded-hot-plate, for practical reasons the specimen conductance should be less than 16 W/(m2K).  
1.7 This test method is applicable to the measurement of a wide variety of sp...

General Information

• Status: Published
• Publication Date: 31-Dec-2018
• ICS: 17.200.10 - Heat. Calorimetry
• Technical Committee: C16 - Thermal Insulation
• Drafting Committee: C16.30 - Thermal Measurement

Relations

• Refers: ASTM C168-24 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2024
• Refers: ASTM C1044-24 - Standard Practice for Using a Guarded-Hot-Plate Apparatus or Thin-Heater Apparatus in the Single-Sided Mode
• Effective Date: 01-Mar-2024
• Refers: ASTM C1043-24 - Standard Practice for Guarded-Hot-Plate Design Using Circular Line-Heat Sources
• Effective Date: 01-Mar-2024
• Refers: ASTM C1363-24 - Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus
• Effective Date: 01-Mar-2024
• Refers: ASTM C1363-19 - Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus
• Effective Date: 01-Sep-2019
• Refers: ASTM C1045-19 - Standard Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
• Effective Date: 01-Apr-2019
• Refers: ASTM C1043-19 - Standard Practice for Guarded-Hot-Plate Design Using Circular Line-Heat Sources
• Effective Date: 01-Mar-2019
• Refers: ASTM C687-18 - Standard Practice for Determination of Thermal Resistance of Loose-Fill Building Insulation
• Effective Date: 01-Sep-2018
• Refers: ASTM C168-18 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2018
• Refers: ASTM C168-17 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Jun-2017
• Refers: ASTM C687-17 - Standard Practice for Determination of Thermal Resistance of Loose-Fill Building Insulation
• Effective Date: 15-Mar-2017
• Refers: ASTM C1043-16 - Standard Practice for Guarded-Hot-Plate Design Using Circular Line-Heat Sources
• Effective Date: 01-Mar-2016
• Refers: ASTM C168-15a - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Oct-2015
• Refers: ASTM C518-15 - Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
• Effective Date: 01-Sep-2015
• Refers: ASTM C168-15 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Jun-2015

Overview

ASTM C177-19e1: Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus defines procedures for laboratory measurement of steady-state heat flux and thermal transmission properties of flat, homogeneous specimens using the guarded-hot-plate apparatus. This method is recognized for its precision in testing materials with low thermal conductivity, commonly termed thermal insulators. Adopted by ASTM International, the standard aligns with globally recognized principles for developing international standards and is referenced by industries and regulatory agencies worldwide.

Key goals of ASTM C177 are to ensure accurate and reproducible measurements of thermal conductivity, resistance, and related properties under controlled laboratory conditions, providing reliable data for product development, quality control, and regulatory compliance.

Key Topics

• Measurement Principle: The test method uses the guarded-hot-plate apparatus, an absolute or primary method, to establish a steady temperature gradient across a flat specimen sandwiched between a heated plate and isothermal cold assemblies.
• Specimen Selection: Optimal accuracy requires flat, homogeneous specimens with large area-to-thickness ratios, minimizing the effects of edge losses and non-uniform heat flow.
• Heat Transfer Modes: The standard addresses complications arising from conduction, convection, and radiation within test specimens, noting the need for special precautions with inhomogeneous or anisotropic materials.
• Steady-State Requirement: Accurate heat flux measurements rely on reaching and maintaining thermal steady-state, which may require significant time depending on specimen characteristics and environmental conditions.
• Apparatus Design: General requirements for construction, operation, and calibration of the guarded-hot-plate apparatus are outlined. Both single-sided and double-sided measurement modes are permitted, as well as vertical and horizontal configurations.
• Uncertainty and Reporting: Users must estimate and report uncertainties for all measured variables, describe test conditions comprehensively, and ensure periodic calibration against reference materials.
• Applicability: Although primarily intended for materials with thermal conductance less than 16 W/(m²·K), ASTM C177 is suitable for a wide variety of opaque, porous, or transparent specimens, and for use across a wide range of temperatures and environments.

Applications

ASTM C177 is widely used across industries that require precise thermal insulation measurements, including:

• Building & Construction: Assessing insulation materials for walls, roofs, and envelopes to meet energy efficiency standards and codes.
• Manufacturing & Product Development: Evaluating new or existing insulative materials and systems for appliances, HVAC equipment, and protective clothing.
• Aerospace & Automotive: Testing internal linings, composites, and insulation assemblies exposed to extreme or fluctuating temperatures.
• Quality Assurance & Certification: Supporting product labeling, certification, and compliance with national and international regulations.
• Research & Development: Generating reference data for new material innovations and validating computational models of heat transfer.

Related Standards

For comprehensive thermal transmission testing and complementary measurement techniques, consider these related ASTM and ISO standards:

• ASTM C518 - Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus: A comparative method often used for routine testing.
• ASTM C1043 - Practice for Guarded-Hot-Plate Design Using Circular Line-Heat Sources: Details on alternative heater configurations.
• ASTM C1044 - Practice for Using Guarded-Hot-Plate or Thin-Heater Apparatus in the Single-Sided Mode: Guidance for single-sided specimen testing.
• ISO 8302 - Thermal Insulation - Determination of Steady-State Areal Thermal Resistance and Related Properties - Guarded Hot Plate Apparatus: Closely comparable to ASTM C177.
• ASTM C168 - Terminology Relating to Thermal Insulation: Standard definitions of terms used in heat flow measurement.

