ASTM C335/C335M-23

Standard Test Method for Steady-State Heat Transfer Properties of Pipe Insulation

SIGNIFICANCE AND USE
4.1 As determined by this test method, the pipe insulation lineal thermal resistance or conductance (and, when applicable, the thermal resistivity or conductivity) are means of comparing insulations which include the effects of the insulation and its fit upon the pipe, circumferential and longitudinal jointing, and variations in construction, but do not include the effects of the outer surface resistance or heat transfer coefficient. They are thus appropriate when the insulation outer-surface temperature and the pipe temperature are known or specified. However, since the thermal properties determined by this test method include the effects of fit and jointing, they are not true material properties. Therefore, properties determined by this test method are somewhat different from those obtained on apparently similar material in flat form using the guarded hot plate, Test Method C177, or the heat flow meter apparatus, Test Method C518.  
4.2 The pipe insulation lineal thermal transference incorporates both the effect of the insulation and its fit upon the pipe and also the effect of the surface heat-transfer coefficient. It is appropriate when the ambient conditions and the pipe temperature are known or specified and the thermal effects of the surface are to be included.  
4.3 Because of the test condition requirements prescribed in this test method, recognize that the thermal transfer properties obtained will not necessarily be the value pertaining under all service conditions. As an example, this test method provides that the thermal properties shall be obtained by tests on dry or conditioned specimens, while such conditions are not necessarily realized in service. The results obtained are strictly applicable only for the conditions of test and for the product construction tested, and must not be applied without proper adjustment when the material is used at other conditions, such as mean temperatures that differ appreciably from those of the test. With these quali...
SCOPE
1.1 This test method covers the measurement of the steady-state heat transfer properties of pipe insulations. Specimen types include rigid, flexible, and loose fill; homogeneous and nonhomogeneous; isotropic and nonisotropic; circular or non-circular cross section. Measurement of metallic reflective insulation and mass insulations with metal jackets or other elements of high axial conductance is included; however, additional precautions must be taken and specified special procedures must be followed.  
1.2 The test apparatus for this purpose is a guarded-end or calibrated-end pipe apparatus. The guarded-end apparatus is a primary (or absolute) method. The guarded-end method is comparable, but not identical to ISO 8497. The ISO method does not use the calculation procedure in Practice C1045.  
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.  
1.4 When appropriate, or as required by specifications or other test methods, the following thermal transfer properties for the specimen can be calculated from the measured data (see 3.2):  
1.4.1 The pipe insulation lineal thermal resistance and conductance,  
1.4.2 The pipe insulation lineal thermal transference,  
1.4.3 The surface areal resistance and heat transfer coefficient,  
1.4.4 The thermal resistivity and conductivity,  
1.4.5 The areal thermal resistance and conductance, and  
1.4.6 The areal thermal transference.
Note 1: In this test method the preferred resistance, conductance, and transference are the lineal values computed for a unit length of pipe. These must not be confused with the corresponding areal properties computed on a unit area basis which are more applicable to flat slab geometr...

General Information

• Status: Published
• Publication Date: 28-Feb-2023
• ICS: 91.100.60 - Thermal and sound insulating materials
• 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 C870-24 - Standard Practice for Conditioning of Thermal Insulating Materials
• Effective Date: 01-Mar-2024
• Refers: ASTM C680-23a - Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical Systems by Use of Computer Programs
• Effective Date: 01-Nov-2023
• Refers: ASTM C1045-19 - Standard Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
• Effective Date: 01-Apr-2019
• 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 C870-11(2017) - Standard Practice for Conditioning of Thermal Insulating Materials
• Effective Date: 15-Apr-2017
• 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
• Refers: ASTM C680-14 - Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical Systems by Use of Computer Programs
• Effective Date: 01-Sep-2014
• Refers: ASTM C302-13 - Standard Test Method for Density and Dimensions of Preformed Pipe-Covering-Type Thermal Insulation
• Effective Date: 01-Nov-2013
• Refers: ASTM C1045-07(2013) - Standard Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
• Effective Date: 01-Sep-2013
• Refers: ASTM C168-13 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Apr-2013
• Refers: ASTM C870-11 - Standard Practice for Conditioning of Thermal Insulating Materials
• Effective Date: 01-Apr-2011

Overview

ASTM C335/C335M-23 establishes the standard test method for measuring the steady-state heat transfer properties of pipe insulation. Developed by ASTM International, this test method applies to a variety of pipe insulation types, including rigid, flexible, loose-fill, homogeneous, nonhomogeneous, isotropic, and nonisotropic materials. The method is especially relevant for determining the lineal and areal thermal resistance, conductance, transference, and conductivity of pipe insulation under steady-state conditions, providing critical data for engineers, manufacturers, and building designers concerned with thermal performance in piping systems.

Key Topics

• Scope and Applicability: The standard covers a wide range of insulation materials and pipe geometries, including circular and non-circular (e.g., square, rectangular) cross-sections. It includes considerations for insulation with metallic jackets and high axial conductance elements.
• Test Apparatus: Measurements are conducted using a guarded-end or calibrated-end pipe apparatus. The guarded-end pipe method is a primary, absolute technique and is preferred, especially when testing materials with high axial conductance.
• Measured Properties:
  • Pipe insulation lineal thermal resistance and conductance
  • Pipe insulation lineal thermal transference
  • Surface areal resistance and heat transfer coefficient
  • Thermal resistivity and conductivity
  • Areal thermal resistance, conductance, and transference
• Comparison with Other Methods: Properties measured via this method account for the insulation's fit, jointing, and construction variations, distinguishing them from material properties obtained via flat specimen tests, such as ASTM C177 (guarded hot plate) or ASTM C518 (heat flow meter).
• Test Limitations: Results are strictly valid for the test conditions and construction. Adjustments are required when applying the data to different temperatures, humidity, ambient conditions, or insulation configurations.
• Dual Unit Standard: Measurements can be reported in either SI or inch-pound units, but mixing values from both systems is not permitted.

Applications

• Thermal Design and Analysis: The data derived from ASTM C335/C335M-23 support the selection and comparison of pipe insulation products based on their steady-state heat transfer performance. This is crucial for optimizing energy conservation in building HVAC systems, industrial processing, and district heating/cooling applications.
• Product Development and Quality Assurance: Manufacturers use this standard to verify product performance, ensuring conformance with specifications and competitiveness in the marketplace.
• Engineering Calculations: The standard provides essential input for computational tools and engineering calculations estimating thermal losses or gains in pipework, supporting compliance with energy codes and sustainability goals.
• Specification and Procurement: Architects and engineers reference the standard in project specifications, ensuring the installed insulation meets desired thermal criteria in practice.

Related Standards

• ASTM C177: Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus
• ASTM C518: Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
• ASTM C168: Terminology Relating to Thermal Insulation
• ASTM C302: Test Method for Density and Dimensions of Preformed Pipe-Covering-Type Thermal Insulation
• ASTM C1045: Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
• ASTM C1058: Practice for Selecting Temperatures for Evaluating and Reporting Thermal Properties of Thermal Insulation
• ISO 8497: Thermal insulation - Determination of steady-state thermal transmission properties of thermal insulation for circular pipes

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Utilizing ASTM C335/C335M-23 ensures reliable, comparable, and reproducible data on pipe insulation thermal characteristics, supporting energy efficiency and system performance in a variety of industrial and building applications. For optimal use, reference this standard alongside related ASTM and ISO methods to achieve a comprehensive approach to thermal insulation testing and specification.

