ASTM C835-00
(Test Method)Standard Test Method for Total Hemispherical Emittance of Surfaces up to 1400°C
Standard Test Method for Total Hemispherical Emittance of Surfaces up to 1400°C
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.
1.2 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 and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see Section 7.
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Designation: C 835 – 00
Standard Test Method for
Total Hemispherical Emittance of Surfaces From 20 to
1400°C
This standard is issued under the fixed designation C 835; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope
s = Stefan-Boltzmann constant,
−8 2 4
= 5.669 3 10 W/m ·K ,
1.1 This calorimetric test method covers the determination
Q = heat flow rate, W,
of total hemispherical emittance of metal and graphite surfaces
T = temperature of heated specimen, K,
and coated metal surfaces from approximately 20 to 1400°C.
T = temperature of bell jar inner surface, K,
The upper-use temperature is limited only by the characteris-
A = surface area of specimen over which heat generation
tics (for example, melting temperature, vapor pressure) of the
is measured, m ,
specimen and the design limits of the test facility. This test
A = surface area of bell jar inner surface, m ,
method has been demonstrated for use up to 1400°C.
F = the gray body shape factor, which includes the effect
1.2 This standard does not purport to address all of the
of geometry and the departure of real surfaces from
safety concerns, if any, associated with its use. It is the
blackbody conditions, dimensionless, and
responsibility of the user of this standard to establish appro- 2
Pa = absolute pressure, pascal (N/m ). One pascal is
priate safety and health practices and determine the applica-
equivalent to 0.00750 mm Hg.
bility of regulatory limitations prior to use. For specific hazard
statements, see Section 7.
4. Summary of Test Method
4.1 A strip specimen of the material, approximately 13 mm
2. Referenced Documents
wide and 250 mm long, is placed in an evacuated chamber and
2.1 ASTM Standards:
is directly heated with an electric current to the temperature at
C 168 Terminology Relating to Thermal Insulating Materi-
which the emittance measurement is desired. The power
als
dissipated over a small central region of the specimen and the
E 230 Temperature-Electromotive Force (EMF) Tables for
temperature of this region are measured. Using the Stefan-
Standardized Thermocouples
Boltzmann equation, this power is equated to the radiative heat
E 691 Practice for Conducting an Interlaboratory Study to
transfer to the surroundings and, with the measured tempera-
Determine the Precision of a Test Method
ture, is used to calculate the value of the total hemispherical
emittance of the specimen surface.
3. Terminology
3.1 Definitions—The terms and symbols are as defined in
5. Significance and Use
Terminology C 168 with exceptions included as appropriate.
5.1 The emittance as measured by this test method can be
3.2 Symbols:Symbols:
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.
e = error in the variable i, 6 %,
i
5.2 This test method can be used to determine the effect of
e = total hemispherical emittance of heated specimen,
service conditions on the emittance of materials. In particular,
dimensionless,
the use of this test method with furnace exposure (time at
e = total hemispherical emittance of bell jar inner sur-
temperature) of the materials commonly used in all-metallic
face, dimensionless,
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
This test method is under the jurisdiction of ASTM Committee C-16 on
will provide emittance values that are applicable to materials
Thermal Insulation and is the direct responsibility of Subcommittee C16.30 on
Thermal Measurements. used under other conditions, such as in an air environment.
Current edition approved June 15, 1995. Published August 1995. Originally
However, it must be recognized that surface properties of
e1
published as C 835 – 76. Last previous edition C 835 – 82 (1988) .
materials used in air or other atmospheres may be different. In
Annual Book of ASTM Standards, Vol 04.06.
addition, preconditioned surfaces, as described in 5.2, may be
Annual Book of ASTM Standards, Vol 14.03.
Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
C 835
altered in a vacuum environment because of vacuum stripping and current measurements, thermocouples and voltmeter or
of absorbed gases and other associated vacuum effects. Thus, other readout, vacuum system, and specimen holders. A sche-
emittances measured under vacuum may have values that differ matic of the test arrangement is shown in Fig. 1. Means must
from those that exist in air, and the user must be aware of this be provided for electrically heating the specimen, and instru-
situation. With these qualifications in mind, emittance obtained ments are required to measure the electrical power input to the
by this test method may be applied to predictions of thermal specimen and the temperatures of the specimen and surround-
transference. ing surface.