By following ASTM C177-19e1, organizations benefit from recognized best practices in the determination of steady-state heat flux and thermal transmission properties, ensuring reliable, actionable thermal data for product development, safety, energy management, and compliance across global markets.

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.
´1
Designation: C177 − 19
Standard Test Method for
Steady-State Heat Flux Measurements and Thermal
Transmission Properties by Means of the Guarded-Hot-Plate
Apparatus
This standard is issued under the fixed designation C177; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
ε NOTE—Updated 2.3 editorially in February 2023.
1. Scope however, since the test results from the two orientations may be
different if convective heat flow occurs within the specimens.
1.1 This test method establishes the criteria for the labora-
tory measurement of the steady-state heat flux through flat,
1.6 Although no definitive upper limit can be given for the
homogeneous specimen(s) when their surfaces are in contact
magnitude of specimen conductance that is measurable on a
with solid, parallel boundaries held at constant temperatures guarded-hot-plate, for practical reasons the specimen conduc-
using the guarded-hot-plate apparatus.
tance should be less than 16 W/(m K).
1.2 The test apparatus designed for this purpose is known as
1.7 This test method is applicable to the measurement of a
a guarded-hot-plate apparatus and is a primary (or absolute)
wide variety of specimens, ranging from opaque solids to
method. This test method is comparable, but not identical, to
porous or transparent materials, and a wide range of environ-
ISO 8302.
mental conditions including measurements conducted at ex-
tremes of temperature and with various gases and pressures.
1.3 This test method sets forth the general design require-
ments necessary to construct and operate a satisfactory
1.8 Inhomogeneities normal to the heat flux direction, such
guarded-hot-plate apparatus. It covers a wide variety of appa-
as layered structures, can be successfully evaluated using this
ratus constructions, test conditions, and operating conditions.
test method. However, testing specimens with inhomogeneities
Detailed designs conforming to this test method are not given
in the heat flux direction, such as an insulation system with
but must be developed within the constraints of the general
thermal bridges, can yield results that are location specific and
requirements. Examples of analysis tools, concepts and proce-
shall not be attempted with this type of apparatus. See Test
dures used in the design, construction, calibration and opera-
Method C1363 for guidance in testing these systems.
tion of a guarded-hot-plate apparatus are given in Refs (1-41).
1.9 Calculations of thermal transmission properties based
1.4 This test method encompasses both the single-sided and
upon measurements using this method shall be performed in
the double-sided modes of measurement. Both distributed and
conformance with Practice C1045.
line source guarded heating plate designs are permitted. The
1.10 In order to ensure the level of precision and accuracy
user should consult the standard practices on the single-sided
expected, persons applying this standard must possess a
mode of operation, Practice C1044, and on the line source
knowledge of the requirements of thermal measurements and
apparatus, Practice C1043, for further details on these heater
testing practice and of the practical application of heat transfer
designs.
theory relating to thermal insulation materials and systems.
1.5 The guarded-hot-plate apparatus can be operated with
Detailed operating procedures, including design schematics
either vertical or horizontal heat flow. The user is cautioned
and electrical drawings, should be available for each apparatus
to ensure that tests are in accordance with this test method. In
addition, automated data collecting and handling systems
This test method is under the jurisdiction of ASTM Committee C16 on Thermal
connected to the apparatus must be verified as to their
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
accuracy. This can be done by calibration and inputting data
Measurement.
sets, which have known results associated with them, into
Current edition approved Jan. 1, 2019. Published January 2019. Originally
computer programs.
approved in 1942. Last previous edition approved in 2013 as C177 – 13. DOI:
10.1520/C0177-19E01.
1.11 It is not practical for a test method of this type to
The boldface numbers given in parentheses refer to the list of references at the
end of this standard. establish details of design and construction and the procedures
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
C177 − 19
to cover all contingencies that might offer difficulties to a mine the applicability of regulatory limitations prior to use.
person without technical knowledge concerning theory of heat Specific precautionary statements are given in Note 22.
1.17 This international standard was developed in accor-
flow, temperature measurements and general testing practices.
The user may also find it necessary, when repairing or dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
modifying the apparatus, to become a designer or builder, or
Development of International Standards, Guides and Recom-
both, on whom the demands for fundamental understanding
mendations issued by the World Trade Organization Technical
and careful experimental technique are even greater. Standard-
Barriers to Trade (TBT) Committee.
ization of this test method is not intended to restrict in any way
the future development of new or improved apparatus or
2. Referenced Documents
procedures.
2.1 ASTM Standards:
1.12 This test method does not specify all details necessary
C168 Terminology Relating to Thermal Insulation
for the operation of the apparatus. Decisions on sampling,
C518 Test Method for Steady-State Thermal Transmission
specimen selection, preconditioning, specimen mounting and
Properties by Means of the Heat Flow Meter Apparatus
positioning, the choice of test conditions, and the evaluation of
C687 Practice for Determination of Thermal Resistance of
test data shall follow applicable ASTM Test Methods, Guides,
