ASTM C835 - 06 (2020)

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Designation: C835 −06 (Reapproved 2020)
Standard Test Method for
Total Hemispherical Emittance of Surfaces up to 1400°C
1
This standard is issued under the ﬁxed designation C835; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This calorimetric test method covers the determination
of total hemispherical emittance of metal and graphite surfaces
and coated metal surfaces up to approximately 1400°C. The
upper-use temperature is limited only by the characteristics (for
example, melting temperature, vapor pressure) of the specimen
and the design limits of the test facility. This test method has
been demonstrated for use up to 1400 °C. The lower-use
temperature is limited by the temperature of the bell jar.
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
standard.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
For speciﬁc hazard statements, see Section 7.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
2
C168 Terminology Relating to Thermal Insulation
E230 Speciﬁcation for Temperature-Electromotive Force
(emf) Tables for Standardized Thermocouples
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
3. Terminology
3.1 Deﬁnitions—The terms and symbols are as deﬁned in
Terminology C168 with exceptions included as appropriate.
3.2 Symbols:
e
i
= error in the variable i,6%,
ε
1
= total hemispherical emittance of heated specimen,
dimensionless,
ε
2
= total hemispherical emittance of bell jar inner surface,
dimensionless,
σ= Stefan-Boltzmann constant,
= 5.669 × 10
−8
W/m
2
·K
4
,
Q= heat ﬂow rate, W,
T
1
= temperature of heated specimen, K,
T
2
= temperature of bell jar inner surface, K,
A
1
= surface area of specimen over which heat generation is
measured, m
2
,
A
2
= surface area of bell jar inner surface, m
2
,
F= the gray body shape factor, which includes the effect
of geometry and the departure of real surfaces from
blackbody conditions, dimensionless, and
Pa = absolute pressure, pascal (N/m
2
). One pascal is
equivalent to 0.00750 mm Hg.
4. Summary of Test Method
4.1 A strip specimen of the material, approximately 13 mm
wide and 250 mm long, is placed in an evacuated chamber and
is directly heated with an electric current to the temperature at
which the emittance measurement is desired. The power
dissipated over a small central region of the specimen and the
temperature of this region are measured. Using the Stefan-
Boltzmann equation, this power is equated to the radiative heat
transfer to the surroundings and, with the measured
temperature, is used to calculate the value of the total hemi-
spherical emittance of the specimen surface.
5. Signiﬁcance and Use
5.1 The emittance as measured by this test method can be
used in the calculation of radiant heat transfer from surfaces
that are representative of the tested specimens, and that are
within the temperature range of the tested specimens.
5.2 This test method can be used to determine the effect of
service conditions on the emittance of materials. In particular,
the use of this test method with furnace exposure (time at
1
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 March 1, 2020. Published March 2020. Originally
approved in 1976. Last previous edition approved in 2013 as C835 – 06 (2013)
ε1
.
DOI: 10.1520/C0835-06R20.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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.

temperature) of the materials commonly used in all-metallic

insulations can determine the effects of oxidation on emittance.

5.3 The measurements described in this test method are

conducted in a vacuum environment. Usually this condition

will provide emittance values that are applicable to materials

used under other conditions, such as in an air environment.

However, it must be recognized that surface properties of

materials used in air or other atmospheres may be different. In

addition, preconditioned surfaces, as described in 5.2, may be

altered in a vacuum environment because of vacuum stripping

of absorbed gases and other associated vacuum effects. Thus,

emittances measured under vacuum may have values that differ

from those that exist in air, and the user must be aware of this

situation. With these qualiﬁcations in mind, emittance obtained

by this test method may be applied to predictions of thermal

transference.

5.4 Several assumptions are made in the derivation of the

emittance calculation as described in this test method. They are

that:

5.4.1 The enclosure is a blackbody emitter at a uniform

temperature,

5.4.2 The total hemispherical absorptance of the completely

diffuse blackbody radiation at the temperature of the enclosure

is equal to the total hemispherical emittance of the specimen at

its temperature, and

5.4.3 There is no heat loss from the test section by convec-

tion or conduction. For most materials tested by the procedures

as described in this test method, the effects of these assump-

tions are small and either neglected or corrections are made to

the measured emittance.

5.5 For satisfactory results in conformance with this test

method, the principles governing the size, construction, and

use of apparatus described in this test method should be

followed. If these principles are followed, any measured value

obtained by the use of this test method is expected to be

accurate to within 65 %. If the results are to be reported as

having been obtained by this test method, all of the require-

ments prescribed in this test method shall be met.

5.6 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 transfer,

temperature measurements, and general testing practices. Stan-

dardization of this test method does not reduce the need for

such technical knowledge. It is recognized also that it would be

unwise to restrict in any way the development of improved or

new methods or procedures by research workers because of

standardization of this test method.

6. Apparatus

6.1 In general, the apparatus shall consist of the following

equipment: a bell jar, power supply and multi-meter for voltage

and current measurements, thermocouples and voltmeter or

other readout, vacuum system, and specimen holders. A

schematic of the test arrangement is shown in Fig. 1. Means

must be provided for electrically heating the specimen, and

instruments are required to measure the electrical power input

to the specimen and the temperatures of the specimen and

surrounding surface.