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: C335/C335M − 23
Standard Test Method for
Steady-State Heat Transfer Properties of Pipe Insulation
This standard is issued under the fixed designation C335/C335M; 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.
must not be confused with the corresponding areal properties computed on
1. Scope
a unit area basis which are more applicable to flat slab geometry. If these
1.1 This test method covers the measurement of the steady-
areal properties are computed, the area used in their computation must be
state heat transfer properties of pipe insulations. Specimen reported.
NOTE 2—Discussions of the appropriateness of these properties to
types include rigid, flexible, and loose fill; homogeneous and
particular specimens or materials may be found in Test Method C177, Test
nonhomogeneous; isotropic and nonisotropic; circular or non-
Method C518, and in the literature (1).
circular cross section. Measurement of metallic reflective
1.5 This test method allows for operation over a wide range
insulation and mass insulations with metal jackets or other
of temperatures. The upper and lower limit of the pipe surface
elements of high axial conductance is included; however,
temperature is determined by the maximum and minimum
additional precautions must be taken and specified special
service temperature of the specimen or of the materials used in
procedures must be followed.
constructing the apparatus. In any case, the apparatus must be
1.2 The test apparatus for this purpose is a guarded-end or
operated such that the temperature difference between the
calibrated-end pipe apparatus. The guarded-end apparatus is a
exposed surface and the ambient is sufficiently large enough to
primary (or absolute) method. The guarded-end method is
provide the precision of measurement desired. Normally the
comparable, but not identical to ISO 8497. The ISO method
apparatus is operated in closely controlled still air ambient
does not use the calculation procedure in Practice C1045.
from 15 to 30°C, but other temperatures, other gases, and other
1.3 The values stated in either SI units or inch-pound units
velocities are acceptable. It is also acceptable to control the
are to be regarded separately as standard. The values stated in
outer specimen surface temperature by the use of a heated or
each system may not be exact equivalents; therefore, each
cooled outer sheath or blanket or by the use of an additional
system shall be used independently of the other. Combining
uniform layer of insulation.
values from the two systems may result in non-conformance
1.6 The use any size or shape of test pipe is allowable
with the standard.
provided that it matches the specimens to be tested. Normally
1.4 When appropriate, or as required by specifications or
the test method is used with circular pipes; however, its use is
other test methods, the following thermal transfer properties
permitted with pipes or ducts of noncircular cross section
for the specimen can be calculated from the measured data (see
(square, rectangular, hexagonal, etc.). One common size used
3.2):
for interlaboratory comparison is a pipe with a circular cross
1.4.1 The pipe insulation lineal thermal resistance and
section of 88.9-mm diameter (standard nominal 80-mm [3-in.]
conductance,
pipe size), although several other sizes are reported in the
1.4.2 The pipe insulation lineal thermal transference,
literature (2-4).
1.4.3 The surface areal resistance and heat transfer
1.7 The test method applies only to test pipes with a
coefficient,
horizontal or vertical axis. For the horizontal axis, the literature
1.4.4 The thermal resistivity and conductivity,
includes using the guarded-end, the calibrated, and the
1.4.5 The areal thermal resistance and conductance, and
calibrated-end cap methods. For the vertical axis, no experi-
1.4.6 The areal thermal transference.
ence has been found to support the use of the calibrated or
NOTE 1—In this test method the preferred resistance, conductance, and calibrated-end methods. Therefore the method is restricted to
transference are the lineal values computed for a unit length of pipe. These
using the guarded-end pipe apparatus for vertical axis mea-
surements.
This test method is under the jurisdiction of ASTM Committee C16 on Thermal
1.8 This test method covers two distinctly different types of
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
pipe apparatus, the guarded-end and the calibrated or
Measurement.
Current edition approved March 1, 2023. Published April 2023. Originally
approved in 1954. Last previous edition approved in 2017 as C335/C335M – 17. The boldface numbers in parentheses refer to the references at the end of this
DOI: 10.1520/C0335_C0335M-23. test method.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C335/C335M − 23
calculated-end types, which differ in the treatment of axial heat 2.2 ISO Standards:
transfer at the end of the test section. ISO 8497 Thermal Insulation-Dermination of Steady State
Thermal Transmission Properties of Thermal Insulation
1.8.1 The guarded-end apparatus utilizes separately heated
for Circular Pipes
guard sections at each end, which are controlled at the same
temperature as the test section to limit axial heat transfer. This
3. Terminology
type of apparatus is preferred for all types of specimens within
3.1 Definitions—For definitions of terms used in this test
the scope of this test method and must be used for specimens
method, refer to Terminology C168.
incorporating elements of high axial conductance.
3.2 Definitions of Terms Specific to This Standard:
1.8.2 The calibrated or calculated-end apparatus utilizes
3.2.1 areal thermal conductance, C—the steady-state time
insulated end caps at each end of the test section to minimize
rate of heat flow per unit area of a specified surface (Note 3)
axial heat transfer. Corrections based either on the calibration
divided by the difference between the average pipe surface
of the end caps under the conditions of test or on calculations
temperature and the average insulation outer surface tempera-
using known material properties, are applied to the measured
ture. It is the reciprocal of the areal thermal resistance, R.
test section heat transfer. These apparatuses are not applicable
Q 1
for tests on specimens with elements of high axial conductance
C 5 5 (1)
A t 2 t R
~ !
such as reflective insulations or metallic jackets. There is no o 2
known experience on using these apparatuses for measure-
where the surface of the area, A, must be specified (usu-
ments using a vertical axis.
ally the pipe surface or sometimes the insulation outer sur-
face).
1.9 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
NOTE 3—The value of C, the areal thermal conductance, is arbitrary
responsibility of the user of this standard to establish appro-
since it depends upon an arbitrary choice of the area, A. For a homoge-
neous material for which the thermal conductivity is defined as in 3.2.7
priate safety, health, and environmental practices and deter-
(Eq 8), the areal conductance, C, is given as follows:
mine the applicability of regulatory limitations prior to use.
1.10 This international standard was developed in accor- 2πLλ
p
C 5 (2)
Aln r /r
dance with internationally recognized principles on standard- ~ !
2 o
ization established in the Decision on Principles for the
If the area is specially chosen to be the “log mean area,”
Development of International Standards, Guides and Recom-
equal to 2πL (r − r )/l n(r /r ), then C = λ /(r − r ). Since
2 o 2 o p 2 o
mendations issued by the World Trade Organization Technical
(r − r ) is equal to the insulation thickness measured from
2 o
Barriers to Trade (TBT) Committee.
the pipe surface, this is analogous to the relation between
conductance and conductivity for flat slab geometry. Similar
2. Referenced Documents
relations exist for the areal thermal resistance defined in
3.2.2. Since these areal coefficients are arbitrary, and since
2.1 ASTM Standards:
the area used is often not stated, thus leading to possible
C168 Terminology Relating to Thermal Insulation
confusion, it is recommended that these areal coefficients not
C177 Test Method for Steady-State Heat Flux Measure-
be used unless specifically requested.
ments and Thermal Transmission Properties by Means of
the Guarded-Hot-Plate Apparatus 3.2.2 areal thermal resistance, R—the average temperature
C302 Test Method for Density and Dimensions of Pre-
difference between the pipe surface and the insulation outer
formed Pipe-Covering-Type Thermal Insulation surface required to produce a steady-state unit rate of heat flow
C518 Test Method for Steady-State Thermal Transmission
per unit area of a specified surface (Note 3). It is the reciprocal
Properties by Means of the Heat Flow Meter Apparatus of the areal thermal conductance, C.
C680 Practice for Estimate of the Heat Gain or Loss and the
A t 2 t
~ o 2!
Surface Temperatures of Insulated Flat, Cylindrical, and R 5 5 (3)
Q C
Spherical Systems by Use of Computer Programs
where the surface of the area, A, must be specified (usu-
C870 Practice for Conditioning of Thermal Insulating Ma-
ally the pipe surface or sometimes the insulation outer sur-
terials
face).
C1045 Practice for Calculating Thermal Transmission Prop-
erties Under Steady-State Conditions
3.2.3 areal thermal transference, T —the time rate of heat
r
C1058 Practice for Selecting Temperatures for Evaluating
flow per unit surface area of the insulation divided by the
and Reporting Thermal Properties of Thermal Insulation
difference between the average pipe surface temperature and
E230 Specification for Temperature-Electromotive Force
the average air ambient temperature.
(emf) Tables for Standardized Thermocouples
Q
T 5 (4)
r 2
2πr L t 2 t
~ !
o a
3.2.4 pipe insulation lineal thermal conductance, C —the
L
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
steady-state time rate of heat flow per unit pipe insulation
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
length divided by the difference between the average pipe
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. surface temperature and the average insulation outer surface
C335/C335M − 23
temperature. It is the reciprocal of the pipe insulation lineal
λ = pipe insulation thermal conductivity, W/m·K
p
thermal resistance, R .
L [Btu · in ⁄F · hr · ft ],
r = pipe insulation thermal resistivity, K·m/W [F · hr · ft ],
Q 1 L
C 5 5 (5)
h = surface areal heat transfer coefficient of insulation
L
L t 2 t R
~ !
o 2 L
2 2
outer surface, W/m ·K [Btu · in ⁄F · hr · ft ],
3.2.5 pipe insulation lineal thermal resistance, R —the
L
C = areal thermal conductance, W/m ·K
average temperature difference between the pipe surface and 2
[Btu · in ⁄F · hr · ft ],
2 2
the insulation outer surface required to produce a steady-state
R = areal thermal resistance, K·m /W [F · hr · ft ],