Loose-Fill Building Insulation
Practices or Product Specifications or governmental regula-
C1043 Practice for Guarded-Hot-Plate Design Using Circu-
tions. If no applicable standard exists, sound engineering
lar Line-Heat Sources
judgment that reflects accepted heat transfer principles must be
C1044 Practice for Using a Guarded-Hot-Plate Apparatus or
used and documented.
Thin-Heater Apparatus in the Single-Sided Mode
C1045 Practice for Calculating Thermal Transmission Prop-
1.13 This test method allows a wide range of apparatus
erties Under Steady-State Conditions
design and design accuracy to be used in order to satisfy the
C1058 Practice for Selecting Temperatures for Evaluating
requirements of specific measurement problems. Compliance
and Reporting Thermal Properties of Thermal Insulation
with this test method requires a statement of the uncertainty of
C1363 Test Method for Thermal Performance of Building
each reported variable in the report. A discussion of the
Materials and Envelope Assemblies by Means of a Hot
significant error factors involved is included.
Box Apparatus
1.14 Major sections within this test method are arranged as
E230 Specification for Temperature-Electromotive Force
follows:
(emf) Tables for Standardized Thermocouples
Section Section
E691 Practice for Conducting an Interlaboratory Study to
Scope 1
Determine the Precision of a Test Method
Referenced Documents 2
Terminology 3
2.2 ISO Standard:
Summary of Test Method 4
ISO 8302 Thermal Insulation—Determination of Steady-
Significance and Use 5
State Areal Thermal Resistance and Related Properties—
Apparatus 6
Specimen Preparation and Conditioning 7
Guarded-Hot-Plate Apparatus
Procedure 8
2.3 ASTM Adjuncts:ASTM
Calculation of Results 9
Report 10
Table of Theoretical Maximum Thickness of Specimens and
Precision and Bias 11
Associated Errors
Keywords 12
Descriptions of Three Guarded-Hot-Plate Designs
Figures
General Arrangement of the Mechanical Components of the Guarded- Fig. 1
Line-Heat-Source Guarded Hot-Plate Apparatus
Hot-Plate Apparatus
Illustration of Heat Flow in the Guarded-Hot-Plate Apparatus Fig.2
3. Terminology
Example Report Form Fig. 3
Annexes
3.1 For definitions of terms and symbols used in this test
Importance of Thickness A1.1
Measuring Thickness A1.2 method, refer to Terminology C168 and the following subsec-
Limitations Due to Apparatus A1.3
tions.
Limitations Due to Temperature A1.4
Limitations Due to Specimen A1.5
3.2 Definitions of Terms Specific to This Standard:
Random and Systematic Error Components A1.6
3.2.1 auxiliary cold surface assembly, n—the plate that
Error Components for Variables A1.7
provides an isothermal boundary at the outside surface of the
Thermal Conductance or Thermal Resistance Error Analysis A1.8
Thermal Conductivity or Thermal Resistivity Error Analysis A1.9
auxiliary insulation.
Uncertainty Verification A1.10
1.15 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
standard.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
1.16 This standard does not purport to address all of the
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
safety concerns, if any, associated with its use. It is the
4th Floor, New York, NY 10036, http://www.ansi.org.
responsibility of the user of this standard to establish appro-
Available from ASTM Headquarters, Order Adjunct: ADJC0177-E-PDF.
priate safety, health, and environmental practices and deter- Available from ASTM Headquarters, Order Adjunct: ADJC1043.
´1
C177 − 19
3.2.2 auxiliary insulation, n—insulation placed on the back
side of the hot-surface assembly, in place of a second test
specimen, when the single sided mode of operation is used.
(Synonym—backflow specimen.)
3.2.3 cold surface assembly, n—the plates that provide an
isothermal boundary at the cold surfaces of the test specimen.
3.2.4 controlled environment, n—the environment in which
an apparatus operates.
3.2.5 guard, n—promotes one-dimensional heat flow. Pri-
mary guards are planar, additional coplanar guards can be used
and secondary or edge guards are axial.
3.2.6 guarded-hot-plate apparatus, n—an assembly, con-
sisting of a hot surface assembly and two isothermal cold
surface assemblies.
3.2.7 guarded-hot-plate, n—the inner (rectangular or circu-
lar) plate of the hot surface assembly, that provides the heat
input to the metered section of the specimen(s).
3.2.8 hot surface/assembly, n—the complete center assem-
bly providing heat to the specimen(s) and guarding for the
meter section.
3.2.9 metered section, n—the portion of the test specimen
(or auxiliary insulation) through which the heat input to the FIG. 1 General Arrangement of the Mechanical Components of
the Guarded-Hot-Plate Apparatus
guarded-hot-plate flows under ideal guarding conditions.
3.2.10 mode, double-sided, n—operation of the guarded-
hot-plate apparatus for testing two specimens, each specimen
placed on either side of the hot surface assembly.
3.2.11 mode, single-sided, n—operation of the guarded-hot-
plate apparatus for testing one specimen, placed on one side of
the hot-surface assembly.
3.2.12 thermal transmission properties, n—those properties
of a material or system that define the ability of a material or
system to transfer heat such as thermal resistance, thermal
conductance, thermal conductivity and thermal resistivity, as
defined by Terminology C168.
3.3 Symbols—The symbols used in this test method have
the following significance:
3.3.1 ρ —specimen metered section density, kg/m .
m
3.3.2 ρ —specimen density, kg/m .
s
3.3.3 λ—specimen thermal conductivity, W/(m K).
FIG. 2 Illustration of Idealized Heat Flow in a Guarded-Hot-Plate
3.3.4 λ —thermal conductivity of the material in the
guard
Apparatus
primary guard region, W/(m K).
2 4
3.3.5 σ—Stefan-Boltzmann constant, W/m K .
3.3.13 L—in-situ specimen thickness, m.
3.3.6 A—metered section area normal to heat flow, m .
3.3.14 m—mass of the specimen in the metered section, kg.
3.3.7 A —area of the gap between the metered section and
g
3.3.15 m —the mass of the ith component, kg.
the primary guard, m . i
3.3.16 m —mass of the specimen, kg.
s
3.3.8 A —area of the physical metered section (identified as
m
guarded hot plate in Fig. 1 and Fig. 2), m . 3.3.17 Q—heat flow rate in the metered section, W.
