1、Designation: C 835 06Standard Test Method forTotal Hemispherical Emittance of Surfaces up to 1400C1This standard is issued under the fixed designation C 835; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision
2、. 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 calorimetric test method covers the determinationof total hemispherical emittance of metal and graphite surfacesand coated me
3、tal surfaces up to approximately 1400C. Theupper-use temperature is limited only by the characteristics (forexample, melting temperature, vapor pressure) of the specimenand the design limits of the test facility. This test method hasbeen demonstrated for use up to 1400 C. The lower-usetemperature is
4、 limited by the temperature of the bell jar.1.2 This standard does not purport to address all of thesafety concerns, 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 regulat
5、ory limitations prior to use. For specific hazardstatements, see Section 7.2. Referenced Documents2.1 ASTM Standards:2C 168 Terminology Relating to Thermal InsulationE 230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE 691 Practice for Conducting an Int
6、erlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 DefinitionsThe terms and symbols are as defined inTerminology C 168 with exceptions included as appropriate.3.2 Symbols:ei= error in the variable i, 6 %,e1= total hemispherical emittance of heated specimen,dimensionless,e
7、2= total hemispherical emittance of bell jar inner sur-face, dimensionless,s = Stefan-Boltzmann constant,= 5.669 3 108W/m2K4,Q = heat flow rate, W,T1= temperature of heated specimen, K,T2= temperature of bell jar inner surface, K,A1= surface area of specimen over which heat generationis measured, m2
8、,A2= surface area of bell jar inner surface, m2,F = the gray body shape factor, which includes the effectof geometry and the departure of real surfaces fromblackbody conditions, dimensionless, andPa = absolute pressure, pascal (N/m2). One pascal isequivalent to 0.00750 mm Hg.4. Summary of Test Metho
9、d4.1 A strip specimen of the material, approximately 13 mmwide and 250 mm long, is placed in an evacuated chamber andis directly heated with an electric current to the temperature atwhich the emittance measurement is desired. The powerdissipated over a small central region of the specimen and thetem
10、perature of this region are measured. Using the Stefan-Boltzmann equation, this power is equated to the radiative heattransfer to the surroundings and, with the measured tempera-ture, is used to calculate the value of the total hemisphericalemittance of the specimen surface.5. Significance and Use5.
11、1 The emittance as measured by this test method can beused in the calculation of radiant heat transfer from surfacesthat are representative of the tested specimens, and that arewithin the temperature range of the tested specimens.5.2 This test method can be used to determine the effect ofservice con
12、ditions on the emittance of materials. In particular,the use of this test method with furnace exposure (time attemperature) of the materials commonly used in all-metallicinsulations can determine the effects of oxidation on emittance.5.3 The measurements described in this test method areconducted in
13、 a vacuum environment. Usually this conditionwill provide emittance values that are applicable to materialsused under other conditions, such as in an air environment.However, it must be recognized that surface properties of1This test method is under the jurisdiction ofASTM Committee C16 on ThermalIn
14、sulation and is the direct responsibility of Subcommittee C16.30 on ThermalMeasurement.Current edition approved Nov. 1, 2006. Published December 2006. Originallyapproved in 1976. Last previous edition approved in 2006 as C 835 01(2006).2For referenced ASTM standards, visit the ASTM website, www.astm
15、.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.materials
16、used in air or other atmospheres may be different. Inaddition, preconditioned surfaces, as described in 5.2, may bealtered in a vacuum environment because of vacuum strippingof absorbed gases and other associated vacuum effects. Thus,emittances measured under vacuum may have values that differfrom t
17、hose that exist in air, and the user must be aware of thissituation. With these qualifications in mind, emittance obtainedby this test method may be applied to predictions of thermaltransference.5.4 Several assumptions are made in the derivation of theemittance calculation as described in this test
18、method. They arethat:5.4.1 The enclosure is a blackbody emitter at a uniformtemperature,5.4.2 The total hemispherical absorptance of the completelydiffuse blackbody radiation at the temperature of the enclosureis equal to the total hemispherical emittance of the specimen atits temperature, and5.4.3