unit time rate of heat flow per unit of pipe insulation length. It
T = areal thermal transference, W/m ·K
r
is the reciprocal of the pipe insulation lineal thermal
[Btu · in ⁄F · hr · ft ],
conductance, C .
Q = time rate of heat flow to the test section of length L, W
L
[Btu/hr],
L t 2 t
~ ! 1
o 2
R 5 5 (6)
t = temperature of pipe surface, K [F],
L o
Q C
L
t = temperature of insulation inside surface, K [F],
3.2.6 pipe insulation lineal thermal transference, T —the
r t = temperature of insulation outside surface, K [F],
p
steady-state time rate of heat flow per unit pipe insulation
t = temperature of ambient air or gas, K [F],
a
length divided by the difference between the average pipe
r = outer radius of circular pipe, m [ft],
o
surface temperature and the average air ambient temperature. It r = inner radius of circular insulation, m [ft],
is a measure of the heat transferred through the insulation to the r = outer radius of circular insulation, m ft],
L = length of test section (see 8.1.1), m [ft], and
ambient environment.
2 2
A = area of specified surface, m [ft ].
Q
T 5 (7)
r
p
L~t 2 t !
4. Significance and Use
o a
3.2.7 pipe insulation thermal conductivity,λ —of homoge-
p
4.1 As determined by this test method, the pipe insulation
neous material, the ratio of the steady-state time rate of heat
lineal thermal resistance or conductance (and, when applicable,
flow per unit area to the average temperature gradient (tem-
the thermal resistivity or conductivity) are means of comparing
perature difference per unit distance of heat flow path). It
insulations which include the effects of the insulation and its fit
includes the effect of the fit upon the test pipe and is the
upon the pipe, circumferential and longitudinal jointing, and
reciprocal of the pipe insulation thermal resistivity, r . For pipe
L variations in construction, but do not include the effects of the
insulation of circular cross section, the pipe insulation thermal
outer surface resistance or heat transfer coefficient. They are
conductivity is:
thus appropriate when the insulation outer-surface temperature
and the pipe temperature are known or specified. However,
Q 1n r /r
~ ! 1
2 o
λ 5 5 (8)
p since the thermal properties determined by this test method
L2π t 2 t r
~ !
o 2 L
include the effects of fit and jointing, they are not true material
3.2.8 pipe insulation thermal resistivity, r —of homoge-
L
properties. Therefore, properties determined by this test
neous material, the ratio of the average temperature gradient
method are somewhat different from those obtained on appar-
(temperature difference per unit distance of heat flow path) to
ently similar material in flat form using the guarded hot plate,
the steady-state time rate of heat flow per unit area. It includes
Test Method C177, or the heat flow meter apparatus, Test
the effect of the fit upon the test pipe and is the reciprocal of the
Method C518.
pipe insulation thermal conductivity, λ . For pipe insulation of
p
4.2 The pipe insulation lineal thermal transference incorpo-
circular cross section, the pipe insulation thermal resistivity is
rates both the effect of the insulation and its fit upon the pipe
calculated as follows:
and also the effect of the surface heat-transfer coefficient. It is
2πL t 2 t 1
~ !
o 2
r 5 5 (9) appropriate when the ambient conditions and the pipe tempera-
L
Q 1n ~r /r ! λ
2 o p
ture are known or specified and the thermal effects of the
3.2.9 surface areal heat transfer coeffıcient, h —the ratio of
surface are to be included.
the steady-state time rate of heat flow per unit surface area to
4.3 Because of the test condition requirements prescribed in
the average temperature difference between the surface and the
this test method, recognize that the thermal transfer properties
ambient surroundings. The inverse of the surface heat transfer
obtained will not necessarily be the value pertaining under all
coefficient is the surface resistance. For circular cross sections:
service conditions. As an example, this test method provides
Q
that the thermal properties shall be obtained by tests on dry or
h 5 (10)
2 2
2πr L~t 2 t !
2 a
conditioned specimens, while such conditions are not neces-
3.3 Symbols: see 1.3: sarily realized in service. The results obtained are strictly
applicable only for the conditions of test and for the product
C = pipe insulation lineal thermal conductance, W/m·K
L
construction tested, and must not be applied without proper
[Btu · in ⁄F · hr · ft ],
adjustment when the material is used at other conditions, such
R = pipe insulation lineal thermal resistance, K·m/W
L
as mean temperatures that differ appreciably from those of the
[Btu · in ⁄F · hr · ft ],
test. With these qualifications in mind, the following apply:
T = pipe insulation lineal thermal transference, W/m·K
r
p
4.3.1 For horizontal or vertical pipes of the same size and
[Btu · in ⁄F · hr · ft ],
temperature, operating in the same ambient environment,
C335/C335M − 23
values obtained by this test method can be used for the direct method, then all the pertinent requirements prescribed in this
comparison of several specimens, for comparison to specifica- test method shall be met or any exceptions shall be described
tion values, and for engineering data for estimating heat loss of in the report.
actual applications of specimens identical to those tested
4.8 It is not practical in a test method of this type to
(including any jackets or surface treatments). When
establish details of construction and procedure to cover all
appropriate, correct for the effect of end joints and other
contingencies that might offer difficulties to a person without
recurring irregularities (4.4).
technical knowledge concerning the theory of heat flow,
4.3.2 When applying the results to insulation sizes different
temperature measurements, and general testing practices. Stan-
from those used in the test, an appropriate mathematical
dardization of this test method does not reduce the need for
analysis is required. For homogeneous materials, this consists
such technical knowledge. It is recognized also that it would be
of the use of the thermal conductivity or resistivity values
unwise to restrict the further development of improved or new
(corrected for any changes in mean temperature) plus the use of
methods or procedures by research workers because of stan-
the surface heat transfer coefficient when the ambient tempera-
dardization of this test method.
ture is considered (for example, see Practice C680). For
NOTE 4—When testing at ambient temperatures below normal room
nonhomogeneous and reflective insulation materials, a more temperatures, theoretical analysis shows that the experimental heat flow
direction is unimportant for a perfectly homogenous material. However, if
detailed mathematical model is required which properly ac-
the properties of the insulation vary in the radical direction, the experi-
counts for the individual modes of heat transfer (conduction,
mental heat flow direction will significantly affect the measured thermal
convection, radiation) and the variation of each mode with
conductivity. Exercise great care when using data from a radial heat flow
changing pipe size, insulation thickness, and temperature.
outward experiment for a radial heat flow inward application.
4.4 It is difficult to measure the thermal performance of
5. Apparatus
reflective insulation that incorporate air cavities, since the
geometry and orientation of the air cavities can affect convec-
5.1 The apparatus shall consist of the heated test pipe and
tive heat transfer. While it is always desirable to test full-length instrumentation for measuring the pipe and insulation surface
pipe sections, this is not always possible due to size limitations
temperatures, the average ambient air temperature, and the
of existing pipe insulation testers. If insulation sections are average power dissipated in the test section heater. The pipe
tested less than full length, internal convective heat transfer are shall be uniformly heated by an internal electric heater (Notes
usually altered, which would affect the measured performance. 5 and 6). In large apparatus, give some consideration on
Therefore, it must be recognized that the measured thermal providing internal circulating fans or to filling the pipe with a
performance of less than full-length insulation sections is not heat transfer fluid to achieve uniform temperatures. The
necessarily representative of full-length sections. guarded end design also requires, a short section of pipe at each
end of the test section, with its own separately controlled heater
4.5 The design of the guarded-end pipe apparatus is based
(see 5.3 and Fig. 1). The calibrated or calculated-end design
upon negligible axial heat flow in the specimen, the test pipe,
requires suitable insulated caps at each end (see 5.4 and Fig. 2).
heaters, and other thermal conductive paths between the
An essential requirement of the test is an enclosure or room
metering and guard sections. Some nonhomogeneous and
equipped to control the temperature of the air surrounding the
reflective insulation are usually modified at the end over the
apparatus. The apparatus shall conform to the principles and
guard gap in order to prevent axial heat flow. Avoid these
limitations prescribed in the following sections, but it is not
modifications where possible, but for some nonhomogeneous
intended in this test method to include detailed requirements
insulation designs, they provide the only means to satisfy the
for the construction or operation of any particular apparatus.
negligible heat flow assumption across the guard gaps.
Therefore, thermal performance measured on insulation speci- NOTE 5—Experiments have been reported that use an electrically heated
cylindrical screen rather than an internally heated pipe (5). An extension
mens with modified ends are not necessarily representative of
of the heated screen technique has been reported (6) for testing below
the performance of standard insulation sections.
normal temperatures using the radial heat flow inward, similar to some
insulation system applications. While these designs and the accompanying
4.6 It is acceptable to use this test method to determine the
analysis are not included in this test method, their findings are pertinent to
effect of end joints or other isolated irregularities by comparing
this standard.
tests of two specimens, one of which is uniform throughout its
NOTE 6—The most commonly used heater consists of electrical
length and the other which contains the joint or other irregu-
resistance wire or ribbon on the surface or in the grooves of a tubular
larity within the test section. The difference in heat loss
ceramic core that is internal to the test pipe. If the heater fits snugly inside
the test pipe, the contact must be uniform to achieve uniform test pipe
between these two tests, corrected for the uniform area covered