6.2 Bell Jar:

6.2.1 The bell jar may be either metal or glass with an inner

surface that presents a blackbody environment to the specimen

located near the center. This blackbody effect is achieved by

providing a highly absorbing surface and by making the

surface area much larger than the specimen surface area. The

relationship between bell jar size and its required surface

emittance is estimated from the following equation for the gray

body shape factor for a surface completely enclosed by another

surface:

F51

(1)

For this test method to apply, the following condition must

exist:

..A

(2)

This condition can be satisﬁed for all possible values of

specimen emittance by an apparatus design in which A

has

a value less than 0.01 and ε

has a value greater than 0.8. To

FIG. 1 System Arrangement

C835 − 06 (2020)

ensure that the inner surface has an emittance greater than 0.8,

metal and glass bell jars shall be coated with a black paint (1).

It is permissible to leave small areas in the glass bell jars

uncoated for visual monitoring of the specimen during a test.

Metal bell jars can be provided with small-area glass view

ports for sample observation.

6.2.2 The bell jar must be opaque to external high energy

radiation sources (such as open furnaces, sunlight, and other

emittance apparatuses) if they are in view of the specimen.

Both the coated metal and coated glass bell jars meet this

requirement.

6.2.3 The need for bell jar cooling is determined by the

lower-use temperature of the particular apparatus and by the

maximum natural heat dissipation of the bell jar. A bell jar

operating at room temperature (20°C) may be used for speci-

men temperatures down to about 120°C. At least a 100°C

difference between the specimen and the bell jar is recom-

mended to achieve the desired method accuracy. Therefore, for

lower specimen temperatures, bell jar cooling is required. If the

natural heat dissipation of the bell jar is not sufficient to

maintain its temperature at the desired level for any other

operating condition, auxiliary cooling of the bell jar is also

required. An alternative to bell jar cooling is the use of a cooled

shroud (for example, cooled by liquid nitrogen) between the

specimen and the bell jar.

6.3 Power Supply— The power supply may be either ac or

dc and is used to heat the test specimen electrically by making

it a resistive part of the circuit. The true electrical power to the

test section must be measured within a proven uncertainty of6

1 % or better.

6.4 Thermocouples, are used for measuring the surface

temperature of the specimen. The thermocouple materials must

have a melting point signiﬁcantly above the highest test

temperature of the specimen. To minimize temperature mea-

surement errors due to wire conduction losses, the use of

high-thermal conductivity materials such as copper should be

avoided. The size of the thermocouple wire should be the

minimum practical. Experience indicates that diameters less

than 0.13 mm provide acceptable results.

6.4.1 The test section is deﬁned by two thermocouples

equally spaced from the specimen holders. A third thermo-

couple is located at the center of the specimen. Spot welding

has been found to be the most acceptable method of attachment

because it results in minimum disturbance of the specimen

surface. Swaging and peening are alternative methods pre-

scribed for specimens that do not permit spot welding.

6.4.2 The number of thermocouples used to measure the

temperature of the absorbing surface shall be sufficient to

provide a representative average. Four thermocouples have

been found to be sufficient for the system shown in Fig. 1.

Thermocouple locations include three on the bell jar and one

on the baseplate.

6.4.3 The voltage drop in the measurement area of the

specimen is measured by tapping to similar elements of each of

the two thermocouples that bound the test section. A

potentiometer, or equivalent instrument, having a sensitivity of

2µV or less is required for measuring the thermocouple emf’s

from which the test section temperatures are obtained.

6.4.4 Temperature sensors must be calibrated to within the

uncertainty allowed by the apparatus design accuracy. For

information concerning sensitivity and accuracy of

thermocouples, see Table 1 of Tables E230. For a comprehen-

sive discussion on the use of thermocouples, see Ref (2). For

low temperature thermocouple reference tables, see Ref (3).

6.5 Vacuum System— A vacuum system is required to

reduce the pressure in the bell jar to 1.3 mPa or less to

minimize convection and conduction through the residual gas.

This effect is illustrated in Fig. 2, which shows the measured

emittance of oxidized Inconel versus system pressure. This

curve is based upon the assumption that all heat transfer from

the specimen is by radiation. As pressure increases, gas

conduction becomes important.

6.5.1 For the speciﬁed pressure level, a pumping system

consisting of a diffusion or ion pump and mechanical pump is

required. If backstreaming is a problem, cold trapping is

required. The speciﬁcations of an existing system are included

in Table 1 and photographs of a system are included in Fig. 3

The boldface numbers in parentheses refer to the list of references at the end of

this standard.

FIG. 2 Example of Effect of Air Pressure on Measured Emittance of Oxidized Inconel

C835 − 06 (2020)

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摘要：

ASTM C835 - 06 (2020) 是一项由美国材料与试验协会发布的标准化测试方法，专门用于测定材料在高温条件下的总半球发射率。该标准适用于金属、涂层及其他固体材料，通过稳态热流法在真空或可控气氛环境中进行测量，以评估材料在高温环境中的热辐射性能。总半球发射率是衡量材料表面辐射热能能力的关键参数，广泛应用于航空航天、能源、军工及高温工业设备领域，如热防护系统、隔热涂层和炉衬材料的性能验证。ASTM C835 - 06 (2020) 在原先2006年版基础上进行了确认与更新，确保其测试程序

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