3.3.9 A —area of the entire specimen, m . 3.3.18 q—heat flux (heat flow rate per unit area), Q, through
s
area, A, W/m .
3.3.10 C—thermal conductance, W/(m K).
3.3.19 Q —lateral edge heat flow rate between primary
ge
3.3.11 C —the specific heat of the ith component of the
i
Guard and Controlled Environment, W.
metered section, J/(kg K).
3.3.20 Q —lateral heat flow rate across the gap, W.
gp
3.3.12 dT/dt—potential or actual drift rate of the metered
section, K/s. 3.3.21 Q —guard heat flow through Specimen, W.
grd
´1
C177 − 19
3.3.22 Q —edge heat flow between Specimen and Con- metered section is not exactly equal to that which flows
se
trolled Environment, W. through the specimen in the metered section. The resulting
qualitative heat flows are depicted in Fig. 2.
3.3.23 R—thermal resistance, m K/W.
4.2 The three heating/cooling assemblies are designed to
3.3.24 ΔT—temperature difference across the specimen,
create isothermal surfaces on the faces of the specimens within
T − T .
h c
the metered section. The two surfaces designated as the cold
3.3.25 T —cold surface temperature, K.
c
surface assemblies are adjusted to the same temperature for the
3.3.26 T —hot surface temperature, K.
h
double-sided mode of operation. In practice, because the plates
3.3.27 T —mean temperature, K, (T + T )/2. and specimens are of finite dimensions, and because the
m h c
3.3.27.1 Discussion— The Guarded-Hot-Plate Apparatus
external controlled environment is often at a temperature
provides a means for measurement of steady state heat flux different from the edge of the metered section, some lateral
through insulation materials, that consists of a guarded heater
heat flow occurs. The primary guard for the guarded hot plate
unit, comprised of a center metering area and concentric limits the magnitude of the lateral heat flow in the metered
separately heated guards, and an opposite, similarly sized
section. The effectiveness of the primary guard is determined,
cooling plate. Specimens are placed in the space between the in part, by the ratio of its lateral dimension to that of the
heater plate and the cooling plate for testing. The guarded-hot- metered section and to the specimen thickness (6,7,8,20,31).
plate is operated as a single or double sided apparatus.
4.3 Compliance with this test method requires: the estab-
Insulation thermal properties are calculated from measure-
lishment of steady-state conditions, and the measurement of
ments of metering area, energy input, temperatures, and
the unidirectional heat flow Q in the metered section, the
thickness. The guarded-hot-plate, which provides an absolute
metered section area A, the temperature gradient across the
measurement of heat flux, has been shown to be applicable for
specimen, in terms of the temperature T of the hot surface and
h
most insulating materials over a wide range of temperature
the temperature T of the cold surface, (or equivalently, the
c
conditions.
temperature T between the two surfaces), the thickness’ L and
L of each specimen, and guard balance between the metered
4. Summary of Test Method
section and primary guard.
4.1 Fig. 1 illustrates the main components of the idealized
5. Significance and Use
system: two isothermal cold surface assemblies and a guarded-
hot-plate. It is possible that some apparatuses will have more 5.1 This test method covers the measurement of heat flux
than one guard. The guarded-hot-plate is composed of a
and associated test conditions for flat specimens. The guarded-
metered section thermally isolated from a concentric primary hot-plate apparatus is generally used to measure steady-state
guard by a definite separation or gap. Some apparatus may
heat flux through materials having a “low” thermal conductiv-
have more than one guard. The test specimen is sandwiched ity and commonly denoted as “thermal insulators.” Acceptable
between these three units as shown in Fig. 1. In the double-
measurement accuracy requires a specimen geometry with a
sided mode of measurement, the specimen is actually com- large ratio of area to thickness.
posed of two pieces. The measurement in this case produces a
5.2 Two specimens are selected with their thickness, areas,
result that is the average of the two pieces and therefore it is
and densities as identical as possible, and one specimen is
important that the two pieces be closely identical. For guidance
placed on each side of the guarded-hot-plate. The faces of the
in the use of the one-sided mode of measurement, the user is
specimens opposite the guarded-hot-plate and primary guard
directed to Practice C1044. For guidance in the use of a
are placed in contact with the surfaces of the cold surface
guarded-hot-plate incorporating the use of a line source heater,
assemblies.
refer to Practice C1043.
5.3 Steady-state heat transmission through thermal insula-
4.1.1 The guarded-hot-plate provides the power (heat flow
tors is not easily measured, even at room temperature. This is
per unit time) for the measurement and defines the actual test
due to the fact heat transmission through a specimen occurs by
volume, that is, that portion of the specimen that is actually
any or all of three separate modes of heat transfer (radiation,
being measured. The function of the primary guard, and
conduction, and convection). It is possible that any inhomoge-
additional coplanar guard where applicable, of the guarded-
neity or anisotropy in the specimen will require special
hot-plate apparatus is to provide the proper thermal conditions
experimental precautions to measure that flow of heat. In some
within the test volume to reduce lateral heat flow within the
cases it is possible that hours or even days will be required to
apparatus. The proper (idealized) conditions are illustrated in
achieve the thermal steady-state. No guarding system can be
Fig. 1 by the configuration of the isothermal surfaces and lines
constructed to force the metered heat to pass only through the
of constant heat flux within the specimen.
test area of insulation specimen being measured. It is possible
4.1.2 Deviations from the idealized configuration are caused
that moisture content within the material will cause transient
by: specimen inhomogeneities, temperature differences be-
behavior. It is also possible that and physical or chemical