temperatures. If the heater core is smaller than the inside diameter of the
by the joint or other irregularity, is the extra heat loss
pipe, then fill the gap with a material such as sand to provide uniform heat
introduced. Care must be taken that the tests are performed
transfer. In this standard the combination of heater winding and heater
under the same conditions of pipe and ambient temperature and
pipe will be called either a “heater” or a “heater pipe.”
that sufficient length exists between the joint or irregularity and
5.2 Apparatus Pipe, no restriction is placed on the cross
the test section ends to prevent appreciable end loss.
section size or shape, but the length of the test section must be
4.7 For satisfactory results in conformance with this test sufficient to ensure that the total measured heat flow is large
method, the principles governing construction and use of enough, when compared to end losses and to the accuracy of
apparatus described in this test method must be followed. If the the power measurement, to achieve the desired test accuracy
results are to be reported as having been obtained by this test (see 5.3 and 8.4). A test section length of approximately 0.5 m
C335/C335M − 23
FIG. 1 Guarded-End Apparatus
FIG. 2 Calibrated or Calculated-End Apparatus
[24 in.] has proven satisfactory for an apparatus with a circular cross-section of 88.9 mm (nominal 80-mm, [nominal 3-in.]
C335/C335M − 23
pipe size) that is often used for inter-laboratory comparisons. usually necessary to provide supplementary internal heaters at
Do not assume that this length is satisfactory for all sizes of each end to compensate for the end heat loss. The power to
apparatus and for all test conditions. Estimates of the required
such heaters must be included in the measured test section
length must be made from an appropriate error analysis. As a
power.
convenience, it is recommended that the apparatus be con-
5.4.1 For the calibrated-end apparatus, the end caps shall be
structed to accept an integral number of standard lengths of
of the same cross-section as the test specimen and have
insulation.
approximately the same thermal transfer properties. Each end
5.3 Guarded-End Apparatus (Fig. 1), uses separately heated cap shall have a cavity of minimum depth equal to one half the
pipe sections at each end of the test section to accomplish the
test pipe diameter (or one half the major cross-section diagonal
purposes of minimizing axial heat flow in the apparatus, of
of noncircular pipes) and of a size and shape to accept the end
aiding in achieving uniform temperatures in the test section,
of the test pipe. The calibrator pipe shall consist of a short
and of extending these temperatures beyond the test section
section of the same pipe used to construct the test pipe of a
length so that all heat flow in the test section is in the radial
length equal to the combined cavity depth of the two end caps.
direction. Both test and guard section heaters shall be designed
It shall be fitted with internal heaters similar to those used in
to achieve uniform temperatures over the length of each
the end sections of the test pipe including any supplementary
section. This typically requires the use of auxiliary heaters at
end heaters. A minimum of four thermocouples spaced 90°
the outside ends of single guards or the use of double guards.
apart shall be provided in the surface of the calibrator pipe to
5.3.1 The length of the guard section (or the combined
measure its temperature. They shall meet the requirements of
length of double guards) shall be sufficient to limit at each end
5.5.1 and be of a wire size as small as possible but in no case
of the test section the combined axial heat flow in both
larger than 0.64 mm diameter [0.025 in. (22 Awg)].
apparatus and specimen to less than 1% of the test section
5.4.2 For the calculated-end apparatus, the end caps shall be
measured heat flow. A guard section length of approximately
as large or larger than the test specimen. They shall be made of
200 mm [4 in.] has been found satisfactory for apparatus of
homogeneous insulation material of low conductivity. It is
88.9 mm ( nominal 80-mm [nominal 3-in.] pipe size) when
acceptable to have a cavity for the test pipe end. The thermal
testing specimens that are essentially homogeneous, are only
conductivity of the end cap material shall be determined by
moderately nonisotropic, and are of a thickness not greater than
Test Method C177 or Test Method C518 over the temperature
the pipe diameter. Longer guard sections are usually required
range of contemplated use. If the material is not isotropic, the
when testing thicker specimens or when the specimen pos-
thermal conductivity must be determined in different directions
sesses a high axial conductance.
as needed.
5.3.2 A gap shall be provided between the guards and the
test section, and between each guard section if double-guarded,
5.5 Thermocouples, for measuring the surface temperature
in both the heater pipe and the test pipe (except for small
of the test pipe shall meet the requirements of 5.5.1 and be of
bridges necessary for structural support). It is highly desirable
a wire size as small as possible, but in no case larger than 0.64
that all support bridges of high conductance be limited to the
mm [0.025 in. (22 Awg)] in diameter. At least four
test pipe since any bridges in heater pipes or internal support
thermocouples, or one for each 150 mm of length of the test
members make it difficult or impossible to achieve uniform
section, whichever is greater, shall be located to sense equally
surface temperatures while at the same time minimizing end
the temperature of all areas of the test section surface. They
losses in the apparatus. Internal barriers shall be installed at
shall be applied either by peening the individual wires into
each gap to minimize convection and radiation heat transfer
small holes drilled into the pipe surface not more than 3 mm
between sections.
apart or by joining the wires by a welded bead and cementing
5.3.3 Thermocouples of wire as small as possible but not
them into grooves so that the bead is tangent to the outer
larger than 0.64 mm [0.025 in. (22 Awg)] and meeting the
surface of the pipe, but does not project above the surface. For
requirements of 5.11, shall be installed in the test pipe surface
direct averaging, it is acceptable to connect the thermocouples
on both sides of each gap, and not more than 25 mm [1 in.]
in parallel, provided their junctions are electrically isolated and
from the gap, for the purpose of monitoring the temperature
the total resistances are essentially equal.
difference across each gap. It is acceptable to connect the
thermocouples in series and use as a differential thermopile.
5.5.1 Thermocouples used for this method shall be made of
Similar thermocouples shall also be installed on any heater
special grade wire as specified in Tables E230 or shall be
pipes or support members that provide a highly conductive
individually calibrated to the same tolerance. Generally, ther-
path from test section to guard sections.
mocouples made from wire taken from the same spool will be
found to agree with each other within the required tolerance
5.4 Calibrated or Calculated-End Apparatus (Fig. 2), uses
and thus only one calibration will be required from each spool
insulated caps at each end of the test section to minimize axial
of wire. Calibration must extend for the lowest to the highest
heat flow. The measured test section heat loss is then corrected
operating range of the apparatus.
for the end cap loss, that has been determined either by direct
calibration under the conditions of test (the calibrated-end
apparatus) or by calculation, using known material properties
(the calculated-end apparatus). Internal electric heaters shall be
Any temperature-measuring sensor can be used, but thermocouples are used
provided to heat the test section uniformly over its length. It is predominantly.
C335/C335M − 23
5.6 Temperature-Measuring System, excluding the sensor, material (in the case of homogeneous materials), then appro-
with an accuracy equivalent to 60.1 K. A d-c potentiometer or priate sampling plans must be followed. In the absence of such
digital microvoltmeter is normally used for thermocouple plans, the test results can be considered to represent only the
readout. specimens tested.
5.7 Power Supplies, use a closely regulated a-c or d-c
6.3 The intended purpose of the test must be considered in
supply for operating the test section heater. Power supplies for
determining details of the specimen and its applications to the
guard heaters, if used, need not be regulated if automatic
test pipe (Note 7). Some considerations are:
controllers are used.
6.3.1 The means of securing the specimen to the test pipe.
6.3.2 The use of sealants or other materials in the joints.
5.8 Power-Measuring System, capable of measuring the
average power to the test section heater with an accuracy of 6 6.3.3 Whether jackets, covers, bands, reflective sheaths,
0.5% shall be provided. If power input is steady, this is etc., are included.
typically a calibrated wattmeter or a voltage-measuring system
6.3.4 For the testing of reflective insulation, there are
for voltage and amperage (using a standard resistance). If
additional considerations. It is recommended that at least two
power input is variable or fluctuating, an integrating type of
insulation sections be mounted within the central test section.
power measurement, using an integrating period long enough
While the use of full length specimens within the central test
to assure a reliable determination of average power, is required.
section is preferred, this may not be practical within the limits
In all cases, care must be taken that the measured power is only
of existing apparatus. Air exchange must not occur between the
that dissipated in the test section. This requires that corrections
test and guard sections. Install a fibrous or other airtight, low
be applied for power dissipated in leads, dropping resistors, or
conductivity, nonmetallic insulation seal, not more than 25 mm
uncompensated wattmeters.
wide, between the hot pipe and specimen inner casing to
prevent air exchange within this annular space. This seal must
5.9 For a given set of observations as defined in 8.4 the
be installed in the guard region adjacent to the guard gap and
ambient air temperature shall be maintained within 6 1% of
not in the central test section.
the smallest temperature difference between the test pipe and
the ambient or to 6 1°C [62°F], whichever is greater. The
NOTE 7—Unless another purpose is intended, secure the specimen to the
apparatus shall be located in a region of essentially still air and
test pipe in accordance with normal application practice. Include jackets
and other features when desired.
shall not be close to other objects that would alter the pattern
