ASTM E434-71(2002)
(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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E434–71 (Reapproved 2002)
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 deformation under vacuum conditions. The radiant energy
absorbedbythespecimenfromthesolarsourceandemittedby
1.1 This test method covers measurement techniques for
the specimen to the surroundings cause the specimen to reach
calorimetrically determining the ratio of solar absorptance to
anequilibriumtemperaturethatisdependentuponthe a/´ratio
hemispherical emittance using a steady-state method, and for
of its surface.
calorimetrically determining the total hemispherical emittance
3.3 In the dynamic radiative method of measuring total
using a transient technique.
hemispherical emittance, the specimen is heated with a solar
1.2 This standard does not purport to address all of the
simulation source and then allowed to cool by radiation to an
safety concerns, if any, associated with its use. It is the
evacuated space chamber with an inside effective emittance of
responsibility of the user of this standard to establish appro-
unity.Fromaknowledgeofthespecificheatofthespecimenas
priate safety and health practices and determine the applica-
a function of temperature, the area of the test specimen, its
bility of regulatory limitations prior to use.
mass,itscoolingrate,andthetemperatureofthewalls,itstotal
2. Referenced Documents
hemispherical emittance may be calculated as a function of
temperature.
2.1 ASTM Standards:
E349 Terminology Relating to Space Simulation
4. Apparatus
3. Summary of Test Method 4.1 The main elements of the apparatus include a vacuum
system, a cold shroud within the vacuum chamber, instrumen-
3.1 In calorimetric measurements of the radiative properties
tation for temperature measurement, and a solar simulator.
of materials, the specimen under evaluation is placed in a
4.2 Theareaofthethermalshroudshallnotbelessthan100
vacuumenvironmentundersimulatedsolarradiationwithcold
timesthespecimenarea(controlledbythespecimensize).The
surroundings. By observation of the thermal behavior of the
inner surfaces of the chamber shall have a high solar absorp-
specimen the thermophysical properties may be determined by
tance (not less than 0.96) and a total hemispherical emittance
an equation that relates heat balance considerations to measur-
of at least 0.88 (painted with a suitable black paint), and shall
able test parameters.
be diffuse. Suitable insulated standoffs shall be provided for
3.2 Inatypicalmeasurement,todetermine a/´asdefinedin
suspending the specimen. Thermocouple wires shall be con-
Definitions E349, the side of the specimen in question is
nected to a vacuumtight fitting where the temperature of
exposed to a simulated solar source, through a port having
feedthrough is uniform. Outside of the chamber, all thermo-
suitable transmittance over the solar spectrum. This port, or
couples shall connect with a fixed cold junction.
window, must be of sufficient diameter that the specimen and
4.3 The chamber shall be evacuated to a pressure of
radiation monitor will be fully irradiated and must be of
−6
1 310 torr (0.1 mPa) or less at all times.
sufficient thickness that it will maintain its strength without
4.4 The walls of the inner shroud shall be in contact with
coolant so that their temperature can be maintained uniform at
This test method is under the jurisdiction of ASTM Committee E21 on Space all times.
Simulation andApplications of SpaceTechnology and is the direct responsibility of
4.5 A shutter shall be provided in one end of the chamber
Subcommittee E21.04 on Space Simulation Test Methods.
which can be opened to admit a beam of radiant energy from
Current edition approved April 10, 2002. Published April 2002. Originally
a solar simulator. When open, this shutter shall provide an
approved in 1971. Last previous edition approved in 1996 as E434–71 (1996).
DOI: 10.1520/E0434-71R02.
apertureadmittingthefullsimulatorbeam.Whentheshutteris
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E434–71 (2002)
closed,allraysemittedbythespecimenshallbeinterceptedby 6.6 Alow-emittancecoatingshallbeappliedtothebackand
a blackened surface at the coolant temperature (the shutter sidesofthesubstrateandtothethermocouplewiresforseveral
must be at least conductively coupled to the shroud). inches at the specimen end.
