ASTM E434-71(1996)e1

Standard Test Method for Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance Using Solar Simulation

SCOPE
1.1 This test method covers measurement techniques for calorimetrically determining the ratio of solar absorptance to hemispherical emittance using a steady-state method, and for calorimetrically determining the total hemispherical emittance using a transient technique.
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.

General Information

• Status: Historical
• Publication Date: 20-Jun-1971
• Technical Committee: E21 - Space Simulation and Applications of Space Technology
• Drafting Committee: E21.04 - Space Simulation Test Methods

Relations

• Replaced By: ASTM E434-71(2002) - Standard Test Method for Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance Using Solar Simulation
• Effective Date: 21-Jun-1971
• Referred By: ASTM E781-86(2023) - Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar Collectors with Cover Plates
• Effective Date: 21-Jun-1971
• Referred By: ASTM E512-94(2020) - Standard Practice for Combined, Simulated Space Environment Testing of Thermal Control Materials with Electromagnetic and Particulate Radiation
• Effective Date: 21-Jun-1971
• Referred By: ASTM E744-07(2022) - Standard Practice for Evaluating Solar Absorptive Materials for Thermal Applications
• Effective Date: 21-Jun-1971

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
e1
Designation: E 434 – 71 (Reapproved 1996)
Standard Test Method for
Calorimetric Determination of Hemispherical Emittance and
the Ratio of Solar Absorptance to Hemispherical Emittance
Using Solar Simulation
This standard is issued under the fixed designation E 434; 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.
e NOTE—Section 11 was added editorially in May 1996.
1. Scope 3.3 In the dynamic radiative method of measuring total
hemispherical emittance, the specimen is heated with a solar
1.1 This test method covers measurement techniques for
simulation source and then allowed to cool by radiation to an
calorimetrically determining the ratio of solar absorptance to
evacuated space chamber with an inside effective emittance of
hemispherical emittance using a steady-state method, and for
unity. From a knowledge of the specific heat of the specimen as
calorimetrically determining the total hemispherical emittance
a function of temperature, the area of the test specimen, its
using a transient technique.
mass, its cooling rate, and the temperature of the walls, its total
1.2 This standard does not purport to address all of the
hemispherical emittance may be calculated as a function of
safety concerns, if any, associated with its use. It is the
temperature.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Apparatus
bility of regulatory limitations prior to use.
4.1 The main elements of the apparatus include a vacuum
2. Referenced Documents system, a cold shroud within the vacuum chamber, instrumen-
tation for temperature measurement, and a solar simulator.
2.1 ASTM Standards:
2 4.2 The area of the thermal shroud shall not be less than 100
E 349 Terminology Relating to Space Simulation
times the specimen area (controlled by the specimen size). The
3. Summary of Test Method inner surfaces of the chamber shall have a high solar absorp-
tance (not less than 0.96) and a total hemispherical emittance
3.1 In calorimetric measurements of the radiative properties
of at least 0.88 (painted with a suitable black paint), and shall
of materials, the specimen under evaluation is placed in a
be diffuse. Suitable insulated standoffs shall be provided for
vacuum environment under simulated solar radiation with cold
suspending the specimen. Thermocouple wires shall be con-
surroundings. By observation of the thermal behavior of the
nected to a vacuumtight fitting where the temperature of
specimen the thermophysical properties may be determined by
feedthrough is uniform. Outside of the chamber, all thermo-
an equation that relates heat balance considerations to measur-
couples shall connect with a fixed cold junction.
able test parameters.
4.3 The chamber shall be evacuated to a pressure of
3.2 In a typical measurement, to determine a/e as defined in
−6
1 3 10 torr (0.1 mPa) or less at all times.
Definitions E 349, the side of the specimen in question is
4.4 The walls of the inner shroud shall be in contact with
exposed to a simulated solar source, through a port having
coolant so that their temperature can be maintained uniform at
suitable transmittance over the solar spectrum. This port, or
all times.
window, must be of sufficient diameter that the specimen and
4.5 A shutter shall be provided in one end of the chamber
radiation monitor will be fully irradiated and must be of
which can be opened to admit a beam of radiant energy from
sufficient thickness that it will maintain its strength without
a solar simulator. When open, this shutter shall provide an
deformation under vacuum conditions. The radiant energy
aperture admitting the full simulator beam. When the shutter is
absorbed by the specimen from the solar source and emitted by
closed, all rays emitted by the specimen shall be intercepted by
the specimen to the surroundings cause the specimen to reach