of natural convection around the heated specimen. All surfaces
6.4 After the specimen is mounted on the test pipe, mea-
or objects that exchange radiation with the specimen shall have
surements of the outside dimensions needed to describe the
a total hemispherical emittance of at least 0.85 and shall be at
shape shall be made to within 6 0.5% (both before and after
approximately the same temperature as the ambient air. Addi-
testing). For circular shapes, use a flexible steel tape to measure
tional optional equipment is required to use gases other than air
the circumference then divide by 2π to obtain the radius r . The
and to simulate wind effects by establishing forced air veloci-
test section length shall be divided into at least four equal parts,
ties of the direction and magnitude desired.
and dimension measurements shall be taken at the center of
each, except that any irregularity being investigated shall be
5.10 An optional temperature-controlled jacket is an accept-
able procedure to control the outer surface of the specimen to avoided. Additional measurements shall be taken to describe
the irregularities. For guarded-end apparatuses, additional
a temperature different than that of the ambient air. An
measurements at the center of each guard section are also
alternative procedure for raising the outer surface temperature
required. Specimens intended to be of uniform cross section
of a specimen is to surround it with an additional layer of
dimensions throughout their length must be rejected if any
thermal insulation. In either case the thermocouples specified
individual dimension measurement (test section or guard)
in 6.5 for the measurement of the specimen outer surface
differs from the average of the test section measurements by
temperature must be installed prior to placement of the jacket
more than 5%.
or additional insulation layer. Moreover, the emittance of the
inner surface of the jacket or added insulation (facing the
6.5 Thermocouples for the measurement of the average
specimen) must be greater than 0.8 in order not to reduce any
outside surface temperature, t , shall be attached to the insu-
radiation transfer within the specimen. In such cases it is not
lation surface in accordance with the following:
possible to measure directly the thermal transference for the
6.5.1 The test section length shall be divided into at least
specimen.
four equal parts and surface thermocouples shall be longitudi-
nally located at the center of each. Large apparatuses will
6. Test Specimen
require a greater number of thermocouples. For circular shapes,
6.1 Specimens types include rigid, semi-rigid, or flexible
the thermocouples shall also be circumferentially equally
(blanket-type), or loose-fill, suitably contained. Specimens
spaced to form helical patterns with an integral number of
shall be uniform in size and shape throughout their length
complete revolutions and with the angular spacing between
(except for any intentional irregularities that occur well within
adjacent locations from 45 to 90°. For non-circular shapes, the
the test section) and shall be designed for use on pipes of the
thermocouples shall be spaced around in much the same
same size and shape as the available test apparatus.
manner but located to obtain an area-weighted average. Any of
6.2 If test results are to be considered as representative of a the above specified locations shall, whenever possible, be
type of product or of a particular production lot, etc., or of a offset a distance equal to the specimen thickness from any joint
C335/C335M − 23
or other irregularity, and additional thermocouples shall be (t − t ), whichever is greater. Run the test in essentially still air
o a
used as necessary to record the surface temperature. In such (or other desired gas) unless appreciable velocity is needed to
situations the individual temperatures and locations shall be attain uniform temperatures or when the effect of air velocity is
reported (see 11.1.6). to be included as part of the test conditions. Measure any
6.5.2 Thermocouples shall be made of wire not larger forced velocity and report its magnitude and direction.
than0.40 mm [0.016 in. (26 Awg)] and shall meet the require-
8. Procedure
ments of 5.5.1. They shall be fastened to the surface by any
means that will hold the junction and the required length of
8.1 Measure the test section length, L, and the specimen
adjacent wire in intimate thermal contact with the surface but
outside circumference or other dimensions needed to describe
does not alter the radiation emittance characteristics of the
the shape. Normally dimensions used in this method shall be
adjacent surface.
those measured at ambient temperatures of 10 to 35°C. If
6.5.2.1 For nonmetallic surfaces, a minimum of 100 mm [4
properties based upon actual dimensions at operating tempera-
in.] of adjacent wire shall be held in contact with the surface.
ture are desired, determine the dimensions by calculation from
One satisfactory method of fastening is to use masking tape
those measured at ambient temperature using previously mea-
either adhered to the specimen surface or wrapped around the
sured or known coefficients of thermal expansion, or directly
specimen and adhered to itself.
measure the dimensions at operating temperature. Any proper-
6.5.2.2 For metallic surfaces, a minimum of 10 mm of
ties based upon dimensions at operating temperature must be
adjacent lead wire shall be held in contact with the surface.
so identified.
Acceptable means of fastening thermocouple junctions are by
8.1.1 For guarded-end pipes, the test length, L, is the
peening, welding, soldering or brazing, or by use of metallic
distance between the centerlines of the gaps at the ends of the
tape of the same emittance as the surface. Capacitive discharge
test section. For calibrated or calculated-end pipes, the test
welding is especially recommended. Small thin strips of metal
length, L, is the distance between the end caps.
similar to the surface metal shall be welded to the surface to
8.1.2 Take outside dimensions of the specimen at locations
hold the lead wire in contact with the surface.
described in 6.4.
6.5.3 The average surface temperature is calculated by
8.2 Adjust the temperature of the test pipe (or the test
averaging the individual readings of the surface thermo-
section of a guarded-end apparatus) to the desired temperature.
couples. If desired, measure the average by directly connecting
Refer to Practice C1058 for recommended temperatures.
the thermocouples in parallel, provided that the junctions are
8.3 When using the guarded-end method, adjust the tem-
electrically isolated and the total electrical resistances are
perature of each guard so that the temperature difference across
essentially equal.
the gap between the test section and the guard (measured on the
6.6 Thermocouples meeting the requirements of 5.5.1 shall
surface of the test pipe) is zero or not greater than the amount
be installed on elements of high axial heat conductance such as
that will introduce an error of 1% in the measured heat flow.
metallic jackets or accessible liners (specimens with such
Ideally, the axial temperature gradient across the gaps between
elements must be tested on a guarded-end apparatus) in order
the test and guard sections of both the outer test pipe and the
to measure axial temperature gradients needed to compute
internal heater pipe and along any internal support members is
axial heat transfer. These thermocouples shall be installed at
zero to eliminate all axial heat flow within the pipe. In some
both top and bottom locations, and shall be located an equal
designs, it is impossible to balance both surface and internal
distance of approximately 45 mm [0.08] on each side of the
elements at the same time, and it will be necessary to correct
gap between the test section and each guard.
for internal apparatus axial losses. When the only support
bridges are in the outer test pipe, it is sufficient to bring the test
7. Conditioning
pipe surface gap balance (between test section and guards) to
7.1 In general, specimens shall be dried or otherwise con-
zero and no corrections are needed. When the apparatus uses
ditioned to stable conditions immediately prior to test unless it
internal support bridges, it is necessary to use the readings of
has been shown that such procedures are unnecessary to
the internal thermocouples specified in 5.3, along with the
achieve reproducible results for the material being tested.
known dimensions and properties of the support bridges, to
When applicable, follow the conditioning procedures of the
estimate the internal axial losses that must be added to (or
materials specification; otherwise, the normal procedure is to
subtracted from) the measured power input to the test section.
dry to constant weight at a temperature of 102 to 120°C [215
In either case it is often desirable to run two tests, one with the
to 250 °F], unless the specimen is adversely affected, in which
temperature of the guards slightly higher than the test section
case drying in a desiccator from 55 to 60°C [130 to 140 °F] is
and one with it slightly lower. Interpolation between these
recommended (see Practice C870). Report any weight changes
gives an accurate value for the zero balance heat flow along the
due to conditioning. Determine specimen density by Test
internal bridges and for the test section power input and
Method C302 or report the method used to calculate the
provides information on the maximum allowable imbalance
density.
that still meets the 1% criterion. One criterion which has often
7.2 During the experimentation, operate the apparatus in a been used is that the allowable imbalance is no greater than
controlled room or enclosure so that the ambient temperature 0.5% of the temperature drop through the specimen, (t − t ).
2 1
does not vary during a test by more than 6 1°C [6 2°] or 6 This must be verified using the above procedure at the
1% of the difference between the test pipe and the ambient conditions of the test.
C335/C335M − 23
8.3.1 When evaluating reflective insulation, measure the separate calibration curves for each ambient temperature. If the
temperature gradients along the interior and exterior casings test apparatus is to be used at only one set of conditions, then
with thermocouples detailed in 6.6. Compute the axial heat it is acceptable to interpolate between two tests run in the same
conduction along the inner and outer casings from the average ambient but with the calibrator pipe slightly above and slightly
gradients for that casing. Using the average of the four below the desired temperature. The procedure for end cap
gradients, compute the total axial heat conduction for all calibration is as follows:
internal liners. The total axial heat flow for each end of the test 9.2.1 Assemble the end caps to the ca
...

