ASTM E434-10(2020)
(Test Method)Standard Test Method for Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance Using Solar Simulation
Standard Test Method for Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance Using Solar Simulation
ABSTRACT
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. The main elements of the apparatus include a vacuum system, a cold shroud within the vacuum chamber, instrumentation for temperature measurement, and a solar simulator. Any type of coating may be tested by this test method provided its structure remains stable in vacuum over the temperature range of interest. The substrate shall be machined from flat stock and to a size proportioned to the working area of the chamber.
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.3 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.
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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: E434 − 10 (Reapproved 2020)
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 E434; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2 In a typical measurement, to determine α/ε as defined in
Definitions E349, the side of the specimen in question is
1.1 This test method covers measurement techniques for
exposed to a simulated solar source, through a port having
calorimetrically determining the ratio of solar absorptance to
suitable transmittance over the solar spectrum. This port, or
hemispherical emittance using a steady-state method, and for
window, must be of sufficient diameter that the specimen and
calorimetrically determining the total hemispherical emittance
radiation monitor will be fully irradiated and must be of
using a transient technique.
sufficient thickness that it will maintain its strength without
1.2 This standard does not purport to address all of the
deformation under vacuum conditions. The radiant energy
safety concerns, if any, associated with its use. It is the
absorbedbythespecimenfromthesolarsourceandemittedby
responsibility of the user of this standard to establish appro-
the specimen to the surroundings cause the specimen to reach
priate safety, health, and environmental practices and deter-
anequilibriumtemperaturethatisdependentupontheα/εratio
mine the applicability of regulatory limitations prior to use.
of its surface.
1.3 This international standard was developed in accor-
3.3 In the dynamic radiative method of measuring total
dance with internationally recognized principles on standard-
hemispherical emittance, the specimen is heated with a solar
ization established in the Decision on Principles for the
simulation source and then allowed to cool by radiation to an
Development of International Standards, Guides and Recom-
evacuated space chamber with an inside effective emittance of
mendations issued by the World Trade Organization Technical
unity.Fromaknowledgeofthespecificheatofthespecimenas
Barriers to Trade (TBT) Committee.
a function of temperature, the area of the test specimen, its
mass,itscoolingrate,andthetemperatureofthewalls,itstotal
2. Referenced Documents
hemispherical emittance may be calculated as a function of
2.1 ASTM Standards:
temperature.
E349Terminology Relating to Space Simulation
4. Apparatus
3. Summary of Test Method
4.1 The main elements of the apparatus include a vacuum
3.1 In calorimetric measurements of the radiative properties
system, a cold shroud within the vacuum chamber, instrumen-
of materials, the specimen under evaluation is placed in a
tation for temperature measurement, and a solar simulator.
vacuumenvironmentundersimulatedsolarradiationwithcold
surroundings. By observation of the thermal behavior of the
4.2 Theareaofthethermalshroudshallnotbelessthan100
specimen the thermophysical properties may be determined by
timesthespecimenarea(controlledbythespecimensize).The
an equation that relates heat balance considerations to measur-
inner surfaces of the chamber shall have a high solar absorp-
able test parameters.
tance (not less than 0.96) and a total hemispherical emittance
of at least 0.88 (painted with a suitable black paint), and shall
be diffuse. Suitable insulated standoffs shall be provided for
suspending the specimen. Thermocouple wires shall be con-
This test method is under the jurisdiction of ASTM Committee E21 on Space
Simulation andApplications of SpaceTechnology and is the direct responsibility of
nected to a vacuumtight fitting where the temperature of
Subcommittee E21.04 on Space Simulation Test Methods.
feedthrough is uniform. Outside of the chamber, all thermo-
Current edition approved Nov. 1, 2020. Published December 2020. Originally
couples shall connect with a fixed cold junction.
approvedin1971.Lastpreviouseditionapprovedin2015asE434–10(2015).DOI:
10.1520/E0434-10R20.
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 Nextel Brand Velvet Coating 401-C10 Black, available from Reflective
the ASTM website. Products Div., 3M Co., has been found to be satisfactory.
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E434 − 10 (2020)
4.3 The chamber shall be evacuated to a pressure of 6.2 The substrate shall be machined from flat stock and to a
−6
1×10 torr (0.1 mPa) or less at all times. size proportioned to the working area of the chamber.
4.4 The walls of the inner shroud shall be in contact with 6.3 Each specimen shall be drilled with a set of holes, near
coolant so that their temperature can be maintained uniform at the edge, through which suspension strings are to be inserted.
all times.