a blackened surface at the coolant temperature (the shutter
an equilibrium temperature that is dependent upon the a/e ratio
must be at least conductively coupled to the shroud).
of its surface.
4.6 The vacuum chamber shall be provided with a fused
silica window large enough to admit the simulator beam and
This test method is under the jurisdiction of ASTM Committee E-21 on Space
Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.04 on Space Simulation Test Methods.
Current edition approved June 21, 1971. Published August 1971. Nextel Brand Velvet Coating 401-C10 Black, available from Reflective
Annual Book of ASTM Standards, Vol 15.03. Products Div., 3M Co., has been found to be satisfactory.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 434
uniformly irradiate the entire specimen projected area. This 6.8 The specimens shall be suspended from the top of the
window shall have high transmittance through the solar spec- shroud by means of thread or string. These strings shall be of
trum wavelength region. The chamber shall be provided with a small diameter, low thermal conductivity, and low emittance in
vacuumtight sleeve for opening and closing the shutter and order to minimize heat losses through the leads.
standard vacuum fittings for gaging, bleeding, leak testing, and 6.9 An alternative method of specimen mounting (mass
pumping. If low a/e specimens are to be measured, the solid dependent) shall be to suspend the specimens by their own
angle subtended by the port from the specimen should be small small wire thermocouple leads. In this case the thermocouple
(dependent upon desired accuracy). If flat specular specimens holes shall be drilled as before but radially around the edge.
are to be measured, the port plane should be canted with The suspension holes may also be eliminated in this case.
respect to the specimen plane to eliminate multiple reflections
7. Procedure
of the simulator beam. Multiple reflections could result in as
much as a 7 % apparent increase in a/e.
7.1 Suspend the test specimen in the chamber normal to the
4.7 The solar simulator should duplicate the extraterrestrial
incident solar radiation, but geometrically removed from the
solar spectrum as closely as possible. A beam irradiance of at
central axis of the chamber so that radiation from the specimen
least 7000 W/m at the specimen plane shall be available from
to the chamber walls is not specularly reflected back to the
the solar simulator (;5 solar constants). This irradiance may
specimen. Since the chamber walls are designed to be cold and
be required to raise the temperature of certain specimens to a
highly absorbing, first reflections from the walls are usually all
desired level.
that need be considered.
7.2 Determine the simulated solar irradiance incident on the
5. Coating Requirements
specimen with a suitable radiometric device such as a com-
5.1 Any type of coating may be tested by this test method
mercial thermopile radiometer or a black monitor sample of
provided its structure remains stable in vacuum over the
known a/e which may be suspended similarly to the test
temperature range of interest.
specimen within the incident beam of simulated solar radiation.
5.2 For high emittance specimens the accuracy of the
Take care in the latter case that the irradiance and spectral
measurements is increased if only one surface of the substrate
distribution of the incident energy is the same for both
is coated with the specimen coating in question. The remaining
specimen and monitor.
area of the substrate shall be coated with a low emittance
7.3 Then close the system and start the evacuation and
material of known hemispherical emittance (such as evapo-
cooling of the shroud (see Ref (3) for a typical system).
−6
rated aluminum or evaporated gold).
Maintain a pressure of 1 3 10 torr (0.1 mPa) or less and the
5.3 The thickness and density of the coating shall be
walls of the chamber must be at coolant temperature. Record
measured and its heat capacity calculated from existing refer-
the specimen, monitor, and shroud temperatures.
ences (see Refs (1) and (2)).
7.4 When the specimen has reached thermal equilibrium,
that is, when the specimen temperature becomes constant with
6. Specimen Preparation
constant surrounding conditions, shut off the solar simulator.
6.1 The substrates used for the measurements described When specimens of large thermal mass are used, carefully
here shall be of a material whose specific heat as a function of
evaluate the DT/Dt 5 0 conditions, that is, the Dt chosen
temperature can be found in standard references (for example, should be dependent on the specimen time constant.
OFHC copper or a common aluminum alloy such as 6061-T6)
7.5 Close the moveable door in the shroud and allow the
(Ref (1)). specimens to cool to a desired temperature. Measure the
6.2 The substrate shall be machined from flat stock and to a specimen temperature as a function of time and calculate the
size proportioned to the working area of the chamber.
rates of change of the temperature.
6.3 Each specimen shall be drilled with a set of holes, near
8. Calculation
the edge, through which suspension strings are to be inserted.
6.4 Each substrate shall be drilled with two small shallow
