1、Designation: C835 06 (Reapproved 2013)1Standard Test Method forTotal Hemispherical Emittance of Surfaces up to 1400C1This standard is issued under the fixed designation C835; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTESection 16 was editorially revised in April 2014.1. Scope1.1 This calorimetric test method covers the determinationof tot
3、al hemispherical emittance of metal and graphite surfacesand coated metal 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
4、hasbeen demonstrated for use up to 1400 C. The lower-usetemperature is limited by the temperature of the bell jar.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety
5、 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 regulatory limitations prior to use. For specific hazardstatements, see Section 7.2. Referenced Documents2.1 ASTM
6、 Standards:2C168 Terminology Relating to Thermal InsulationE230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 DefinitionsThe terms and symbo
7、ls are as defined inTerminology C168 with exceptions included as appropriate.3.2 Symbols:ei= error in the variable i, 6 %,1= total hemispherical emittance of heated specimen,dimensionless,2= total hemispherical emittance of bell jar inner surface,dimensionless, = Stefan-Boltzmann constant,= 5.669 10
8、8W/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 generation ismeasured, m2,A2= surface area of bell jar inner surface, m2,F = the gray body shape factor, which includes the effectof geometry
9、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 Method4.1 A strip specimen of the material, approximately 13 mmwide and 250 mm long, is placed in an evacuated chamber and
10、is 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 thetemperature of this region are measured. Using the Stefan-Boltzmann equation, this power is equated to the radiative hea
11、ttransfer to the surroundings and, with the measuredtemperature, is used to calculate the value of the total hemi-spherical emittance of the specimen surface.5. Significance and Use5.1 The emittance as measured by this test method can beused in the calculation of radiant heat transfer from surfacest
12、hat 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 conditions on the emittance of materials. In particular,the use of this test method with furnace exposure (time attemper
13、ature) 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 a vacuum environment. Usually this condition1This test method is under the jurisdiction ofASTM Committee C16 on Ther
14、malInsulation and is the direct responsibility of Subcommittee C16.30 on ThermalMeasurement.Current edition approved Sept. 1, 2013. Published April 2014. Originallyapproved in 1976. Last previous edition approved in 2006 as C835 06. DOI:10.1520/C0835-06R13E01.2For referenced ASTM standards, visit th
15、e ASTM website, www.astm.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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. U
16、nited States1will 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 ofmaterials used in air or other atmospheres may be different. Inaddition, preconditioned surfaces, as describe
17、d 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 those that exist in air, and the user must be aware of thissituation. With these qualifications in m
18、ind, 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 method. They arethat:5.4.1 The enclosure is a blackbody emitter at a uniformtemperature,5.4.2 The t
19、otal 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 There is no heat loss from the test section by convec-tion or conduction. For most materials tested
20、 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 conformance with this testmethod, the principles governing the size, construction, anduse of apparatus
21、 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 are to be reported ashaving been obtained by this test method, all of the require-ments prescribed in
22、 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 person withouttechnical knowledge concerning the theory of heat transfer,temperature measurements, and g
23、eneral 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 of improved ornew methods or procedures by research workers because ofstandardization of this test
24、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 orother readout, vacuum system, and specimen holders. Aschematic of the test arrangement is shown in Fig
25、. 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 surface.6.2 Bell Jar:6.2.1 The bell jar may be either metal or glass with an innersurface that presen
26、ts 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 surface area. Therelationship between bell jar size and its required surfaceemittance is estimated f
27、rom the following equation for the graybody shape factor for a surface completely enclosed by anothersurface:F 51111A1A2S122 1D(1)For this test method to apply, the following condition mustexist:11A1A2S122 1D(2)This condition can be satisfied for all possible values ofspecimen emittance by an appara
28、tus design in which A1/A2hasa value less than 0.01 and 2has 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 with a black paint (1).33The boldface numbers in parentheses refer to the list of references at the end of
29、this standard.FIG. 1 System ArrangementC835 06 (2013)12It 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.6.2.2 The bell jar must be opaque to
30、 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 thisrequirement.6.2.3 The need for bell jar cooling is determined by thelower-use temperature of the part
31、icular 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 100Cdifference between the specimen and the bell jar is recom-mended to achieve the desired method accuracy
32、. 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 otheroperating condition, auxiliary cooling of the bell jar is alsorequired.An alternative to bell jar c
33、ooling 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 specimen electrically by makingit a resistive part of the circuit. The true electrical power to thete
34、st 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 point significantly above the highest testtemperature of the specimen. To minimize temperature mea-
35、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 indicates that diameters lessthan 0.13 mm provide acceptable results.6.4.1 The test section is de
36、fined 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 attachmentbecause it results in minimum disturbance of the specimensurface. Swaging and peening are alter
37、native 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 representative average. Four thermocouples havebeen found to be sufficient for the system shown in Fig. 1
38、.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 thermocouples that bound the test section. Apotentiometer, or equivalent instrument, having a sensi
39、tivity of2V or less is required for measuring the thermocouple emfsfrom which the test section temperatures are obtained.6.4.4 Temperature sensors must be calibrated to within theuncertainty allowed by the apparatus design accuracy. Forinformation concerning sensitivity and accuracy ofthermocouples,
40、 see Table 1 of Tables E230. For a comprehen-sive discussion on the use of thermocouples, see Ref (2). Forlow temperature thermocouple reference tables, see Ref (3).6.5 Vacuum System A vacuum system is required toreduce the pressure in the bell jar to 1.3 mPa or less tominimize convection and conduc
41、tion 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 transfer fromthe specimen is by radiation. As pressure increases, gasconduction becomes important.6.5
42、.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 an existing system are includedin Table 1 and photographs of a system are included in Fig. 3and Fi
43、g. 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. If measured elsewhere in the pumping system, suchas in the diffusion pump inlet, the pressure dr
44、op between themeasuring location and the bell jar must be accounted for. TheFIG. 2 Example of Effect of Air Pressure on Measured Emittance of Oxidized InconelC835 06 (2013)13vacuum system should also be checked for gross leakage thatcould allow incoming gas to sweep over the specimen.6.6 Specimen Ho
45、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.Holdersshould be positioned off-center within the bell jar to minimizenormal reflections betwe
46、en 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 dimensions (width and thickness) of the testspecimen and the length between voltage taps and th
47、ermo-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 between thermocouples should alsobe measured to the nearest 0.5 mm.6.8 All instruments shall be
48、 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.7.2 Power leads to the apparatus should be well insulatedand fused.7.3 Power to the specim
49、en 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 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