19、There is no heat loss from the test section by convec-tion or conduction. For most materials tested by the proceduresas described in this test method, the effects of these assump-tions are small and either neglected or corrections are made tothe measured emittance.5.5 For satisfactory results in con
20、formance with this testmethod, the principles governing the size, construction, anduse of apparatus described in this test method should befollowed. If these principles are followed, any measured valueobtained by the use of this test method is expected to beaccurate to within 65 %. If the results ar
21、e to be reported ashaving been obtained by this test method, all of the require-ments prescribed in this test method shall be met.5.6 It is not practical in a test method of this type toestablish details of construction and procedure to cover allcontingencies that might offer difficulties to a perso
22、n withouttechnical knowledge concerning the theory of heat transfer,temperature measurements, and general testing practices. Stan-dardization of this test method does not reduce the need forsuch technical knowledge. It is recognized also that it would beunwise to restrict in any way the development
23、of improved ornew methods or procedures by research workers because ofstandardization of this test method.6. Apparatus6.1 In general, the apparatus shall consist of the followingequipment: a bell jar, power supply and multi-meter for voltageand current measurements, thermocouples and voltmeter oroth
24、er readout, vacuum system, and specimen holders. Aschematic of the test arrangement is shown in Fig. 1. Meansmust be provided for electrically heating the specimen, andinstruments are required to measure the electrical power inputto the specimen and the temperatures of the specimen andsurrounding su
25、rface.6.2 Bell Jar:6.2.1 The bell jar may be either metal or glass with an innersurface that presents a blackbody environment to the specimenlocated near the center. This blackbody effect is achieved byproviding a highly absorbing surface and by making thesurface area much larger than the specimen s
26、urface area. Therelationship between bell jar size and its required surfaceemittance is estimated from the following equation for the graybody shape factor for a surface completely enclosed by anothersurface:F 511e11A1A2S1e22 1D(1)For this test method to apply, the following condition mustexist:1e1A
27、1A2S1e22 1D(2)This condition can be satisfied for all possible values ofspecimen emittance by an apparatus design in which A1/A2hasa value less than 0.01 and e2has a value greater than 0.8. Toensure that the inner surface has an emittance greater than 0.8,metal and glass bell jars shall be coated wi
28、th a black paint (1).3It is permissible to leave small areas in the glass bell jarsuncoated for visual monitoring of the specimen during a test.Metal bell jars can be provided with small-area glass viewports for sample observation.3The boldface numbers in parentheses refer to the list of references
29、at the end ofthis standard.FIG. 1 System ArrangementC8350626.2.2 The bell jar must be opaque to external high energyradiation sources (such as open furnaces, sunlight, and otheremittance apparatuses) if they are in view of the specimen.Both the coated metal and coated glass bell jars meet thisrequir
30、ement.6.2.3 The need for bell jar cooling is determined by thelower-use temperature of the particular apparatus and by themaximum natural heat dissipation of the bell jar. A bell jaroperating at room temperature (20C) may be used for speci-men temperatures down to about 120C. At least a 100Cdifferen
31、ce between the specimen and the bell jar is recom-mended to achieve the desired method accuracy. Therefore, forlower specimen temperatures, bell jar cooling is required. If thenatural heat dissipation of the bell jar is not sufficient tomaintain its temperature at the desired level for any otheroper
32、ating condition, auxiliary cooling of the bell jar is alsorequired.An alternative to bell jar cooling is the use of a cooledshroud (for example, cooled by liquid nitrogen) between thespecimen and the bell jar.6.3 Power Supply The power supply may be either ac ordc and is used to heat the test specim
33、en electrically by makingit a resistive part of the circuit. The true electrical power to thetest section must be measured within a proven uncertainty of61 % or better.6.4 Thermocouples, are used for measuring the surfacetemperature of the specimen. The thermocouple materials musthave a melting poin
34、t significantly above the highest testtemperature of the specimen. To minimize temperature mea-surement errors due to wire conduction losses, the use ofhigh-thermal conductivity materials such as copper should beavoided. The size of the thermocouple wire should be theminimum practical. Experience in