tween the metered section and the guard (gap imbalance), and
change in the material with time or environmental condition
temperature differences between the outer edge of the assembly
will permanently alter the specimen.
and the surrounding controlled environment (edge imbalance).
These experimental realities lead to heat flow measurements 5.4 Application of this test method on different test insula-
that are too small or too large because the power supplied to the tions requires that the designer make choices in the design
´1
C177 − 19
selection of materials of construction and measurement and Method C518 will be comparable with that of this test method,
control systems. Thus it is possible that there will be different Test Method C518 cannot be more accurate. In cases of
designs for the guarded-hot-plate apparatus when used at dispute, this test method is the recommended procedure.
ambient versus cryogenic or high temperatures. Test thickness,
temperature range, temperature difference range, ambient con-
6. Apparatus
ditions and other system parameters must also be selected
6.1 A general arrangement of the mechanical components of
during the design phase. Annex A1 is referenced to the user,
such a guarded-hot-plate apparatus is illustrated in Fig. 1. This
which addresses such issues as limitations of the apparatus,
consists of a hot surface assembly comprised of a metered
thickness measurement considerations and measurement
section and a primary guard, two cold surface assemblies, and
uncertainties, all of which must be considered in the design and
secondary guarding in the form of edge insulation, a
operation of the apparatus.
temperature-controlled secondary guard(s), and often an envi-
5.5 Apparatus constructed and operated in accordance with ronmental chamber. Some of the components illustrated in Fig.
this test method should be capable of accurate measurements
1 are omitted in systems designed for ambient conditions,
for its design range of application. Since this test method is although a controlled laboratory environment is still required;
applicable to a wide range of specimen characteristics, test
edge insulation and the secondary guard are typically used only
conditions, and apparatus design, it is impractical to give an at temperatures that are more than 6 10°C from ambient. At
all-inclusive statement of precision and bias for the test
ambient conditions, the environmental chamber is recom-
method. Analysis of the specific apparatus used is required to mended to help eliminate the effects of air movement within
specify a precision and bias for the reported results. For this
the laboratory and to help ensure that a dry environment is
reason, conformance with the test method requires that the user maintained.
must estimate and report the uncertainty of the results under the
6.1.1 The purpose of the hot surface assembly is to produce
reported test conditions.
a steady-state, one-dimensional heat flux through the speci-
mens. The purpose of the edge insulation, secondary guard,
5.6 Qualification of a new apparatus. When a new or
and environmental chamber is to restrict heat losses from the
modified design is developed, tests shall be conducted on at
outer edge of the primary guard. The cold surface assemblies
least two materials of known thermal stability and having
are isothermal heat sinks for removing the energy generated by
verified or calibrated properties traceable to a national stan-
the heating units; the cold surface assemblies are adjusted so
dards laboratory. Tests shall be conducted for at least two sets
they are at the same temperature.
of temperature conditions that cover the operating range for the
apparatus. If the differences between the test results and the
6.2 Design Criteria—Establish specifications for the follow-
national standards laboratory characterization are determined
ing specifications prior to the design. Various parameters
to be significant, then the source of the error shall, if possible,
influence the design of the apparatus and shall be considered
be identified. Only after successful comparison with the
throughout the design process, maximum specimen thickness;
certified samples, can the apparatus claim conformance with
range of specimen thermal conductances; range of hot surface
this test method. It is recommended that checks be continued
and cold surface temperatures; characteristics of the specimens
on a periodic basis to confirm continued conformance of the
(that is, rigidity, density, hardness); orientation of the apparatus
apparatus.
(vertical or horizontal heat flow); and required accuracy.
5.7 The thermal transmission properties of a specimen of
6.3 Hot Surface Assembly—The hot surface assembly con-
material have the potential to be affected due to the following
sists of a central metered section and a primary guard. The
factors: (a) composition of the material (b) moisture or other
metered section consists of a metered section heater sand-
environmental conditions (c) time or temperature exposure (d)
wiched between metered section surface plates. The primary
thickness (e) temperature difference across the specimen (f)
guard is comprised of one or more guard heaters sandwiched
mean temperature. It must be recognized, therefore, that the
between primary guard surface plates. The metered section and
selection of a representative value of thermal transmission
primary guard shall be thermally isolated from each other by
properties for a material must be based upon a consideration of
means of a physical space or gap located between the sections.
these factors and an adequate amount of test information. The hot surface assembly using a line-heat-source is covered in
Practice C1043.
5.8 Since both heat flux and its uncertainty may be depen-
dent upon environmental and apparatus test conditions, as well
NOTE 1—The primary guard, in some cases, is further divided into two
concentric sections (double guard) with a gap separator to improve the
as intrinsic characteristics of the specimen, the report for this
guard effectiveness.
test method shall include a thorough description of the speci-
men and of the test conditions.
6.3.1 Requirements—The hot surface assembly shall be
designed and constructed to satisfy the following minimum
