ASTM C835-01(2006)
(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 – 01 (Reapproved 2006)
Standard Test Method for
Total Hemispherical Emittance of Surfaces up to 1400°C
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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope
Q = heat flow rate, W,
T = temperature of heated specimen, K,
1.1 This calorimetric test method covers the determination 1
T = temperature of bell jar inner surface, K,
oftotalhemisphericalemittanceofmetalandgraphitesurfaces 2
A = surface area of specimen over which heat generation
and coated metal surfaces up to approximately 1400°C. The
is measured, m ,
upper-usetemperatureislimitedonlybythecharacteristics(for
A = surface area of bell jar inner surface, m ,
example,meltingtemperature,vaporpressure)ofthespecimen
F = the gray body shape factor, which includes the effect
and the design limits of the test facility. This test method has
of geometry and the departure of real surfaces from
been demonstrated for use up to 1400°C.
blackbody conditions, dimensionless, and
1.2 This standard does not purport to address all of the
Pa = absolute pressure, pascal (N/m ). One pascal is
safety concerns, if any, associated with its use. It is the
equivalent to 0.00750 mm Hg.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Summary of Test Method
bility of regulatory limitations prior to use. For specific hazard
4.1 Astrip specimen of the material, approximately 13 mm
statements, see Section 7.
wide and 250 mm long, is placed in an evacuated chamber and
is directly heated with an electric current to the temperature at
2. Referenced Documents
which the emittance measurement is desired. The power
2.1 ASTM Standards:
dissipated over a small central region of the specimen and the
C168 Terminology Relating to Thermal Insulation
temperature of this region are measured. Using the Stefan-
E230 Specification and Temperature-Electromotive Force
Boltzmannequation,thispowerisequatedtotheradiativeheat
(EMF) Tables for Standardized Thermocouples
transfer to the surroundings and, with the measured tempera-
E691 Practice for Conducting an Interlaboratory Study to
ture, is used to calculate the value of the total hemispherical
Determine the Precision of a Test Method
emittance of the specimen surface.
3. Terminology
5. Significance and Use
3.1 Definitions—The terms and symbols are as defined in
5.1 The emittance as measured by this test method can be
Terminology C168 with exceptions included as appropriate.
used in the calculation of radiant heat transfer from surfaces
3.2 Symbols:
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
e = error in the variable i, 6 %,
i
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, insulationscandeterminetheeffectsofoxidationonemittance.
s = Stefan-Boltzmann constant,
5.3 The measurements described in this test method are
−8 2 4
= 5.669 310 W/m ·K ,
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.
ThistestmethodisunderthejurisdictionofASTMCommitteeC16onThermal
However, it must be recognized that surface properties of
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
materials used in air or other atmospheres may be different. In
Measurements.
addition, preconditioned surfaces, as described in 5.2, may be
Current edition approved March 1, 2006. Published March 2006. Originally
altered in a vacuum environment because of vacuum stripping
approved in 1976. Last previous edition approved in 2001 as C835–01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
of absorbed gases and other associated vacuum effects. Thus,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
emittancesmeasuredundervacuummayhavevaluesthatdiffer
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.
C 835 – 01 (2006)
from those that exist in air, and the user must be aware of this schematic of the test arrangement is shown in Fig. 1. Means
situation.Withthesequalificationsinmind,emittanceobtained must be provided for electrically heating the specimen, and
by this test method may be applied to predictions of thermal instruments are required to measure the electrical power input
transference. to the specimen and the temperatures of the specimen and
5.4 Several assumptions are made in the derivation of the surrounding surface.
emittancecalculationasdescribedinthistestmethod.Theyare 6.2 Bell Jar:
that: 6.2.1 The bell jar may be either metal or glass with an inner
5.4.1 The enclosure is a blackbody emitter at a uniform surface that presents a blackbody environment to the specimen
temperature, located near the center. This blackbody effect is achieved by
5.4.2 Thetotalhemisphericalabsorptanceofthecompletely providing a highly absorbing surface and by making the
diffuse blackbody radiation at the temperature of the enclosure surface area much larger than the specimen surface area. The
isequaltothetotalhemisphericalemittanceofthespecimenat relationship between bell jar size and its required surface
its temperature, and emittanceisestimatedfromthefollowingequationforthegray
5.4.3 There is no heat loss from the test section by convec- bodyshapefactorforasurfacecompletelyenclosedbyanother
tionorconduction.Formostmaterialstestedbytheprocedures surface:
as described in this test method, the effects of these assump-
F 5 (1)
tions are small and either neglected or corrections are made to
1 A 1
1 21
S D
the measured emittance.
e A e
1 2 2
5.5 For satisfactory results in conformance with this test
method, the principles governing the size, construction, and
For this test method to apply, the following condition must
use of apparatus described in this test method should be
exist:
followed. If these principles are followed, any measured value
1 A 1
obtained by the use of this test method is expected to be
.. 21 (2)
S D
e A e
1 2 2
accurate to within 65%. If the results are to be reported as
This condition can be satisfied for all possible values of
having been obtained by this test method, all of the require-
specimen emittance by an apparatus design in which A /A has
ments prescribed in this test method shall be met. 1 2
a value less than 0.01 and e has a value greater than 0.8. To
5.6 It is not practical in a test method of this type to 2
ensure that the inner surface has an emittance greater than 0.8,
establish details of construction and procedure to cover all
metalandglassbelljarsshallbecoatedwithablackpaint (1).
contingencies that might offer difficulties to a person without
It is permissible to leave small areas in the glass bell jars
technical knowledge concerning the theory of heat transfer,
uncoated for visual monitoring of the specimen during a test.
temperaturemeasurements,andgeneraltestingpractices.Stan-
Metal bell jars can be provided with small-area glass view
dardization of this test method does not reduce the need for
ports for sample observation.
suchtechnicalknowledge.Itisrecognizedalsothatitwouldbe
6.2.2 The bell jar must be opaque to external high energy
unwise to restrict in any way the development of improved or
radiation sources (such as open furnaces, sunlight, and other
new methods or procedures by research workers because of
emittance apparatuses) if they are in view of the specimen.
standardization of this test method.
