1、Designation: E 434 71 (Reapproved 2002)Standard Test Method forCalorimetric Determination of Hemispherical Emittance andthe Ratio of Solar Absorptance to Hemispherical EmittanceUsing Solar Simulation1This standard is issued under the fixed designation E 434; the number immediately following the desi
2、gnation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers measu
3、rement techniques forcalorimetrically determining the ratio of solar absorptance tohemispherical emittance using a steady-state method, and forcalorimetrically determining the total hemispherical emittanceusing a transient technique.1.2 This standard does not purport to address all of thesafety conc
4、erns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 349 Terminology Relating to Space Simu
5、lation23. Summary of Test Method3.1 In calorimetric measurements of the radiative propertiesof materials, the specimen under evaluation is placed in avacuum environment under simulated solar radiation with coldsurroundings. By observation of the thermal behavior of thespecimen the thermophysical pro
6、perties may be determined byan equation that relates heat balance considerations to measur-able test parameters.3.2 In a typical measurement, to determine a/e as defined inDefinitions E 349, the side of the specimen in question isexposed to a simulated solar source, through a port havingsuitable tra
7、nsmittance over the solar spectrum. This port, orwindow, must be of sufficient diameter that the specimen andradiation monitor will be fully irradiated and must be ofsufficient thickness that it will maintain its strength withoutdeformation under vacuum conditions. The radiant energyabsorbed by the
8、specimen from the solar source and emitted bythe specimen to the surroundings cause the specimen to reachan equilibrium temperature that is dependent upon the a/e ratioof its surface.3.3 In the dynamic radiative method of measuring totalhemispherical emittance, the specimen is heated with a solarsim
9、ulation source and then allowed to cool by radiation to anevacuated space chamber with an inside effective emittance ofunity. From a knowledge of the specific heat of the specimen asa function of temperature, the area of the test specimen, itsmass, its cooling rate, and the temperature of the walls,
10、 its totalhemispherical emittance may be calculated as a function oftemperature.4. Apparatus4.1 The main elements of the apparatus include a vacuumsystem, a cold shroud within the vacuum chamber, instrumen-tation for temperature measurement, and a solar simulator.4.2 The area of the thermal shroud s
11、hall not be less than 100times the specimen area (controlled by the specimen size). Theinner surfaces of the chamber shall have a high solar absorp-tance (not less than 0.96) and a total hemispherical emittanceof at least 0.88 (painted with a suitable black paint),3and shallbe diffuse. Suitable insu
12、lated standoffs shall be provided forsuspending the specimen. Thermocouple wires shall be con-nected to a vacuumtight fitting where the temperature offeedthrough is uniform. Outside of the chamber, all thermo-couples shall connect with a fixed cold junction.4.3 The chamber shall be evacuated to a pr
13、essure of1 3 106torr (0.1 mPa) or less at all times.4.4 The walls of the inner shroud shall be in contact withcoolant so that their temperature can be maintained uniform atall times.4.5 A shutter shall be provided in one end of the chamberwhich can be opened to admit a beam of radiant energy froma s
14、olar simulator. When open, this shutter shall provide anaperture admitting the full simulator beam. When the shutter isclosed, all rays emitted by the specimen shall be intercepted bya blackened surface at the coolant temperature (the shuttermust be at least conductively coupled to the shroud).4.6 T
15、he vacuum chamber shall be provided with a fusedsilica window large enough to admit the simulator beam and1This test method is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.04 on Space Simulati
16、on Test Methods.Current edition approved June 21, 1971. Published August 1971.2Annual Book of ASTM Standards, Vol 15.03.3Nextel Brand Velvet Coating 401-C10 Black, available from ReflectiveProducts Div., 3M Co., has been found to be satisfactory.1Copyright ASTM International, 100 Barr Harbor Drive,
17、PO Box C700, West Conshohocken, PA 19428-2959, United States.uniformly irradiate the entire specimen projected area. Thiswindow shall have high transmittance through the solar spec-trum wavelength region. The chamber shall be provided with avacuumtight sleeve for opening and closing the shutter ands
18、tandard vacuum fittings for gaging, bleeding, leak testing, andpumping. If low a/e specimens are to be measured, the solidangle subtended by the port from the specimen should be small(dependent upon desired accuracy). If flat specular specimensare to be measured, the port plane should be canted with