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: C335/C335M − 17 C335/C335M − 23
Standard Test Method for
Steady-State Heat Transfer Properties of Pipe Insulation
This standard is issued under the fixed designation C335/C335M; 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.
1. Scope
1.1 This test method covers the measurement of the steady-state heat transfer properties of pipe insulations. Specimen types
include rigid, flexible, and loose fill; homogeneous and nonhomogeneous; isotropic and nonisotropic; circular or non-circular cross
section. Measurement of metallic reflective insulation and mass insulations with metal jackets or other elements of high axial
conductance is included; however, additional precautions must be taken and specified special procedures must be followed.
1.2 The test apparatus for this purpose is a guarded-end or calibrated-end pipe apparatus. The guarded-end apparatus is a primary
(or absolute) method. The guarded-end method is comparable, but not identical to ISO 8497. The ISO method does not use the
calculation procedure in Practice C1045.
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
1.4 When appropriate, or as required by specifications or other test methods, the following thermal transfer properties for the
specimen can be calculated from the measured data (see 3.2):
1.4.1 The pipe insulation lineal thermal resistance and conductance,
1.4.2 The pipe insulation lineal thermal transference,
1.4.3 The surface areal resistance and heat transfer coefficient,
1.4.4 The thermal resistivity and conductivity,
1.4.5 The areal thermal resistance and conductance, and
1.4.6 The areal thermal transference.
NOTE 1—In this test method the preferred resistance, conductance, and transference are the lineal values computed for a unit length of pipe. These must
not be confused with the corresponding areal properties computed on a unit area basis which are more applicable to flat slab geometry. If these areal
properties are computed, the area used in their computation must be reported.
This test method is under the jurisdiction of ASTM Committee C16 on Thermal Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
Measurement.
Current edition approved May 1, 2017March 1, 2023. Published October 2017April 2023. Originally approved in 1954. Last previous edition approved in 20102017 as
ε1
C335/C335M – 10C335/C335M – 17. . DOI: 10.1520/C0335_C0335M-17.10.1520/C0335_C0335M-23.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C335/C335M − 23
NOTE 2—Discussions of the appropriateness of these properties to particular specimens or materials may be found in Test Method C177, Test Method
C518, and in the literature (1).
1.5 This test method allows for operation over a wide range of temperatures. The upper and lower limit of the pipe surface
temperature is determined by the maximum and minimum service temperature of the specimen or of the materials used in
constructing the apparatus. In any case, the apparatus must be operated such that the temperature difference between the exposed
surface and the ambient is sufficiently large enough to provide the precision of measurement desired. Normally the apparatus is
operated in closely controlled still air ambient from 15 to 30°C, but other temperatures, other gases, and other velocities are
acceptable. It is also acceptable to control the outer specimen surface temperature by the use of a heated or cooled outer sheath
or blanket or by the use of an additional uniform layer of insulation.
1.6 The use any size or shape of test pipe is allowable provided that it matches the specimens to be tested. Normally the test
method is used with circular pipes; however, its use is permitted with pipes or ducts of noncircular cross section (square,
rectangular, hexagonal, etc.). One common size used for interlaboratory comparison is a pipe with a circular cross section of
88.9-mm diameter (standard nominal 80-mm [3-in.] pipe size), although several other sizes are reported in the literature (2-4).
1.7 The test method applies only to test pipes with a horizontal or vertical axis. For the horizontal axis, the literature includes using
the guarded-end, the calibrated, and the calibrated-end cap methods. For the vertical axis, no experience has been found to support
the use of the calibrated or calibrated-end methods. Therefore the method is restricted to using the guarded-end pipe apparatus for
vertical axis measurements.
1.8 This test method covers two distinctly different types of pipe apparatus, the guarded-end and the calibrated or calculated-end
types, which differ in the treatment of axial heat transfer at the end of the test section.
1.8.1 The guarded-end apparatus utilizes separately heated guard sections at each end, which are controlled at the same
temperature as the test section to limit axial heat transfer. This type of apparatus is preferred for all types of specimens within the
scope of this test method and must be used for specimens incorporating elements of high axial conductance.
1.8.2 The calibrated or calculated-end apparatus utilizes insulated end caps at each end of the test section to minimize axial heat
transfer. Corrections based either on the calibration of the end caps under the conditions of test or on calculations using known
material properties, are applied to the measured test section heat transfer. These apparatuses are not applicable for tests on
specimens with elements of high axial conductance such as reflective insulations or metallic jackets. There is no known experience
on using these apparatuses for measurements using a vertical axis.
1.9 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.
1.10 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:
C168 Terminology Relating to Thermal Insulation
C177 Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the
Guarded-Hot-Plate Apparatus
C302 Test Method for Density and Dimensions of Preformed Pipe-Covering-Type Thermal Insulation
C518 Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
C680 Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical
Systems by Use of Computer Programs
C870 Practice for Conditioning of Thermal Insulating Materials
C1045 Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
The boldface numbers in parentheses refer to the references at the end of this test method.
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.
C335/C335M − 23
C1058 Practice for Selecting Temperatures for Evaluating and Reporting Thermal Properties of Thermal Insulation
E230 Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
2.2 ISO Standards:
ISO 8497 Thermal Insulation-Dermination of Steady State Thermal Transmission Properties of Thermal Insulation for Circular
Pipes
2.3 ASTM Adjuncts:
Guarded-end Apparatus
Calibrated-end Apparatus
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology C168.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 areal thermal conductance, C—the steady-state time rate of heat flow per unit area of a specified surface (Note 3) divided
by the difference between the average pipe surface temperature and the average insulation outer surface temperature. It is the
reciprocal of the areal thermal resistance, R.
Q 1
C 5 5 (1)
A t 2 t R
~ !
o 2
where the surface of the area, A, must be specified (usually the pipe surface or sometimes the insulation outer surface).
NOTE 3—The value of C, the areal thermal conductance, is arbitrary since it depends upon an arbitrary choice of the area, A. For a homogeneous material
for which the thermal conductivity is defined as in 3.2.7 (Eq 8), the areal conductance, C, is given as follows:
2πLλ
p
C 5 (2)
Aln~r /r !
2 o
If the area is specially chosen to be the “log mean area,” equal to 2πL (r − r )/l n(r /r ), then C = λ /(r − r ). Since
2 o 2 o p 2 o
(r − r ) is equal to the insulation thickness measured from the pipe surface, this is analogous to the relation between conduc-
2 o
tance and conductivity for flat slab geometry. Similar relations exist for the areal thermal resistance defined in 3.2.2. Since
these areal coefficients are arbitrary, and since the area used is often not stated, thus leading to possible confusion, it is recom-
mended that these areal coefficients not be used unless specifically requested.
3.2.2 areal thermal resistance, R—the average temperature difference between the pipe surface and the insulation outer surface
required to produce a steady-state unit rate of heat flow per unit area of a specified surface (Note 3). It is the reciprocal of the areal
thermal conductance, C.
A t 2 t
~ ! 1
o 2
R 5 5 (3)
Q C
where the surface of the area, A, must be specified (usually the pipe surface or sometimes the insulation outer surface).
3.2.3 areal thermal transference, T —the time rate of heat flow per unit surface area of the insulation divided by the difference
r
between the average pipe surface temperature and the average air ambient temperature.
Q
T 5 (4)
r
2πr L t 2 t
~ !
o a
3.2.4 pipe insulation lineal thermal conductance, C —the steady-state time rate of heat flow per unit pipe insulation length divided
L
by the difference between the average pipe surface temperature and the average insulation outer surface temperature. It is the
reciprocal of the pipe insulation lineal thermal resistance, R .
L
Q 1
C 5 5 (5)
L
L t 2 t R
~ !
o 2 L
3.2.5 pipe insulation lineal thermal resistance, R —the average temperature difference between the pipe surface and the insulation
L
outer surface required to produce a steady-state unit time rate of heat flow per unit of pipe insulation length. It is the reciprocal
of the pipe insulation lineal thermal conductance, C .
L
L t 2 t
~ ! 1
o 2
R 5 5 (6)
L
Q C
L
C335/C335M − 23
3.2.6 pipe insulation lineal thermal transference, T —the steady-state time rate of heat flow per unit pipe insulation length divided
r
p
by the difference between the average pipe surface temperature and the average air ambient temperature. It is a measure of the heat
transferred through the insulation to the ambient environment.
Q
T 5 (7)
r
p
L~t 2 t !
o a
3.2.7 pipe insulation thermal conductivity,λ —of homogeneous material, the ratio of the steady-state time rate of heat flow per
p
unit area to the average temperature gradient (temperature difference per unit distance of heat flow path). It includes the effect of
the fit upon the test pipe and is the reciprocal of the pipe insulation thermal resistivity, r . For pipe insulation of circular cross
L
section, the pipe insulation thermal conductivity is:
Q 1n r /r
~ 2 o!
λ 5 5 (8)
p
L2π~t 2 t ! r
o 2 L
3.2.8 pipe insulation thermal resistivity, r —of homogeneous material, the ratio of the average temperature gradient (temperature
L
difference per unit distance of heat flow path) to the steady-state time rate of heat flow per unit area. It includes the effect of the
fit upon the test pipe and is the reciprocal of the pipe insulation thermal conductivity, λ . For pipe insulation of circular cross
p
section, the pipe insulation thermal resistivity is calculated as follows:
2πL t 2 t 1
~ !
o 2
r 5 5 (9)
L
Q 1n ~r /r ! λ
2 o p
3.2.9 surface areal heat transfer coeffıcient, h —the ratio of the steady-state time rate of heat flow per unit surface area to the
average temperature difference between the surface and the ambient surroundings. The inverse of the surface heat transfer
coefficient is the surface resistance. For circular cross sections:
Q
h 5 (10)
2 2
2πr L~t 2 t !
2 a
3.3 Symbols: see 1.3:
C = pipe insulation lineal thermal conductance, W/m·K [Btu · in ⁄F · hr · ft ],
L
R = pipe insulation lineal thermal resistance, K·m/W [Btu · in ⁄F · hr · ft ],
L
T = pipe insulation lineal thermal transference, W/m·K [Btu · in ⁄F · hr · ft ],
r
p
λ = pipe insulation thermal conductivity, W/m·K [Btu · in ⁄F · hr · ft ],
p
r = pipe insulation thermal resistivity, K·m/W [F · hr · ft ],
L
2 2
h = surface areal heat transfer coefficient of insulation outer surface, W/m ·K [Btu · in ⁄F · hr · ft ],
2 2
C = areal thermal conductance, W/m ·K [Btu · in ⁄F · hr · ft ],
2 2
R = areal thermal resistance, K·m /W [F · hr · ft ],
2 2
T = areal thermal transference, W/m ·K [Btu · in ⁄F · hr · ft ],
r
Q = time rate of heat flow to the test section of length L, W [Btu/hr],
t = temperature of pipe surface, K [F],
o
t = temperature of insulation inside surface, K [F],
t = temperature of insulation outside surface, K [F],
t = temperature of ambient air or gas, K [F],
a
r = outer radius of circular pipe, m [ft],
o
r = inner radius of circular insulation, m [ft],
r = outer radius of circular insulation, m ft],
L = length of test section (see 8.1.1), m [ft], and
2 2
A = area of specified surface, m [ft ].
4. Significance and Use
4.1 As determined by this test method, the pipe insulation lineal thermal resistance or conductance (and, when applicable, the
thermal resistivity or conductivity) are means of comparing insulations which include the effects of the insulation and its fit upon
the pipe, circumferential and longitudinal jointing, and variations in construction, but do not include the effects of the outer surface
resistance or heat transfer coefficient. They are thus appropriate when the insulation outer-surface temperature and the pipe
temperature are known or specified. However, since the thermal properties determined by this test method include the effects of
C335/C335M − 23
fit and jointing, they are not true material properties. Therefore, properties determined by this test method are somewhat different
from those obtained on apparently similar material in flat form using the guarded hot plate, Test Method C177, or the heat flow
meter apparatus, Test Method C518.
4.2 The pipe insulation lineal thermal transference incorporates both the effect of the insulation and its fit upon the pipe and also
the effect of the surface heat-transfer coefficient. It is appropriate when the ambient conditions and the pipe temperature are known
or specified and the thermal effects of the surface are to be included.
4.3 Because of the test condition requirements prescribed in this test method, recognize that the thermal transfer properties