5.9 The results of comparative test methods such as Test
requirements during operation.
Method C518 depend on the quality of the heat flux reference
6.3.1.1 The maximum departure from a plane for any
standards. The apparatus in this test method is one of the
surface plate shall not exceed 0.025 % of the linear dimension
absolute methods used for generation of the reference stan-
of the metered section during operation.
dards. The accuracy of any comparative method can be no
better than that of the referenced procedure. While it is possible
NOTE 2—Planeness of the surface can be checked with a metal
that the precision of a comparative method such as Test straightedge held against the surface and viewed at grazing incidence with
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a light source behind the straightedge. Departures as small as 2.5 μm are
The design of the heating element shall also consider the heat
readily visible, and large departures can be measured using shim-stock,
flux distribution of the surface of the heating element. Most
thickness gages or thin paper.
apparatus incorporate the use of a distributed electric resistance
6.3.1.2 The average temperature difference between the
heating element dispersed uniformly across the metered section
metered section surface plate and the primary guard surface
and the primary guard. The surface plates and heating elements
plate shall not exceed 0.2 K. In addition, the temperature
shall be clamped or bolted together in a uniform manner such
difference across any surface plate in the lateral direction shall
that the temperature difference requirements specified in
be less than 2 % of the temperature difference imposed across
6.3.1.2 are satisfied. Bolting the composite constructions to-
the specimen.
gether has been found satisfactory.
6.3.2.5 The insertion of insulating sheets between the heat-
NOTE 3—When qualifying the apparatus, additional temperature sen-
ing elements and surface plates (that is, to mount a gap
sors shall be applied to the surface plates of the metered section and
primary guards that verify that the requirements of 6.3.1.2 are satisfied.
temperature imbalance detector) is allowed. To satisfy the
requirements of 6.3.1.2, similar sheets shall be mounted
6.3.1.3 The surfaces of the metered and primary guard
between the heating element and the opposing surface plate.
surface plates that are in contact with the test specimen shall be
treated to maintain a total hemispherical emittance greater than 6.3.2.6 Hot Surface Assembly Size—Design criteria estab-
lished in 6.2 will determine the size of the apparatus. The size
0.8 over the entire range of operating conditions.
of the metered section shall be large enough so that the amount
NOTE 4—At high temperatures the importance of high emittance of the
of specimen material in contact with the metered section (and
surfaces adjacent to the specimens cannot be stressed too strongly since
therefore being measured) can be considered representative of
radiative heat transfer predominates in many materials as the temperature
increases. the material being tested.
6.3.2.7 After determining the maximum specimen thickness
6.3.1.4 The metered section and primary guard surface
that will be tested by this design, refer to Adjunct, Table of
plates shall remain planar during the operation of the appara-
Theoretical Maximum Thickness of Specimens and Associated
tus. See 6.3.1.1.
Errors, regarding associated errors attributable to combinations
6.3.2 Materials—The materials used in the construction of
of metered section size, primary guard width, and specimen
the hot surface assembly shall be carefully chosen after
thickness.
considering the following material property criteria.
6.3.2.1 Temperature Stability—Materials are selected for the
NOTE 5—Typically the width of the primary guard equal to approxi-
heaters and surface plates that are dimensionally and chemi-
mately one-half of the linear dimension of the metered section has been
found to reduce edge heat loss to acceptable levels.
cally stable and suitably strong to withstand warpage and
distortion when a clamping force is applied. For modest
6.3.2.8 Heat Capacitance—The heat capacity of the hot
temperatures, electric resistance heaters embedded in silicone
surface assembly will impact the time required to achieve
have been successfully employed; at higher temperatures,
thermal equilibrium. Selecting materials with a low specific
heating elements sandwiched between mica sheets or inserted
heat will increase the responsiveness of the apparatus. The
into a ceramic core have been used. Surface plates for hot
thickness of the surface plates needs to be carefully considered;
surface assemblies used at modest temperatures have been
thick plates assist in reducing lateral temperature distributions
fabricated from copper and aluminum. High purity nickel
but reduce responsiveness. A balance between these require-
alloys have been used for higher temperature applications.
ments is needed.
6.3.2.2 Thermal Conductivity—To reduce the lateral tem-
6.4 The Gap—The metered section and the primary guard
perature differences across the metered and primary guard
shall be physically separated by a gap. The gap provides a
surface plates, fabricate these plates from materials that pos-
lateral thermal resistance between these sections of the hot
sess a high thermal conductivity for the temperature and
surface assembly. The area of the gap in the plane of the
environmental conditions of operation. Copper and aluminum
surface plates shall not be more than 5 % of the metered
are excellent choices for modest temperature applications; at
section area.
higher temperatures consider using nickel, high purity alumina
6.4.1 The heater windings from the metered section and
or aluminum nitride. These are examples of materials used and
primary guard heating elements shall be designed to create a
the operator must fully understand the thermal conductivity
uniform temperature along the gap perimeter.
versus temperature dependency of the materials selected.
6.4.2 The metered section area shall be determined by
6.3.2.3 Emittance—To obtain a uniform and durable high
measurements to the center of the gap that surrounds this area,
surface emittance in the desired range, select a surface plate