6.4 Each substrate shall be drilled with two small shallow
4.5 A shutter shall be provided in one end of the chamber holes in the back for thermocouples.
which can be opened to admit a beam of radiant energy from
6.5 Ideally the back and sides of the substrate shall be
a solar simulator. When open, this shutter shall provide an
buffedandpolishedandoneuninsulatedthermocoupleinserted
apertureadmittingthefullsimulatorbeam.Whentheshutteris
in the back of the specimen (one wire in each hole). One of
closed,allraysemittedbythespecimenshallbeinterceptedby
these wires shall be peened into each hole.
a blackened surface at the coolant temperature (the shutter
6.6 Alow-emittancecoatingshallbeappliedtothebackand
must be at least conductively coupled to the shroud).
sidesofthesubstrateandtothethermocouplewiresforseveral
4.6 The vacuum chamber shall be provided with a fused
inches at the specimen end.
silica window large enough to admit the simulator beam and
6.7 The substrates shall be coated with the material in
uniformly irradiate the entire specimen projected area. This
question.Thecoatingshallbeofsufficientthicknesssoastobe
window shall have high transmittance through the solar spec-
opaque. (This will avoid any substrate effects.)
trumwavelengthregion.Thechambershallbeprovidedwitha
vacuumtight sleeve for opening and closing the shutter and
6.8 The specimens shall be suspended from the top of the
standardvacuumfittingsforgaging,bleeding,leaktesting,and shroud by means of thread or string. These strings shall be of
pumping. If low α/ε specimens are to be measured, the solid
smalldiameter,lowthermalconductivity,andlowemittancein
anglesubtendedbytheportfromthespecimenshouldbesmall order to minimize heat losses through the leads.
(dependent upon desired accuracy). If flat specular specimens
6.9 An alternative method of specimen mounting (mass
are to be measured, the port plane should be canted with
dependent) shall be to suspend the specimens by their own
respect to the specimen plane to eliminate multiple reflections
small wire thermocouple leads. In this case the thermocouple
of the simulator beam. Multiple reflections could result in as
holes shall be drilled as before but radially around the edge.
much as a 7% apparent increase in α/ε.
The suspension holes may also be eliminated in this case.
4.7 The solar simulator should duplicate the extraterrestrial
7. Procedure
solar spectrum as closely as possible. A beam irradiance of at
least 7000W/m at the specimen plane shall be available from 7.1 Suspend the test specimen in the chamber normal to the
the solar simulator (;5 solar constants). This irradiance may
incident solar radiation, but geometrically removed from the
be required to raise the temperature of certain specimens to a centralaxisofthechambersothatradiationfromthespecimen
desired level.
to the chamber walls is not specularly reflected back to the
specimen.Sincethechamberwallsaredesignedtobecoldand
5. Coating Requirements
highly absorbing, first reflections from the walls are usually all
that need be considered.
5.1 Any type of coating may be tested by this test method
provided its structure remains stable in vacuum over the
7.2 Determinethesimulatedsolarirradianceincidentonthe
temperature range of interest.
specimen with a suitable radiometric device such as a com-
mercial thermopile radiometer or a black monitor sample of
5.2 For high emittance specimens the accuracy of the
known α/ε which may be suspended similarly to the test
measurements is increased if only one surface of the substrate
iscoatedwiththespecimencoatinginquestion.Theremaining specimenwithintheincidentbeamofsimulatedsolarradiation.
Take care in the latter case that the irradiance and spectral
area of the substrate shall be coated with a low emittance
material of known hemispherical emittance (such as evapo- distribution of the incident energy is the same for both
specimen and monitor.
rated aluminum or evaporated gold).
7.3 Then close the system and start the evacuation and
5.3 The thickness and density of the coating shall be
cooling of the shroud (see Ref (3) for a typical system).
measured and its heat capacity calculated from existing refer-
−6
Maintain a pressure of 1×10 torr (0.1 mPa) or less and the
ences (see Refs (1) and (2)).
walls of the chamber must be at coolant temperature. Record
6. Specimen Preparation
the specimen, monitor, and shroud temperatures.
6.1 The substrates used for the measurements described
7.4 When the specimen has reached thermal equilibrium,
here shall be of a material whose specific heat as a function of
that is, when the specimen temperature becomes constant with
temperature can be found in standard references (for example,
constant surrounding conditions, shut off the solar simulator.
OFHC copper or a common aluminum alloy such as 6061-T6)
When specimens of large thermal mass are used, carefully
(Ref (1)).
evaluatethe ∆T/∆t=0conditions,thatis,the ∆tchosenshould
be dependent on the specimen time constant.