4.6 The vacuum chamber shall be provided with a fused 6.7 The substrates shall be coated with the material in
silica window large enough to admit the simulator beam and question.Thecoatingshallbeofsufficientthicknesssoastobe
uniformly irradiate the entire specimen projected area. This opaque. (This will avoid any substrate effects.)
window shall have high transmittance through the solar spec- 6.8 The specimens shall be suspended from the top of the
trumwavelengthregion.Thechambershallbeprovidedwitha shroud by means of thread or string. These strings shall be of
vacuumtight sleeve for opening and closing the shutter and smalldiameter,lowthermalconductivity,andlowemittancein
standardvacuumfittingsforgaging,bleeding,leaktesting,and order to minimize heat losses through the leads.
pumping. If low a/´ specimens are to be measured, the solid 6.9 An alternative method of specimen mounting (mass
anglesubtendedbytheportfromthespecimenshouldbesmall dependent) shall be to suspend the specimens by their own
(dependent upon desired accuracy). If flat specular specimens small wire thermocouple leads. In this case the thermocouple
are to be measured, the port plane should be canted with holes shall be drilled as before but radially around the edge.
respect to the specimen plane to eliminate multiple reflections The suspension holes may also be eliminated in this case.
of the simulator beam. Multiple reflections could result in as
much as a 7% apparent increase in a/´. 7. Procedure
4.7 The solar simulator should duplicate the extraterrestrial
7.1 Suspend the test specimen in the chamber normal to the
solar spectrum as closely as possible. A beam irradiance of at
incident solar radiation, but geometrically removed from the
least 7000W/m at the specimen plane shall be available from
centralaxisofthechambersothatradiationfromthespecimen
the solar simulator (;5 solar constants). This irradiance may
to the chamber walls is not specularly reflected back to the
be required to raise the temperature of certain specimens to a
specimen.Sincethechamberwallsaredesignedtobecoldand
desired level.
highly absorbing, first reflections from the walls are usually all
that need be considered.
5. Coating Requirements
7.2 Determinethesimulatedsolarirradianceincidentonthe
5.1 Any type of coating may be tested by this test method
specimen with a suitable radiometric device such as a com-
provided its structure remains stable in vacuum over the mercial thermopile radiometer or a black monitor sample of
temperature range of interest.
known a/´ which may be suspended similarly to the test
5.2 For high emittance specimens the accuracy of the specimenwithintheincidentbeamofsimulatedsolarradiation.
measurements is increased if only one surface of the substrate Take care in the latter case that the irradiance and spectral
iscoatedwiththespecimencoatinginquestion.Theremaining distribution of the incident energy is the same for both
area of the substrate shall be coated with a low emittance specimen and monitor.
material of known hemispherical emittance (such as evapo- 7.3 Then close the system and start the evacuation and
rated aluminum or evaporated gold). cooling of the shroud (see Ref (3) for a typical system).
−6
5.3 The thickness and density of the coating shall be Maintain a pressure of 1 310 torr (0.1 mPa) or less and the
measured and its heat capacity calculated from existing refer- walls of the chamber must be at coolant temperature. Record
ences (see Refs (1) and (2)). the specimen, monitor, and shroud temperatures.
7.4 When the specimen has reached thermal equilibrium,
6. Specimen Preparation that is, when the specimen temperature becomes constant with
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
evaluatethe DT/Dt=0conditions,thatis,the Dtchosenshould
temperature can be found in standard references (for example,
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
the edge, through which suspension strings are to be inserted.
8. Calculation
6.4 Each substrate shall be drilled with two small shallow
holes in the back for thermocouples. 8.1 Calculate the a/´ ratio from the following equation:
6.5 Ideally the back and sides of the substrate shall be
4 4
a/´5 A s~T 2 T !/A E (1)
T 1 0 p
buffedandpolishedandoneuninsulatedthermocoupleinserted
in the back of the specimen (one wire in each hole). One of
where:
these wires shall be peened into each hole. a = effective solar absorptance of the specimen,
´ = hemispherical emittance of the specimen,
s = Stefan-Boltzman constant,
A = projected area of the specimen exposed to solar
p
The boldface numbers in parentheses refer to the list of references appended to
radiation,
this method.