unless detailed calculations or tests are used to define this area
material or suitable surface treatment, or both. For modest
more precisely.
temperature applications, high emittance paints may be em-
ployed. Aluminum can be anodized to provide the necessary 6.4.3 Any connections between the metered section and the
high emittance. For high temperature applications, most ce-
primary guard shall be designed to minimize heat flow across
ramics will inherently satisfy this requirement while nickel the gap. If a mechanical means is used to satisfy the require-
surface plates can be treated with an oxide coating.
ments of 6.3.1.4, these connections shall be fabricated with
6.3.2.4 Temperature Uniformity—Select a heating element materials having a high thermal resistance. Instrumentation or
design that will supply the necessary heat flux density for the heater leads that cross the gap should be fabricated with
range of specimen thermal conductances to be investigated. fine-gage wire and traverse the gap at an oblique angle.
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6.4.4 The gap may be filled with a fibrous insulation. 6.6.1.1 Size—The secondary guard should have an inner
Packing the gap with this insulation has been found to maintain dimension that is at least twice the dimension of the hot surface
the metered section and primary guard surface plates planar. heater and the height should be equal to the thickness of the hot
An additional benefit of this practice for high temperature surface heater plus twice the thickness of the thickest specimen
applications is that the densely packed insulation reduces the that will be tested.
amount of heat conducted across the gap spacing.
6.6.1.2 Materials—The materials used in the construction of
the secondary guard are not as critical as those selected for the
6.5 Cold Surface Assembly—The cold surface assembly
hot and cold surface assemblies. However, the materials used
consists of a single temperature controlled section and is
in the design of the secondary guard shall be selected so that
comprised of a cold surface heater sandwiched between cold
they are thermally stable over the intended temperature range,
surface plates and a heat sink. It is recommended that the size
the heating element shall be capable of producing the necessary
of the cold surface assembly be identical to the hot surface
heat flux density to adjust the ambient temperature, and a
assembly, including the primary guard. It is acceptable to
means of cooling the secondary guard is required if the
construct cold surface assemblies with a gap where operation
apparatus is intended for use at temperatures below the
of the apparatus is susceptible to edge loss effects. This design
laboratory ambient. The use of high thermal conductivity
is the ideal design, however, this assembly has traditionally
metals is recommended for the construction since the second-
been constructed without a gap with great success.
ary guard should be isothermal.
NOTE 6—The temperature of the cold surface assembly may be
maintained through the use of a temperature-controlled bath; in this
NOTE 8—Successful secondary guard designs consist of a sheathed
instance, there is no need to install a cold surface heater. Care must be
heater wire or cable wrapped around an adequately-sized metal tube and
taken in this instance; the flow rate of the bath must be sufficient to satisfy
pressed against the metal tube with another sheet of metal. For low-
the temperature uniformity requirements specified in 6.3.1.2 and 6.5.1.
temperature operation, a cooling coil has been wrapped around the
exterior surface of the secondary guard.
6.5.1 Requirements—The cold surface assemblies shall be
designed and constructed to satisfy all of the requirements of
6.6.1.3 Location—The secondary guard shall be positioned
6.3.1 except that, since only one surface plate of each cold
around the hot surface assembly such that a uniform spacing is
surface assembly is in contact with the test specimens, the
created between the components. The height of the secondary
requirement that specifies the temperature difference between
guard shall be adjusted such that the mid-height of the
the surface plates shall not apply.
secondary guard is aligned with the center of the hot surface
6.5.2 Materials—The criteria to select materials that will be
assembly thickness.
used in the construction of the cold surface assemblies are
6.6.2 Edge Insulation—The interspace between the hot and
identical to the hot surface assembly and are listed in 6.3.2.
cold surface assemblies, specimens and the secondary guard
6.5.3 High Temperature Operation—When the cold surface
shall be filled with an insulating material. Due to the complex
assemblies will be operated at high temperatures, it is accept-
shapes of this interspace, a powder or fibrous insulation is
able to insert several thin sheets of insulation between the heat
recommended.
sink and cold surface heater. The addition of these insulation
6.6.2.1 The selection of an edge insulation material will
sheets will reduce the energy requirements to the cold surface
depend on the test conditions. Vermiculite is easy to use but
heater and extend service life.
should not be employed at temperatures above 540°C because
6.6 Additional Edge Loss Protection—Deviation from one-
it’s thermal conductivity increases dramatically with tempera-
dimensional heat flow in the test specimen is due to non-
ture.
adiabatic conditions at the edges of the hot surface assembly
NOTE 9—Avoid the use of vermiculite when the guarded-hot-plate is
and the specimens. This deviation is greatly increased when the
used to evaluate specimens in different gaseous environments; vermiculite
apparatus is used at temperatures other than ambient. When the
is extremely hygroscopic and the system is difficult to evacuate when it is
guarded-hot-plate apparatus is operated at temperatures that
used.
deviate from ambient by more than 10°C, the apparatus shall
NOTE 10—Care shall be taken to ensure that there are no voids, pockets,
be outfitted with additional components to reduce edge losses. or other extraneous sources of radiative heat transfer occurring at or near
the guarded-hot-plate.
These components are described in the following sections and