obtained will not necessarily be the value pertaining under all service conditions. As an example, this test method provides that
the thermal properties shall be obtained by tests on dry or conditioned specimens, while such conditions are not necessarily realized
in service. The results obtained are strictly applicable only for the conditions of test and for the product construction tested, and
must not be applied without proper adjustment when the material is used at other conditions, such as mean temperatures that differ
appreciably from those of the test. With these qualifications in mind, the following apply:
4.3.1 For horizontal or vertical pipes of the same size and temperature, operating in the same ambient environment, values
obtained by this test method can be used for the direct comparison of several specimens, for comparison to specification values,
and for engineering data for estimating heat loss of actual applications of specimens identical to those tested (including any jackets
or surface treatments). When appropriate, correct for the effect of end joints and other recurring irregularities (4.4).
4.3.2 When applying the results to insulation sizes different from those used in the test, an appropriate mathematical analysis is
required. For homogeneous materials, this consists of the use of the thermal conductivity or resistivity values (corrected for any
changes in mean temperature) plus the use of the surface heat transfer coefficient when the ambient temperature is considered (for
example, see Practice C680). For nonhomogeneous and reflective insulation materials, a more detailed mathematical model is
required which properly accounts for the individual modes of heat transfer (conduction, convection, radiation) and the variation
of each mode with changing pipe size, insulation thickness, and temperature.
4.4 It is difficult to measure the thermal performance of reflective insulation that incorporate air cavities, since the geometry and
orientation of the air cavities can affect convective heat transfer. While it is always desirable to test full-length pipe sections, this
is not always possible due to size limitations of existing pipe insulation testers. If insulation sections are tested less than full length,
internal convective heat transfer are usually altered, which would affect the measured performance. Therefore, it must be
recognized that the measured thermal performance of less than full-length insulation sections is not necessarily representative of
full-length sections.
4.5 The design of the guarded-end pipe apparatus is based upon negligible axial heat flow in the specimen, the test pipe, heaters,
and other thermal conductive paths between the metering and guard sections. Some nonhomogeneous and reflective insulation are
usually modified at the end over the guard gap in order to prevent axial heat flow. Avoid these modifications where possible, but
for some nonhomogeneous insulation designs, they provide the only means to satisfy the negligible heat flow assumption across
the guard gaps. Therefore, thermal performance measured on insulation specimens with modified ends are not necessarily
representative of the performance of standard insulation sections.
4.6 It is acceptable to use this test method to determine the effect of end joints or other isolated irregularities by comparing tests
of two specimens, one of which is uniform throughout its length and the other which contains the joint or other irregularity within
the test section. The difference in heat loss between these two tests, corrected for the uniform area covered by the joint or other
irregularity, is the extra heat loss introduced. Care must be taken that the tests are performed under the same conditions of pipe
and ambient temperature and that sufficient length exists between the joint or irregularity and the test section ends to prevent
appreciable end loss.
4.7 For satisfactory results in conformance with this test method, the principles governing construction and use of apparatus
described in this test method must be followed. If the results are to be reported as having been obtained by this test method, then
all the pertinent requirements prescribed in this test method shall be met or any exceptions shall be described in the report.
4.8 It is not practical in a test method of this type to establish details of construction and procedure to cover all contingencies that
might offer difficulties to a person without technical knowledge concerning the theory of heat flow, temperature measurements, and
general testing practices. Standardization of this test method does not reduce the need for such technical knowledge. It is
C335/C335M − 23
FIG. 1 Guarded-End Apparatus
recognized also that it would be unwise to restrict the further development of improved or new methods or procedures by research
workers because of standardization of this test method.
NOTE 4—When testing at ambient temperatures below normal room temperatures, theoretical analysis shows that the experimental heat flow direction is
unimportant for a perfectly homogenous material. However, if the properties of the insulation vary in the radical direction, the experimental heat flow
direction will significantly affect the measured thermal conductivity. Exercise great care when using data from a radial heat flow outward experiment for
a radial heat flow inward application.
5. Apparatus
5.1 The apparatus shall consist of the heated test pipe and instrumentation for measuring the pipe and insulation surface
temperatures, the average ambient air temperature, and the average power dissipated in the test section heater. The pipe shall be
uniformly heated by an internal electric heater (Notes 5 and 6). In large apparatus, give some consideration on providing internal
circulating fans or to filling the pipe with a heat transfer fluid to achieve uniform temperatures. The guarded end design also
requires, a short section of pipe at each end of the test section, with its own separately controlled heater (see 5.3 and Fig. 1). The
calibrated or calculated-end design requires suitable insulated caps at each end (see 5.4 and Fig. 2). An essential requirement of
the test is an enclosure or room equipped to control the temperature of the air surrounding the apparatus. The apparatus shall
conform to the principles and limitations prescribed in the following sections, but it is not intended in this test method to include
detailed requirements for the construction or operation of any particular apparatus.
NOTE 5—Experiments have been reported that use an electrically heated cylindrical screen rather than an internally heated pipe (5). An extension of the
heated screen technique has been reported (6) for testing below normal temperatures using the radial heat flow inward, similar to some insulation system
applications. While these designs and the accompanying analysis are not included in this test method, their findings are pertinent to this standard.
NOTE 6—The most commonly used heater consists of electrical resistance wire or ribbon on the surface or in the grooves of a tubular ceramic core that
is internal to the test pipe. If the heater fits snugly inside the test pipe, the contact must be uniform to achieve uniform test pipe temperatures. If the heater
core is smaller than the inside diameter of the pipe, then fill the gap with a material such as sand to provide uniform heat transfer. In this standard the
combination of heater winding and heater pipe will be called either a “heater” or a “heater pipe.”
5.2 Apparatus Pipe, no restriction is placed on the cross section size or shape, but the length of the test section must be sufficient
to ensure that the total measured heat flow is large enough, when compared to end losses and to the accuracy of the power
measurement, to achieve the desired test accuracy (see 5.3 and 8.4). A test section length of approximately 0.5 m [24 in.] has
proven satisfactory for an apparatus with a circular cross-section of 88.9 mm (nominal 80-mm, [nominal 3-in.] pipe size) that is
often used for inter-laboratory comparisons. Do not assume that this length is satisfactory for all sizes of apparatus and for all test
conditions. Estimates of the required length must be made from an appropriate error analysis. As a convenience, it is recommended
that the apparatus be constructed to accept an integral number of standard lengths of insulation.
C335/C335M − 23
FIG. 2 Calibrated or Calculated-End Apparatus
5.3 Guarded-End Apparatus (Fig. 1), uses separately heated pipe sections at each end of the test section to accomplish the
purposes of minimizing axial heat flow in the apparatus, of aiding in achieving uniform temperatures in the test section, and of
extending these temperatures beyond the test section length so that all heat flow in the test section is in the radial direction. Both
test and guard section heaters shall be designed to achieve uniform temperatures over the length of each section. This typically
requires the use of auxiliary heaters at the outside ends of single guards or the use of double guards.
5.3.1 The length of the guard section (or the combined length of double guards) shall be sufficient to limit at each end of the test
section the combined axial heat flow in both apparatus and specimen to less than 1% of the test section measured heat flow. A guard
section length of approximately 200 mm [4 in.] has been found satisfactory for apparatus of 88.9 mm ( nominal 80-mm [nominal
3-in.] pipe size) when testing specimens that are essentially homogeneous, are only moderately nonisotropic, and are of a thickness
not greater than the pipe diameter. Longer guard sections are usually required when testing thicker specimens or when the specimen
possesses a high axial conductance.
5.3.2 A gap shall be provided between the guards and the test section, and between each guard section if double-guarded, in both
the heater pipe and the test pipe (except for small bridges necessary for structural support). It is highly desirable that all support
bridges of high conductance be limited to the test pipe since any bridges in heater pipes or internal support members make it
difficult or impossible to achieve uniform surface temperatures while at the same time minimizing end losses in the apparatus.
Internal barriers shall be installed at each gap to minimize convection and radiation heat transfer between sections.
5.3.3 Thermocouples of wire as small as possible but not larger than 0.64 mm [0.025 in. (22 Awg)] and meeting the requirements
of 5.11, shall be installed in the test pipe surface on both sides of each gap, and not more than 25 mm [1 in.] from the gap, for
the purpose of monitoring the temperature difference across each gap. It is acceptable to connect the thermocouples in series and
use as a differential thermopile. Similar thermocouples shall also be installed on any heater pipes or support members that provide
a highly conductive path from test section to guard sections.
C335/C335M − 23
5.4 Calibrated or Calculated-End Apparatus (Fig. 2), uses insulated caps at each end of the test section to minimize axial heat
flow. The measured test section heat loss is then corrected for the end cap loss, that has been determined either by direct calibration
under the conditions of test (the calibrated-end apparatus) or by calculation, using known material properties (the calculated-end
apparatus). Internal electric heaters shall be provided to heat the test section uniformly over its length. It is usually necessary to
provide supplementary internal heaters at each end to compensate for the end heat loss. The power to such heaters must be included
in the measured test section power.
5.4.1 For the calibrated-end apparatus, the end caps shall be of the same cross-section as the test specimen and have approximately
the same thermal transfer properties. Each end cap shall have a cavity of minimum depth equal to one half the test pipe diameter
(or one half the major cross-section diagonal of noncircular pipes) and of a size and shape to accept the end of the test pipe. The
calibrator pipe shall consist of a short section of the same pipe used to construct the test pipe of a length equal to the combined
cavity depth of the two end caps. It shall be fitted with internal heaters similar to those used in the end sections of the test pipe
including any supplementary end heaters. A minimum of four thermocouples spaced 90° apart shall be provided in the surface of
the calibrator pipe to measure its temperature. They shall meet the requirements of 5.5.1 and be of a wire size as small as possible
but in no case larger than 0.64 mm diameter [0.025 in. (22 Awg)].
5.4.2 For the calculated-end apparatus, the end caps shall be as large or larger than the test specimen. They shall be made of
homogeneous insulation material of low conductivity. It is acceptable to have a cavity for the test pipe end. The thermal
conductivity of the end cap material shall be determined by Test Method C177 or Test Method C518 over the temperature range
of contemplated use. If the material is not isotropic, the thermal conductivity must be determined in different directions as needed.
5.5 Thermocouples, for measuring the surface temperature of the test pipe shall meet the requirements of 5.5.1 and be of a wire
size as small as possible, but in no case larger than 0.64 mm [0.025 in. (22 Awg)] in diameter. At least four thermocouples, or
one for each 150 mm of length of the test section, whichever is greater, shall be located to sense equally the temperature of all
areas of the test section surface. They shall be applied either by peening the individual wires into small holes drilled into the pipe
surface not more than 3 mm apart or by joining the wires by a welded bead and cementing them into grooves so that the bead is
tangent to the outer surface of the pipe, but does not project above the surface. For direct averaging, it is acceptable to connect