E434–71 (2002)
E = incident total irradiance, A = area of coating, and
c
T = specimen equilibrium temperature with simulated ´ = total hemispherical emittance of coating.
1 c
solar radiation, 8.6 To obtain an a/´ measurement or an effective solar
T = chamber wall temperature with solar source off, and
absorptance, a, for a specimen coated only on one side, one
A = total radiating area of the specimen.
T must consider the following expression:
8.2 This equation is derived in the following manner: If a
A ´ 5 A ´ 1 A ´ (7)
T T c c s s
specimencoatedonallsideswiththematerialinquestion,with
where:
a projected area as viewed in the direction of irradiation, A,a
p
A ,A ,A = total area, area of the coating, and uncoated
total area, A , effective solar absorptance, a, emittance, ´, and
T c s
T
area of the substrate, respectively, and
specific heat c is suspended in an evacuated high absorptance
p
´ , ´ , ´ = total hemispherical emittance of the speci-
isothermal cold-walled chamber and exposed to a simulated T c s
men, coating, and substrate respectively.
solar irradiance, E, the rate of temperature change can be
Rearrangement shows that:
determined by evaluating the heat balance equation. The
energy balance of an irradiated specimen emitting radiant
´ 5 ~A ´ 1 A ´ !/A (8)
T c c s s T
energy in a vacuum is given by the following equation:
Multiplying the a/´ value obtained from Eq 3 by ´ (at the
T
4 4
mc ~dT/dt! 5 A aE 1 E 2 A ´s~T 2 T ! (2)
sametemperatureofequilibrium)obtainedfromEq8willgive
p p p t 1 0
4 the solar absorptance, a. In order to acquire the (a/´) coating,
where E = A´sT , the thermal radiation from the port. To
p 2
divide the a value by ´ (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)=0, and,
p
9. Report
From Eq 2, solving for the a/´ ratio we obtain
9.1 The report should include the methods used for tem-
4 4
a/´5 A s T 2 T !/A E (3)
~
T 1 0 p
peratureandirradiancemeasurements,andtheactualdataused
Eq 3 is used to calculate the a/´ ratio when the parameters for the calculations.
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/´ type of measurement, the total exposure time
4 4
mc~dT/dt! 5 A ´s~T 2 T ! (4)
T 1 0
andlevelofirradiance,andspectraldistributionoftheincident
8.4 If the term T is neglected, the above equation can be flux shall also be reported.
integrated and expanded into:
10. Uncertainty Analysis
~m c 1 m c ! 1 1
s s c c
´5 2 (5)
S 3 3D
10.1 Many potential errors exist in the calorimetric deter-
3sA Dt
T T T
1 2
mination of radiative properties. If it is assumed that the major
where:
uncertainties encountered in these calorimetric measurements
m = mass of the substrate,
are systematic rather than random, they will add in a linear
s
m = mass of the coating,
c
manner and the total uncertainty can be expressed as:
c = thermal capacitance of the substrate,
s
d´ /´ 5 ~d´ /´ ! 1 ~d´ /´ ! 1 ~d´ /´ ! 1 ~d´ /´ ! (9)
s s s s conv s s q s s R s s HL
c = thermal capacitance of the coating,
c
T = temperature of the specimen, and
for emittance, and as
Dt = changeintimefrom T to T andmagnitudesuchthat
1 2
da/´ da/´ da/´ da/´ da/´
5 1 1 1 (10)
c and c may be assumed constant over small S D S D S D S D
s c
a/´ a/´ a/´ a/´ a/´
conv R HL S
temperature ranges.
fortheratioofsolarabsorptancetohemisphericalemittance.
When the temperature decay is recorded with time, then the
The terms on the right of the emittance uncertainty equation
total hemispherical emittance of the sample can be determined
can be defined as conv the conventional error, q the heat
with Eq 4 or Eq 5. The use of Eq 5 is preferable since Eq 4
measurement error, R the ext
...
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