5.4 Several assumptions are made in the derivation of the 6.2 Bell Jar:
emittance calculation as described in this test method. They are 6.2.1 The bell jar may be either metal or glass with an inner
that: surface that presents a blackbody environment to the specimen
5.4.1 The enclosure is a blackbody emitter at a uniform located near the center. This blackbody effect is achieved by
temperature, providing a highly absorbing surface and by making the
5.4.2 The total hemispherical absorptance of the completely surface area much larger than the specimen surface area. The
diffuse blackbody radiation at the temperature of the enclosure relationship between bell jar size and its required surface
is equal to the total hemispherical emittance of the specimen at emittance is estimated from the following equation for the gray
its temperature, and body shape factor for a surface completely enclosed by another
5.4.3 There is no heat loss from the test section by convec- surface:
tion or conduction. For most materials tested by the procedures
F 5 (1)
as described in this test method, the effects of these assump-
1 A 1
1 2 1
S D
tions are small and either neglected or corrections are made to
e A e
1 2 2
the measured emittance.
5.5 For satisfactory results in conformance with this test
For this test method to apply, the following condition must
method, the principles governing the size, construction, and
exist:
use of apparatus described in this test method should be
1 A 1
followed. If these principles are followed, any measured value
.. 2 1 (2)
S D
e A e
1 2 2
obtained by the use of this test method is expected to be
This condition can be satisfied for all possible values of
accurate to within 65 %. If the results are to be reported as
specimen emittance by an apparatus design in which A /A has
having been obtained by this test method, all of the require- 1 2
a value less than 0.01 and e has a value greater than 0.8. To
ments prescribed in this test method shall be met. 2
ensure that the inner surface has an emittance greater than 0.8,
5.6 It is not practical in a test method of this type to
metal and glass bell jars shall be coated with a black paint (1).
establish details of construction and procedure to cover all
It is permissible to leave small areas in the glass bell jars
contingencies that might offer difficulties to a person without
uncoated for visual monitoring of the specimen during a test.
technical knowledge concerning the theory of heat transfer,
Metal bell jars can be provided with small-area glass view
temperature measurements, and general testing practices. Stan-
ports for sample observation.
dardization of this test method does not reduce the need for
6.2.2 The bell jar must be opaque to external high energy
such technical knowledge. It is recognized also that it would be
radiation sources (such as open furnaces, sunlight, and other
unwise to restrict in any way the development of improved or
emittance apparatuses) if they are in view of the specimen.
new methods or procedures by research workers because of
Both the coated metal and coated glass bell jars meet this
standardization of this test method.
requirement.
6. Apparatus
6.1 In general, the apparatus shall consist of the following
The boldface numbers in parenthesses refer to the list of references at the end
equipment: a bell jar, power supply and multi-meter for voltage of this standard.
FIG. 1 System Arrangement
C 835
6.2.3 The need for bell jar cooling is determined by the ometer, or equivalent instrument, having a sensitivity of 2μV or
lower-use temperature of the particular apparatus and by the less is required for measuring the thermocouple emf’s from
maximum natural heat dissipation of the bell jar. A bell jar which the test section temperatures are obtained.
operating at room temperature (20°C) may be used for speci-
6.4.4 Temperature sensors must be calibrated to within the
men temperatures down to about 120°C. At least a 100°C
uncertainty allowed by the apparatus design accuracy. For
difference between the specimen and the bell jar is recom-
information concerning sensitivity and accuracy of thermo-
mended to achieve the desired method accuracy. Therefore, for
couples, see Table 1 of Tables E 230. For a comprehensive
lower specimen temperatures, bell jar cooling is required. If the
discussion on the use of thermocouples, see Ref (2). For low
natural heat dissipation of the bell jar is not sufficient to
temperature thermocouple reference tables, see Ref (3).
maintain its temperature at the desired level for any other
6.5 Vacuum System— A vacuum system is required to
operating condition, auxiliary cooling of the bell jar is also
reduce the pressure in the bell jar to 1.3 mPa or less to
required.
minimize convection and conduction through the residual gas.