19、respect to the specimen plane to eliminate multiple reflectionsof the simulator beam. Multiple reflections could result in asmuch as a 7 % apparent increase in a/e.4.7 The solar simulator should duplicate the extraterrestrialsolar spectrum as closely as possible. A beam irradiance of atleast 7000 W/
20、m2at the specimen plane shall be available fromthe solar simulator (;5 solar constants). This irradiance maybe required to raise the temperature of certain specimens to adesired level.5. Coating Requirements5.1 Any type of coating may be tested by this test methodprovided its structure remains stabl
21、e in vacuum over thetemperature range of interest.5.2 For high emittance specimens the accuracy of themeasurements is increased if only one surface of the substrateis coated with the specimen coating in question. The remainingarea of the substrate shall be coated with a low emittancematerial of know
22、n hemispherical emittance (such as evapo-rated aluminum or evaporated gold).5.3 The thickness and density of the coating shall bemeasured and its heat capacity calculated from existing refer-ences (see Refs (1) and (2).46. Specimen Preparation6.1 The substrates used for the measurements describedher
23、e shall be of a material whose specific heat as a function oftemperature can be found in standard references (for example,OFHC copper or a common aluminum alloy such as 6061-T6)(Ref (1).6.2 The substrate shall be machined from flat stock and to asize proportioned to the working area of the chamber.6
24、.3 Each specimen shall be drilled with a set of holes, nearthe edge, through which suspension strings are to be inserted.6.4 Each substrate shall be drilled with two small shallowholes in the back for thermocouples.6.5 Ideally the back and sides of the substrate shall bebuffed and polished and one u
25、ninsulated thermocouple insertedin the back of the specimen (one wire in each hole). One ofthese wires shall be peened into each hole.6.6 A low-emittance coating shall be applied to the back andsides of the substrate and to the thermocouple wires for severalinches at the specimen end.6.7 The substra
26、tes shall be coated with the material inquestion. The coating shall be of sufficient thickness so as to beopaque. (This will avoid any substrate effects.)6.8 The specimens shall be suspended from the top of theshroud by means of thread or string. These strings shall be ofsmall diameter, low thermal
27、conductivity, and low emittance inorder to minimize heat losses through the leads.6.9 An alternative method of specimen mounting (massdependent) shall be to suspend the specimens by their ownsmall wire thermocouple leads. In this case the thermocoupleholes shall be drilled as before but radially aro
28、und the edge.The suspension holes may also be eliminated in this case.7. Procedure7.1 Suspend the test specimen in the chamber normal to theincident solar radiation, but geometrically removed from thecentral axis of the chamber so that radiation from the specimento the chamber walls is not specularl
29、y reflected back to thespecimen. Since the chamber walls are designed to be cold andhighly absorbing, first reflections from the walls are usually allthat need be considered.7.2 Determine the simulated solar irradiance incident on thespecimen with a suitable radiometric device such as a com-mercial
30、thermopile radiometer or a black monitor sample ofknown a/e which may be suspended similarly to the testspecimen within the incident beam of simulated solar radiation.Take care in the latter case that the irradiance and spectraldistribution of the incident energy is the same for bothspecimen and mon
31、itor.7.3 Then close the system and start the evacuation andcooling of the shroud (see Ref (3) for a typical system).Maintain a pressure of 1 3 106torr (0.1 mPa) or less and thewalls of the chamber must be at coolant temperature. Recordthe specimen, monitor, and shroud temperatures.7.4 When the speci
32、men has reached thermal equilibrium,that is, when the specimen temperature becomes constant withconstant surrounding conditions, shut off the solar simulator.When specimens of large thermal mass are used, carefullyevaluate the DT/Dt = 0 conditions, that is, the Dt chosen shouldbe dependent on the sp
33、ecimen time constant.7.5 Close the moveable door in the shroud and allow thespecimens to cool to a desired temperature. Measure thespecimen temperature as a function of time and calculate therates of change of the temperature.8. Calculation8.1 Calculate the a/e ratio from the following equation:a/e5
34、ATsT142 T04!/ApE (1)where:a = effective solar absorptance of the specimen,e = hemispherical emittance of the specimen,s = Stefan-Boltzman constant,Ap= projected area of the specimen exposed to solarradiation,E = incident total irradiance,T1= specimen equilibrium temperature with simulatedsolar radia