7.5 Close the moveable door in the shroud and allow the
The boldface numbers in parentheses refer to the list of references appended to
this method. specimens to cool to a desired temperature. Measure the
E434 − 10 (2020)
specimen temperature as a function of time and calculate the α A ε T
~ !
es t 0
4 4
5 σ T 2 T (4)
S D
1 0
rates of change of the temperature. ε ~T ! A E ε ~T !
1 p 1
Eq 4 is used to calculate the α /ε (T ) ratio when the
es 1
8. Calculation
parameters A , E, and A are determined and the equilibrium
T p
temperature is measured.
8.1 Calculate the α /ε (T ) ratio from the following equa-
es 1
tion:
8.3 If the source is blocked by the shutter and the specimen
looses energy only by radiation, the energy balance equation
α A ε T
~ !
es t 0
4 4
5 σ T 2 T (1)
S D
1 0
becomes:
ε ~T ! A E ε ~T !
1 p 1
dT
where: 4 4
mc 5 A ε T σ T 2 A α T σ T 1Q 1Q 2 Q (5)
S D ~ ! ~ !
t 1 1 t trα 0 0 ll rg ts
dt
α = Effective solar absorptance relative to the illumi-
es
Where Q and Q represent the heat losses from the support
ll g
nating source,
leads and the heat lost from the residual gasses in the
ε (T ) = hemispherical emittance of the specimen at Tem-
0 vacuum chamber, respectively. The last term Q is any heat
ts
perature T ,
0 input from the temperature sensor. See Ref (4) and Ref (5)
ε (T ) = hemispherical emittance of the specimen at Tem-
I for a treatment of the lead loss and residual gas heat loss
perature T ,
terms.
σ = Stefan-Boltzmann constant,
8.4 If the term T is neglected, and the parasitic heat losses
A = projected area of the specimen exposed to solar
p
and gains can be ignored, the above equation can be integrated
radiation,
and expanded into:
E = incident total irradiance,
T = specimen equilibrium temperature with simulated
1 ~m c 1m c ! 1 1
s s c c
ε T 5 2 (6)
~ ! S D
1 3 3
solar radiation,
3σA ∆t T T
t 1 2
T = chamber wall temperature with solar source off,
where:
and
A = total radiating area of the specimen. m = mass of the substrate,
T s
m = mass of the coating,
c
8.2 This equation is derived in the following manner: If a
c = thermal capacitance of the substrate,
s
specimencoatedonallsideswiththematerialinquestion,with
c = thermal capacitance of the coating,
c
a projected area as viewed in the direction of irradiation, A,a
p T = temperature of the specimen, and
total area, A , effective simulated solar absorptance, α ,
∆t = change in time from T to T and magnitude such that
T es
1 2
emittance at T , ε (T ), and specific heat c is suspended in an
c and c may be assumed constant over small tem-
1 1 p
s c
evacuated high absorptance isothermal cold-walled chamber perature ranges.
and exposed to a simulated solar irradiance, E, the rate of
When the temperature decay is recorded with time, then the
temperature change can be determined by evaluating the heat
total hemispherical emittance of the sample can be determined
balanceequation.Theenergybalanceofanirradiatedspecimen
with Eq 5 or Eq 6. The use of Eq 6 is preferable since Eq 5
emitting radiant energy in a vacuum is given by the following
involves the experimental determination of two quantities
equation (assuming parasitic heat losses can be ignored): 4
(dT/dtand T ),therebyintroducingmorepossibleerrorsthanin
Eq 6.
dT
4 4
mc 5 A σE 1E 2 A ε T σ T 1A α T σ T (2)
S D ~ ! ~ !
p p p p t 1 1 t tr 0 0
dt
8.5 Data from specimens which are coated on one side only
4 shall be reduced by use of the following equation:
where E = AεσT , the thermal radiation from the port. To
p 2
determine the incident thermal radiation, E , see Ref (3). The ~m c 1m c ! 1 1 ε ~A 2 A !
p s s c c s T c
ε T 5 2 2 (7)
4 ~ ! S D
3 3
c
3σA ∆t T T A
last term, A α (T ) σ T , is the amount of heat energy
c 1 2 c
t tr 0 0
absorbedbythesamplefromthechamberwalls.Kirchoff’slaw
where:
tells us that at a given temperature the infrared absorptance is
ε = total hemispherical emittance of substrate,
s
equal the infrared emittance. This means that it will emit as
A = area of coating, and
c
much heat as it absorbs from a black body at the same
ε = total hemispherical emittance of coating.
c
temperature as the sample. Therefore, to know how much
8.6 To obtain an α/ε measurement or an effective solar
energy is absorbed by the sample from the shroud walls we
absorptance, α, for a specimen coated only on one side, one
must
...
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