6.3 Power Supply— The power supply may be either ac or
This effect is illustrated in Fig. 2, which shows the measured
dc and is used to heat the test specimen electrically by making
emittance of oxidized Inconel versus system pressure. This
it a resistive part of the circuit. The true electrical power to the
curve is based upon the assumption that all heat transfer from
test section must be measured within a proven uncertainty of6
the specimen is by radiation. As pressure increases, gas
1 % or better.
conduction becomes important.
6.4 Thermocouples, are used for measuring the surface
6.5.1 For the specified pressure level, a pumping system
temperature of the specimen. The thermocouple materials must
consisting of a diffusion or ion pump and mechanical pump is
have a melting point significantly above the highest test
required. If backstreaming is a problem, cold trapping is
temperature of the specimen. To minimize temperature mea-
required. The specifications of an existing system are included
surement errors due to wire conduction losses, the use of
in Table 1 and photographs of a system are included in Fig. 3
high-thermal conductivity materials such as copper should be
and Fig. 4. This information is included as a guide to assist in
avoided. The size of the thermocouple wire should be the
the design of a facility and is not intended to be a rigid
minimum practical. Experience indicates that diameters less
specification.
than 0.13 mm provide acceptable results.
6.5.2 The specified pressure (1.3 mPa or less) must exist in
6.4.1 The test section is defined by two thermocouples
the bell jar. If measured elsewhere in the pumping system, such
equally spaced from the specimen holders. A third thermo-
as in the diffusion pump inlet, the pressure drop between the
couple is located at the center of the specimen. Spot welding
measuring location and the bell jar must be accounted for. The
has been found to be the most acceptable method of attachment
vacuum system should also be checked for gross leakage that
because it results in minimum disturbance of the specimen
could allow incoming gas to sweep over the specimen.
surface. Swaging and peening are alternative methods pre-
6.6 Specimen Holders, must be designed to allow for
scribed for specimens that do not permit spot welding.
thermal expansion of the specimen without buckling. The
6.4.2 The number of thermocouples used to measure the
lower specimen holder shown in Fig. 4 is designed to move up
temperature of the absorbing surface shall be sufficient to
and down in its support to allow for thermal expansion.Holders
provide a representative average. Four thermocouples have
should be positioned off-center within the bell jar to minimize
been found to be sufficient for the system shown in Fig. 1.
normal reflections between the specimen and bell jar inner
Thermocouple locations include three on the bell jar and one
surface. Specimen holders require auxiliary cooling if end
on the baseplate.
conduction from the specimen causes overheating.
6.4.3 The voltage drop in the measurement area of the
specimen is measured by tapping to similar elements of each of 6.7 Micrometer Calipers, or other means are needed to
the two thermocouples that bound the test section. A potenti- measure the dimensions (width and thickness) of the test
FIG. 2 Example of Effect of Air Pressure on Measured Emittance of Oxidized Inconel
C 835
TABLE 1 Specifications for the Emittance Test Facility Shown in
Figs. 3 and 4
Vacuum system:
A manual vacuum coater system
Vacuum pumps consisting of an 0.8-m /s diffusion pump (100-mm inlet)
backed
by an 0.0023-m /s mechanical pump
A glass bell jar, 0.46 m in diameter by 0.91 m high with an implosion shield
Vacuum gaging, including two thermocouple-type roughing gages and an ioni-
zation gage
A specimen holder having a movable lower clamp to allow for thermal
expansion
A liquid nitrogen cold trap
Power Supply:
Output voltage—0 to 16 V
Maximum current—100 A
Sample Temperature Range:
Maximum 20 to 1400°C
Sample Size:
Nominal—0.25 by 13 by 250 mm
Maximum length—500 mm
Power Measurement:
Current is determined by measurement of voltage across a precision-
calibrated
resistor (0 to 100 A)
Voltage is measured by a digital voltmeter.
specimen and the length between voltage taps and thermo-
couples at room temperature. The specimen dimensions (width
and thickness) should be measured to the nearest 0.025 mm.
The length between voltage taps should be measured to the
nearest 0.5 mm. The length between thermocoupl
...
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