8.1 Calculate the a/e ratio from the following equation:
holes in the back for thermocouples.
4 4
a/e5 A s~T 2 T !/A E (1)
T 1 0 p
6.5 Ideally the back and sides of the substrate shall be
buffed and polished and one uninsulated thermocouple inserted
where:
in the back of the specimen (one wire in each hole). One of
a5 effective solar absorptance of the specimen,
these wires shall be peened into each hole.
e5 hemispherical emittance of the specimen,
6.6 A low-emittance coating shall be applied to the back and s5 Stefan-Boltzman constant,
sides of the substrate and to the thermocouple wires for several A 5 projected area of the specimen exposed to solar
p
inches at the specimen end. radiation,
E 5 incident total irradiance,
6.7 The substrates shall be coated with the material in
T 5 specimen equilibrium temperature with simulated
question. The coating shall be of sufficient thickness so as to be 1
solar radiation,
opaque. (This will avoid any substrate effects.)
T 5 chamber wall temperature with solar source off, and
A 5 total radiating area of the specimen.
T
8.2 This equation is derived in the following manner: If a
The boldface numbers in parentheses refer to the list of references appended to
this method. specimen coated on all sides with the material in question, with
E 434
a projected area as viewed in the direction of irradiation, A ,a
where:
p
total area, A , effective solar absorptance, a, emittance, e, and
A , A , A 5 total area, area of the coating, and uncoated
T
T c s
specific heat c is suspended in an evacuated high absorptance
area of the substrate, respectively, and
p
isothermal cold-walled chamber and exposed to a simulated e , e , e 5 total hemispherical emittance of the speci-
T c s
solar irradiance, E, the rate of temperature change can be men, coating, and substrate respectively.
determined by evaluating the heat balance equation. The Rearrangement shows that:
energy balance of an irradiated specimen emitting radiant
e 5 A e 1 A e !/A (8)
~
T c c s s T
energy in a vacuum is given by the following equation:
Multiplying the a/e value obtained from Eq 3 by e (at the
T
4 4
mc ~dT/dt! 5 A aE 1 E 2 A es~T 2 T ! (2)
same temperature of equilibrium) obtained from Eq 8 will give
p p p t 1 0
the solar absorptance, a. In order to acquire the (a/e) coating,
where E 5 AesT , the thermal radiation from the port. To
p 2
divide the a value by e (already measured in a transient cool
s c
determine the incident thermal radiation, E , see Ref (3). If E
p p
down).
is eliminated from Eq 2 when an equilibrium temperature is
reached, mc (dT/dt) 5 0, and,
p
9. Report
From Eq 2, solving for the a/e ratio we obtain
9.1 The report should include the methods used for tem-
4 4
a/e5 A s~T 2 T !/A E (3)
T 1 0 p perature and irradiance measurements, and the actual data used
for the calculations.
Eq 3 is used to calculate the a/e ratio when the parameters
9.2 A complete characterization of the specimen shall be
A , E, and A are determined and the equilibrium temperature
T p
given whenever possible. This shall include specimen dimen-
is measured.
sions, specimen composition, coating thickness and composi-
8.3 If the source is blocked by the shutter and the specimen
tion, surface roughness, and surface contamination, and any
looses energy only by radiation, the energy balance equation
other conditions which may be considered pertinent.
becomes:
9.3 In an a/e type of measurement, the total exposure time
4 4
and level of irradiance, and spectral distribution of the incident
mc~dT/dt! 5 A es~T 2 T ! (4)
T 1 0
flux shall also be reported.
8.4 If the term T is neglected, the above equation can be
integrated and expanded into:
10. Uncertainty Analysis
~m c 1 m c ! 1 1
s s c c 10.1 Many potential errors exist in the calorimetric deter-
e5 2 (5)
S 3 3D
3sA Dt
T T T mination of radiative properties. If it is assumed that the major
1 2
uncertainties encountered in these calorimetric measurements
where:
are systematic rather than random, they will add in a linear
m 5 mass of the substrate,
s
manner and the total uncertainty can be expressed as:
m 5 mass of the coating,
c
de /e 5 ~de /e ! 1 ~de /e ! 1 ~de /e ! 1 ~de /e ! (9)
c 5 thermal capacitance of the substrate, s s s s conv s s q s s R s s HL
s
c 5 thermal capacitance of the coating,
c
for emittance, and as
T 5 temperature of the specimen, and
da/e da/e da/e da/e da/e
Dt 5 change in time from T to T and magnitude such that
1 2 5 1 1 1 (10)
S D S D S D S D
a/e a/e a/e a/e a/e
conv R HL S
c and c may be assumed constant over small
s c
for the ratio of solar absorptance to hemispherical emittance.
temperature ranges.
The terms on the right of the emittance uncertainty equation
When the temperature decay is recorded with time, then the
can be defined as conv the conventional error, q the heat
total hemispherical emittance of the sample can be determined
measurement error, R the extraneous radiation error, and HL
with Eq 4 or Eq 5. The use of Eq 5 is preferable since Eq 4
the heat loss error, respectively. In the uncertainty equation for
involves the experimental determination of two quantities
4 a/e the last term, s, is defined as the error due to solar
(dT/dt and T ), thereby introducing more possible errors
...