1、Designation:C 745 92(Reapproved 1999)Standard Test Method forHeat Flux Through Evacuated Insulations Using a GuardedFlat Plate Boiloff Calorimeter1This standard is issued under the fixed designation C 745;the number immediately following the designation indicates the year oforiginal adoption or,in t
2、he 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 the determination,from cryo-genic to near room temperatures,of hea
3、t flux through evacu-ated insulations(Note 1)within the approximate range from0.3 to 30 W/m2.Heat flux values obtained using this methodapply strictly only to the particular specimens as tested.NOTE1This test method is primarily intended for use to assess heatflux through evacuated multilayer insula
4、tions which are highly anisotropicby nature.Characteristically,multilayer insulations exhibit apparent ther-mal conductivity values one or two orders of magnitude lower than thebest available powder,fiber,or foam insulations.Although this testmethod is also technically applicable to these latter ins
5、ulations,otherASTM methods with less stringent requirements are equally applicableand much more economical and practical for such materials.1.2 This shall be a primary test method for measuring heatflux through evacuated insulations(Note 2),since calibrationof the apparatus depends on measurement st
6、andards traceableto the National Institute of Standards and Technology(NIST)for length,force,temperature,time,etc.Traceable standardsare not yet available for heat flux through standard evacuatedreference specimens or transfer standards.NOTE2Values of heat flux for the same materials and environment
7、sspecified in this method may also be obtained by measuring electricalenergy dissipation using a guarded hot plate(Test Method C 177)(1,2)2or a guarded cylindrical apparatus(3,4),or by measuring transientthermal response(5).1.3 Specimens to be tested using this method shall be flatand may be either
8、a circular or a rectangular configuration,asappropriate for the particular apparatus being used(Note 3).Contoured specimens or those of other shapes must be testedby other methods which are outside the scope of this standard.Specimen sizes and thicknesses shall conform to the limitationsspecified in
9、 Section 7.NOTE3Existing guarded flat plate boil-off calorimeters require cir-cular specimens.For highly anisotropic multilayer insulations,this con-figuration somewhat simplifies heat transfer calculations,since the result-ing heat flow is two-dimensional rather than three-dimensional as it wouldbe
10、 for a rectangular specimen.1.4 Environmental and other parameters that can be variedin the application of this method are(1)the hot and coldboundary temperatures,(2)the boundary temperature at theexposed edge of the specimen,(3)the mechanical compressivepressure to be imposed on the specimen,and(4)
11、the speciesand partial pressure of the gas occupying the interlayer cavitiesof the specimen and the test chamber(Note 4).Hot boundarytemperature can be varied within the approximate range from250 to 670 K,while cold boundary temperature can be variedfrom approximately 20 to 300 K(Note 5).Selection o
12、fboundary temperatures to be imposed at the hot and coldsurfaces and at the edge of the specimen shall be subject to thelimitations specified in Section 5.Mechanical compressivepressure values to be imposed using this method can vary in theapproximate range from 5 to 10 kPa(Note 6).NOTE4Although thi
13、s test method is primarily intended for use tomeasure heat flux through evacuated insulations,it is also applicable formeasurements where the specimen contains air or other gases at pressuresranging from fully evacuated to atmospheric.However,where measure-ments are to be made on a specimen that is
14、not evacuated to a pressure of1 mPa or less,the apparatus shall be provided with a low-conductivitypressure diaphragm to maintain high-vacuum conditions in the annularspace between the measuring and guard vessels.Heat transfer through evacuated multilayer insulations can vary signifi-cantly from spe
15、cimen to specimen or from test to test due to the presenceof minute but unknown quantities of outgas components(primarily watervapor)within the interstitial cavities.This effect can be minimized withpreconditioning of the specimen by extended evacuation at room tempera-ture or by a combination of he
16、at and evacuation over a much shorter timespan(see 9.2).NOTE5Cold boundary temperatures down to that of liquid hydrogen(20 K)can be achieved using existing apparatus.Temperatures toapproximately 4 K could be achieved with development of an apparatussuitable for use with liquid helium.NOTE6The lower limit of mechanical compressive pressure that canbe achieved for any particular specimen is the self-compression value dueto the weight of the specimen within the earths gravitational field.1.5 Statin