Both the coated metal and coated glass bell jars meet this
requirement.
6. Apparatus
6.1 In general, the apparatus shall consist of the following
equipment:abelljar,powersupplyandmulti-meterforvoltage
and current measurements, thermocouples and voltmeter or 3
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
other readout, vacuum system, and specimen holders. A this standard.
FIG. 1 System Arrangement
C 835 – 01 (2006)
6.2.3 The need for bell jar cooling is determined by the 6.4.3 The voltage drop in the measurement area of the
lower-use temperature of the particular apparatus and by the specimenismeasuredbytappingtosimilarelementsofeachof
maximum natural heat dissipation of the bell jar. A bell jar
the two thermocouples that bound the test section. A potenti-
operating at room temperature (20°C) may be used for speci- ometer,orequivalentinstrument,havingasensitivityof2µVor
men temperatures down to about 120°C. At least a 100°C
less is required for measuring the thermocouple emf’s from
difference between the specimen and the bell jar is recom-
which the test section temperatures are obtained.
mendedtoachievethedesiredmethodaccuracy.Therefore,for
6.4.4 Temperature sensors must be calibrated to within the
lowerspecimentemperatures,belljarcoolingisrequired.Ifthe
uncertainty allowed by the apparatus design accuracy. For
natural heat dissipation of the bell jar is not sufficient to
information concerning sensitivity and accuracy of thermo-
maintain its temperature at the desired level for any other
couples, see Table1 of Tables E230. For a comprehensive
operating condition, auxiliary cooling of the bell jar is also
discussion on the use of thermocouples, see Ref (2). For low
required.Analternativetobelljarcoolingistheuseofacooled
temperature thermocouple reference tables, see Ref (3).
shroud (for example, cooled by liquid nitrogen) between the
6.5 Vacuum System— A vacuum system is required to
specimen and the bell jar.
reduce the pressure in the bell jar to 1.3 mPa or less to
6.3 Power Supply— The power supply may be either ac or
minimize convection and conduction through the residual gas.
dc and is used to heat the test specimen electrically by making
This effect is illustrated in Fig. 2, which shows the measured
it a resistive part of the circuit.The true electrical power to the
emittance of oxidized Inconel versus system pressure. This
testsectionmustbemeasuredwithinaprovenuncertaintyof6
curve is based upon the assumption that all heat transfer from
1% or better.
the specimen is by radiation. As pressure increases, gas
6.4 Thermocouples, are used for measuring the surface
conduction becomes important.
temperatureofthespecimen.Thethermocouplematerialsmust
6.5.1 For the specified pressure level, a pumping system
have a melting point significantly above the highest test
consisting of a diffusion or ion pump and mechanical pump is
temperature of the specimen. To minimize temperature mea-
required. If backstreaming is a problem, cold trapping is
surement errors due to wire conduction losses, the use of
required. The specifications of an existing system are included
high-thermal conductivity materials such as copper should be
in Table 1 and photographs of a system are included in Fig. 3
avoided. The size of the thermocouple wire should be the
and Fig. 4. This information is included as a guide to assist in
minimum practical. Experience indicates that diameters less
the design of a facility and is not intended to be a rigid
than 0.13 mm provide acceptable results.
specification.
6.4.1 The test section is defined by two thermocouples
6.5.2 The specified pressure (1.3 mPa or less) must exist in
equally spaced from the specimen holders. A third thermo-
thebelljar.Ifmeasuredelsewhereinthepumpingsystem,such
couple is located at the center of the specimen. Spot welding
as in the diffusion pump inlet, the pressure drop between the
hasbeenfoundtobethemostacceptablemethodofattachment
measuring location and the bell jar must be accounted for.The
because it results in minimum disturbance of the specimen
vacuum system should also be checked for gross leakage that
surface. Swaging and peening are alternative methods pre-
could allow incoming gas to sweep over the specimen.
scribed for specimens that do not permit spot welding.
6.4.2 The number of thermocouples used to measure the 6.6 Specimen Holders, must be designed to allow for
thermal expansion of the specimen without buckling. The
temperature of the absorbing surface shall be sufficient to
provide a representative average. Four thermocouples have lower specimen holder shown in Fig. 4 is designed to move up
been found to be sufficient for the system shown in Fig. 1. anddowninitssupporttoallowforthermalexpansion.Holders
Thermocouple locations include three on the bell jar and one should be positioned off-center within the bell jar to minimize
on the baseplate. normal reflections between the specimen and bell jar inner
FIG. 2 Example of Effect of Air Pressure on Measured Emittance of Oxidized Inconel
C 835 – 01 (2006)
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.
surface. Specimen holders require auxiliary cooling if end
conduction from the specimen causes overheating.
6.7 Micrometer Calipers, or other means are needed to
measure the dimensions (width and thickness) of the test
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
nearest0.5mm.Theleng
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