the thermocouples in parallel, provided their junctions are electrically isolated and the total resistances are essentially equal.
5.5.1 Thermocouples used for this method shall be made of special grade wire as specified in Tables E230 or shall be individually
calibrated to the same tolerance. Generally, thermocouples made from wire taken from the same spool will be found to agree with
each other within the required tolerance and thus only one calibration will be required from each spool of wire. Calibration must
extend for the lowest to the highest operating range of the apparatus.
5.6 Temperature-Measuring System, excluding the sensor, with an accuracy equivalent to 60.1 K. A d-c potentiometer or digital
microvoltmeter is normally used for thermocouple readout.
5.7 Power Supplies, use a closely regulated a-c or d-c supply for operating the test section heater. Power supplies for guard heaters,
if used, need not be regulated if automatic controllers are used.
5.8 Power-Measuring System, capable of measuring the average power to the test section heater with an accuracy of 6 0.5% shall
be provided. If power input is steady, this is typically a calibrated wattmeter or a voltage-measuring system for voltage and
amperage (using a standard resistance). If power input is variable or fluctuating, an integrating type of power measurement, using
an integrating period long enough to assure a reliable determination of average power, is required. In all cases, care must be taken
that the measured power is only that dissipated in the test section. This requires that corrections be applied for power dissipated
in leads, dropping resistors, or uncompensated wattmeters.
5.9 For a given set of observations as defined in 8.4 the ambient air temperature shall be maintained within 6 1% of the smallest
temperature difference between the test pipe and the ambient or to 6 1°C [62°F], whichever is greater. The apparatus shall be
located in a region of essentially still air and shall not be close to other objects that would alter the pattern of natural convection
around the heated specimen. All surfaces or objects that exchange radiation with the specimen shall have a total hemispherical
Any temperature-measuring sensor can be used, but thermocouples are used predominantly.
C335/C335M − 23
emittance of at least 0.85 and shall be at approximately the same temperature as the ambient air. Additional optional equipment
is required to use gases other than air and to simulate wind effects by establishing forced air velocities of the direction and
magnitude desired.
5.10 An optional temperature-controlled jacket is an acceptable procedure to control the outer surface of the specimen to a
temperature different than that of the ambient air. An alternative procedure for raising the outer surface temperature of a specimen
is to surround it with an additional layer of thermal insulation. In either case the thermocouples specified in 6.5 for the
measurement of the specimen outer surface temperature must be installed prior to placement of the jacket or additional insulation
layer. Moreover, the emittance of the inner surface of the jacket or added insulation (facing the specimen) must be greater than
0.8 in order not to reduce any radiation transfer within the specimen. In such cases it is not possible to measure directly the thermal
transference for the specimen.
6. Test Specimen
6.1 Specimens types include rigid, semi-rigid, or flexible (blanket-type), or loose-fill, suitably contained. Specimens shall be
uniform in size and shape throughout their length (except for any intentional irregularities that occur well within the test section)
and shall be designed for use on pipes of the same size and shape as the available test apparatus.
6.2 If test results are to be considered as representative of a type of product or of a particular production lot, etc., or of a material
(in the case of homogeneous materials), then appropriate sampling plans must be followed. In the absence of such plans, the test
results can be considered to represent only the specimens tested.
6.3 The intended purpose of the test must be considered in determining details of the specimen and its applications to the test pipe
(Note 7). Some considerations are:
6.3.1 The means of securing the specimen to the test pipe.
6.3.2 The use of sealants or other materials in the joints.
6.3.3 Whether jackets, covers, bands, reflective sheaths, etc., are included.
6.3.4 For the testing of reflective insulation, there are additional considerations. It is recommended that at least two insulation
sections be mounted within the central test section. While the use of full length specimens within the central test section is
preferred, this may not be practical within the limits of existing apparatus. Air exchange must not occur between the test and guard
sections. Install a fibrous or other airtight, low conductivity, nonmetallic insulation seal, not more than 25 mm wide, between the
hot pipe and specimen inner casing to prevent air exchange within this annular space. This seal must be installed in the guard region
adjacent to the guard gap and not in the central test section.
NOTE 7—Unless another purpose is intended, secure the specimen to the test pipe in accordance with normal application practice. Include jackets and other
features when desired.
6.4 After the specimen is mounted on the test pipe, measurements of the outside dimensions needed to describe the shape shall
be made to within 6 0.5% (both before and after testing). For circular shapes, use a flexible steel tape to measure the circumference
then divide by 2π to obtain the radius r . The test section length shall be divided into at least four equal parts, and dimension
measurements shall be taken at the center of each, except that any irregularity being investigated shall be avoided. Additional
measurements shall be taken to describe the irregularities. For guarded-end apparatuses, additional measurements at the center of
each guard section are also required. Specimens intended to be of uniform cross section dimensions throughout their length must
be rejected if any individual dimension measurement (test section or guard) differs from the average of the test section
measurements by more than 5%.
6.5 Thermocouples for the measurement of the average outside surface temperature, t , shall be attached to the insulation surface
in accordance with the following:
6.5.1 The test section length shall be divided into at least four equal parts and surface thermocouples shall be longitudinally
located at the center of each. Large apparatuses will require a greater number of thermocouples. For circular shapes, the
thermocouples shall also be circumferentially equally spaced to form helical patterns with an integral number of complete
revolutions and with the angular spacing between adjacent locations from 45 to 90°. For non-circular shapes, the thermocouples
C335/C335M − 23
shall be spaced around in much the same manner but located to obtain an area-weighted average. Any of the above specified
locations shall, whenever possible, be offset a distance equal to the specimen thickness from any joint or other irregularity, and
additional thermocouples shall be used as necessary to record the surface temperature. In such situations the individual
temperatures and locations shall be reported (see 11.1.6).
6.5.2 Thermocouples shall be made of wire not larger than0.40 mm [0.016 in. (26 Awg)] and shall meet the requirements of 5.5.1.
They shall be fastened to the surface by any means that will hold the junction and the required length of adjacent wire in intimate
thermal contact with the surface but does not alter the radiation emittance characteristics of the adjacent surface.
6.5.2.1 For nonmetallic surfaces, a minimum of 100 mm [4 in.] of adjacent wire shall be held in contact with the surface. One
satisfactory method of fastening is to use masking tape either adhered to the specimen surface or wrapped around the specimen
and adhered to itself.
6.5.2.2 For metallic surfaces, a minimum of 10 mm of adjacent lead wire shall be held in contact with the surface. Acceptable
means of fastening thermocouple junctions are by peening, welding, soldering or brazing, or by use of metallic tape of the same
emittance as the surface. Capacitive discharge welding is especially recommended. Small thin strips of metal similar to the surface
metal shall be welded to the surface to hold the lead wire in contact with the surface.
6.5.3 The average surface temperature is calculated by averaging the individual readings of the surface thermocouples. If desired,
measure the average by directly connecting the thermocouples in parallel, provided that the junctions are electrically isolated and
the total electrical resistances are essentially equal.
6.6 Thermocouples meeting the requirements of 5.5.1 shall be installed on elements of high axial heat conductance such as
metallic jackets or accessible liners (specimens with such elements must be tested on a guarded-end apparatus) in order to measure
axial temperature gradients needed to compute axial heat transfer. These thermocouples shall be installed at both top and bottom
locations, and shall be located an equal distance of approximately 45 mm [0.08] on each side of the gap between the test section
and each guard.
7. Conditioning
7.1 In general, specimens shall be dried or otherwise conditioned to stable conditions immediately prior to test unless it has been
shown that such procedures are unnecessary to achieve reproducible results for the material being tested. When applicable, follow
the conditioning procedures of the materials specification ; specification; otherwise, the normal procedure is to dry to constant
weight at a temperature of 102 to 120°C [215 to 250 °F] , °F], unless the specimen is adversely affected, in which case drying in
a desiccator from 55 to 60°C [130 to 140 °F] is recommended (see Practice C870). When desired, report Report any weight
changes due to conditioning. Determine specimen density by Test Method C302. or report the method used to calculate the density.
7.2 During the experimentation, operate the apparatus in a controlled room or enclosure so that the ambient temperature does not
vary during a test by more than 6 1°C [6 2°] or 6 1% of the difference between the test pipe and the ambient (t − t ), whichever
o a
is greater. Run the test in essentially still air (or other desired gas) unless appreciable velocity is needed to attain uniform
temperatures or when the effect of air velocity is to be included as part of the test conditions. Measure any forced velocity and
report its magnitude and direction.
8. Procedure
8.1 Measure the test section length, L, and the specimen outside circumference or other dimensions needed to describe the shape.
Normally dimensions used in this method shall be those measured at ambient temperatures of 10 to 35°C. If properties based upon
actual dimensions at operating temperature are desired, determine the dimensions by calculation from those measured at ambient
temperature using previously measured or known coefficients of thermal expansion, or directly measure the dimensions at
operating temperature. Any properties based upon dimensions at operating temperature must be so identified.
8.1.1 For guarded-end pipes, the test length, L, is the distance between the centerlines of the gaps at the ends of the test section.
For calibrated or calculated-end pipes, the test length, L, is the distance between the end caps.
8.1.2 Take outside dimensions of the specimen at locations described in 6.4.
8.2 Adjust the temperature of the test pipe (or the test section of a guarded-end apparatus) to the desired temperature. Refer to
Practice C1058 for recommended temperatures.
C335/C335M − 23
8.3 When using the guarded-end method, adjust the temperature of each guard so that the temperature difference across the gap
between the test section and the guard (measured on the surface of the test pipe) is zero or not greater than the amount that will
introduce an error of 1% in the measured heat flow. Ideally, the axial temperature gradient across the gaps between the test and
guard sections of both the outer test pipe and the internal heater pipe and along any internal support members is zero to eliminate
all axial heat flow within the pipe. In some designs, it is impossible to balance both surface and internal elements at the same time,
and it will be necessary to correct for internal apparatus axial losses. When the only support bridges are in the outer test pipe, it
is sufficient to bring the test pipe surface gap balance (between test section and guards) to zero and no corrections are needed. When
the apparatus uses internal support bridges, it is necessary to use the readings of the internal thermocouples specified in 5.3, along
with the known dimensions and properties of the support bridges, to estimate the internal axial losses that must be added to (or
subtracted from) the measured power input to the test section. In either case it is often desirable to run two tests, one with the
temperature of the guards slightly higher than the test section and one with it slightly lower. Interpolation between these gives an
accurate value for the zero balance heat flow along the internal bridges and for the test section power input and provides
information on the maximum allowable imbalance that still meets the 1% criterion. One criterion which has often been used is that
the allowable imbalance is no greater than 0.5% of the temperature drop through the specimen, (t − t ). This must be verified
2 1
using the above procedure at the conditions of the test.
8.3.1 When evaluating reflective insulation, measure the temperature gradients along the interior and exterior casings with
thermocouples detailed in 6.6. Compute the axial heat conduction along the inner and outer casings from the average gradients for
that casing. Using the average of the four gradients, compute the total axial heat conduction for all internal liners. The total axial
heat flow for each end of the test section shou
...