35、dicates that diameters lessthan 0.13 mm provide acceptable results.6.4.1 The test section is defined by two thermocouplesequally spaced from the specimen holders. A third thermo-couple is located at the center of the specimen. Spot weldinghas been found to be the most acceptable method of attachment
36、because it results in minimum disturbance of the specimensurface. Swaging and peening are alternative methods pre-scribed for specimens that do not permit spot welding.6.4.2 The number of thermocouples used to measure thetemperature of the absorbing surface shall be sufficient toprovide a representa
37、tive average. Four thermocouples havebeen found to be sufficient for the system shown in Fig. 1.Thermocouple locations include three on the bell jar and oneon the baseplate.6.4.3 The voltage drop in the measurement area of thespecimen is measured by tapping to similar elements of each ofthe two ther
38、mocouples that bound the test section. A potenti-ometer, or equivalent instrument, having a sensitivity of 2V orless is required for measuring the thermocouple emfs fromwhich the test section temperatures are obtained.6.4.4 Temperature sensors must be calibrated to within theuncertainty allowed by t
39、he apparatus design accuracy. Forinformation concerning sensitivity and accuracy of thermo-couples, see Table 1 of Tables E 230. For a comprehensivediscussion on the use of thermocouples, see Ref (2). For lowtemperature thermocouple reference tables, see Ref (3).6.5 Vacuum System A vacuum system is
40、required toreduce the pressure in the bell jar to 1.3 mPa or less tominimize convection and conduction through the residual gas.This effect is illustrated in Fig. 2, which shows the measuredemittance of oxidized Inconel versus system pressure. Thiscurve is based upon the assumption that all heat tra
41、nsfer fromthe specimen is by radiation. As pressure increases, gasconduction becomes important.6.5.1 For the specified pressure level, a pumping systemconsisting of a diffusion or ion pump and mechanical pump isrequired. If backstreaming is a problem, cold trapping isrequired. The specifications of
42、an existing system are includedin Table 1 and photographs of a system are included in Fig. 3and Fig. 4. This information is included as a guide to assist inthe design of a facility and is not intended to be a rigidspecification.6.5.2 The specified pressure (1.3 mPa or less) must exist inthe bell jar
43、. If measured elsewhere in the pumping system, suchas in the diffusion pump inlet, the pressure drop between themeasuring location and the bell jar must be accounted for. Thevacuum system should also be checked for gross leakage thatcould allow incoming gas to sweep over the specimen.6.6 Specimen Ho
44、lders, must be designed to allow forthermal expansion of the specimen without buckling. Thelower specimen holder shown in Fig. 4 is designed to move upand down in its support to allow for thermal expansion.HoldersFIG. 2 Example of Effect of Air Pressure on Measured Emittance of Oxidized InconelC8350
45、63should be positioned off-center within the bell jar to minimizenormal reflections between the specimen and bell jar innersurface. Specimen holders require auxiliary cooling if endconduction from the specimen causes overheating.6.7 Micrometer Calipers, or other means are needed tomeasure the dimens
46、ions (width and thickness) of the testspecimen and the length between voltage taps and thermo-couples at room temperature. The specimen dimensions (widthand thickness) should be measured to the nearest 0.025 mm.The length between voltage taps should be measured to thenearest 0.5 mm.The length betwee
47、n thermocouples should alsobe measured to the nearest 0.5 mm.6.8 All instruments shall be calibrated initially and recali-brated at reasonable intervals.7. Hazards7.1 Thin metallic specimens provide the possibility for cutsto the handler. Specimens should, therefore, be treated gentlyand with care.7
48、.2 Power leads to the apparatus should be well insulatedand fused.7.3 Power to the specimen should be cut off before disman-tling has begun.7.4 Normal safety precautions dictate that an implosionshield be provided if a glass bell jar is used. One example of aproblem that can occur with a glass bell
49、jar is the local thermalstress resulting from uneven heating of the bell jar.8. Test Specimen8.1 The specimen used for a test must be sufficientlyuniform in surface to represent the sample material from whichit is taken. Caution must be exercised to prevent contaminationof the specimen surface from all sources, and especially fromfingerprints.8.2 The size of the test specimen must be compatible withthe power supply and desired maximum test temperature. Fig.5 shows acceptable overall test specimen dimensions for threematerials in use with a 16-V, 100-A ac power supply. Sp
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