shall be used if edge losses cannot be minimized.
6.6.3 Enclosure—The guarded-hot-plate shall be placed in-
side an enclosure when the apparatus is used in to maintain a
NOTE 7—Another means of assessing whether edge insulation is
required is to attach a temperature sensor to the mid-height of the exterior gaseous environment that is different than the laboratory
edge of the specimen. Sufficient edge insulation is present if the edge
ambient.
temperature, T , satisfies the following requirement.
e
6.6.3.1 For low-temperature operation, a dry gas environ-
~T 2 T !/ΔT,0.05 (1)
ment shall be used to prevent condensation from occurring on
e m
the cold surface assemblies and specimens.
6.6.1 Secondary Guard—To reduce heat exchange between
6.6.3.2 For high temperature operation, it will often be
the edges of the guarded-hot-plate and the environment, the
desirable to protect the apparatus from severe degradation by
guarded-hot-plate shall be outfitted with a co-axial
using a non-oxidizing gas.
temperature-controlled container referred to as the secondary
guard. The secondary guard will be employed to adjust the 6.6.3.3 The enclosure can also be used for substituting
ambient temperature to approximate the mean temperature of different gaseous environments and control of the ambient
the test specimen. pressure.
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6.7 Clamping Force—A means shall be provided for impos- at a distance from the corners equal to one-fourth of the side of
ing a reproducible constant clamping force on the guarded-hot- the metering area. The corners and the axes should be avoided.
plate to promote good thermal contact between the hot and cold For a round plate, the sensors should be spaced equally around
surface assemblies and the specimens and to maintain accurate the gap.
spacing between the hot and cold surface assemblies. It is
6.8.1.4 Electrically isolated gap imbalance sensors should
unlikely that a force greater than 2.5 kPa will be required for
be placed on both surface plates of the guarded heating unit to
the majority of insulating materials. In the case of compressible
average the imbalance on both faces of the heating unit.
materials, a constant pressure arrangement is not needed and it
6.8.1.5 Thermal junctions or other sensitive elements should
is possible that spacers between the plates will be necessary to
each be located in similar areas of the hot surface assembly. It
maintain constant thickness.
is suggested that all junctions should be located at points
6.7.1 A steady force, that will thrust the cold surface
directly adjacent to the centers of the areas between heater
assemblies toward each other can be imposed by using
windings. Any leads crossing the gap should be thermally
constant-force springs or an equivalent method.
anchored to the primary guard to provide a heat sink from
6.7.2 For compressible specimens, spacers are required if
external thermal variations. In some instances it may be
the test thickness can not be measured by other means. The
desirable to provide a heat sink for these leads outside the
spacers shall be small in cross-section and located near the
primary guard to minimize any radial heat flow.
exterior perimeter of the primary guard. Avoid placing spacers
6.8.2 Temperature Sensors—Methods possessing adequate
on surfaces where underlying sensors are being used to
accuracy, such as thermistors, thermocouples, diodes and
measure plate conditions.
precision resistance thermometers may be used for the mea-
surement of temperatures in the apparatus. Thermocouples are
NOTE 11—Because of the changes of specimen thickness possible as a
result of temperature exposure, or compression by the plates, it is
the most widely used detector due to their wide range of
recommended that, when possible, specimen thickness be measured in the
applicability and accuracy. The goal is to measure the tempera-
apparatus at the existing test temperature and compression conditions.
ture gradient within the specimen, and the method chosen
Gaging points, or measuring studs along the outer perimeter of the cold
(sensors mounted on the specimen surface, in grooves, or
surface assemblies, will serve for these measurements. The effective
between interior layers) should be that which yields the highest
combined specimen thickness is determined by the average difference in
the distance between the gaging points when the specimen is in place in
accuracy in the measurement of the temperature gradient. A
the apparatus and when it is not in place.
discussion of these alternatives is provided in 6.8.2.3 and
6.8 Temperature Measurements: 6.8.2.4.
6.8.1 Imbalance Detectors—A suitable means shall be pro-
6.8.2.1 Use of Thermocouples—Precautions should be used
vided to detect the average temperature imbalance between to minimize spurious voltages in temperature control and
surface plates of the metering section and the primary guard.
measuring circuits. Spurious voltages, due to wire
6.8.1.1 Sensors—The gap region shall be instrumented with inhomogeneities, generally increase as the temperature gradi-
temperature sensors to monitor and control the average tem- ents within the measuring leads increase. For the same reason,
perature imbalance across the gap. Fine-gage thermocouples junctions between dissimilar metal leads should not be made in
connected as thermopiles are often used for this purpose, the regions of appreciable temperature gradients. Low thermal
although other temperature control sensors, such as emf switches should be used in the temperature measurement
thermistors, have been used. Highly alloyed thermocouples, circuits. An insulated, isothermal box of heavy sheet metal can
rather than pure metals, should be used to maximize the be used when joining leads of dissimilar metals in the
thermal resistance across the gap. Because of nonuniform heat thermocouple circuit. It is recommended that all connections of
flux within the surface plates, temperature imbalance is not thermocouple wire to copper wire be accomplished within the
always constant along the gap perimeter. It has been found that
isothermal box in order that the junctions are at the same
with proper design the thermal conductance of the wires temperature; then the
...