35、tion,T0= chamber wall temperature with solar source off, andAT= total radiating area of the specimen.8.2 This equation is derived in the following manner: If aspecimen coated on all sides with the material in question, with4The boldface numbers in parentheses refer to the list of references appended
36、 tothis method.E 434 71 (2002)2a projected area as viewed in the direction of irradiation, Ap,atotal area, AT, effective solar absorptance, a, emittance, e, andspecific heat cpis suspended in an evacuated high absorptanceisothermal cold-walled chamber and exposed to a simulatedsolar irradiance, E, t
37、he rate of temperature change can bedetermined by evaluating the heat balance equation. Theenergy balance of an irradiated specimen emitting radiantenergy in a vacuum is given by the following equation:mcpdT/dt! 5 ApaE 1 Ep2 AtesT142 T04! (2)where Ep= AesT24, the thermal radiation from the port. Tod
38、etermine the incident thermal radiation, Ep, see Ref (3). If Epis eliminated from Eq 2 when an equilibrium temperature isreached, mcp(dT/dt) = 0, and,From Eq 2, solving for the a/e ratio we obtaina/e5ATsT142 T04!/ApE (3)Eq 3 is used to calculate the a/e ratio when the parametersAT, E, and Apare dete
39、rmined and the equilibrium temperatureis measured.8.3 If the source is blocked by the shutter and the specimenlooses energy only by radiation, the energy balance equationbecomes:mcdT/dt! 5 ATesT142 T04! (4)8.4 If the term T04is neglected, the above equation can beintegrated and expanded into:e5mscs1
40、 mccc!3sATDtS1T1321T23 D(5)where:ms= mass of the substrate,mc= mass of the coating,cs= thermal capacitance of the substrate,cc= thermal capacitance of the coating,T = temperature of the specimen, andDt = change in time from T1to T2and magnitude such thatcsand ccmay be assumed constant over smalltemp
41、erature ranges.When the temperature decay is recorded with time, then thetotal hemispherical emittance of the sample can be determinedwith Eq 4 or Eq 5. The use of Eq 5 is preferable since Eq 4involves the experimental determination of two quantities(dT/dt and T4), thereby introducing more possible
42、errors than inEq 5.8.5 Data from specimens which are coated on one side onlyshall be reduced by use of the following equation:ec5mscs1 mccc!3sAcDtS1T1321T23 D2esAT2 Ac!Ac(6)where:es= total hemispherical emittance of substrate,Ac= area of coating, andec= total hemispherical emittance of coating.8.6 T
43、o obtain an a/e measurement or an effective solarabsorptance, a, for a specimen coated only on one side, onemust consider the following expression:ATeT5 Acec1 Ases(7)where:AT,Ac,As= total area, area of the coating, and uncoatedarea of the substrate, respectively, andeT, ec, es= total hemispherical e
44、mittance of the speci-men, coating, and substrate respectively.Rearrangement shows that:eT5 Acec1 Ases!/AT(8)Multiplying the a/e value obtained from Eq 3 by eT(at thesame temperature of equilibrium) obtained from Eq 8 will givethe solar absorptance, a. In order to acquire the (a/e) coating,divide th
45、e asvalue by ec(already measured in a transient cooldown).9. Report9.1 The report should include the methods used for tem-perature and irradiance measurements, and the actual data usedfor the calculations.9.2 A complete characterization of the specimen shall begiven whenever possible. This shall inc
46、lude specimen dimen-sions, specimen composition, coating thickness and composi-tion, surface roughness, and surface contamination, and anyother conditions which may be considered pertinent.9.3 In an a/e type of measurement, the total exposure timeand level of irradiance, and spectral distribution of
47、 the incidentflux shall also be reported.10. Uncertainty Analysis10.1 Many potential errors exist in the calorimetric deter-mination of radiative properties. If it is assumed that the majoruncertainties encountered in these calorimetric measurementsare systematic rather than random, they will add in
48、 a linearmanner and the total uncertainty can be expressed as:des/es5 des/es!conv1 des/es!q1 des/es!R1 des/es!HL(9)for emittance, and asda/ea/e5Sda/ea/eDconv1Sda/ea/eDR1Sda/ea/eDHL1Sda/ea/eDS(10)for the ratio of solar absorptance to hemispherical emittance.The terms on the right of the emittance unc
49、ertainty equationcan be defined as conv the conventional error, q the heatmeasurement error, R the extraneous radiation error, and HLthe heat loss error, respectively. In the uncertainty equation fora/e the last term, s, is defined as the error due to solarsimulation where all fractional errors have been previouslydefined. These uncertainties are discussed in the followingparagraphs.10.2 Conventional ErrorThe conventional error contribu-tion to the total uncertainty involves errors in the measurementof the basic physical quantities of a sample such as the area ofthe sa
