ASTM C745-92(1999)

The thermal performance of multilayer insulations will vary from specimen to specimen due to differences in the material properties, such as the emittance of the reflective shields. In addition, it can vary due to environmental conditioning and the presence of foreign matter such as oxygen or water vapor. Finally, it can vary due to aging, settling, or exposure to excessive mechanical pressures which could wrinkle or otherwise affect the surface texture of the layers. For these reasons, it is imperative that specimen materials be selected carefully to obtain representative samples. It is recommended that several specimens of any one material be tested and no less than four data points obtained for each. For specimens where heat transfer measurements under high-vacuum conditions are required, a preconditioning procedure should be employed to remove water vapor and other outgas components from the multilayer materials.

1.1 This test method covers the determination, from cryogenic to near room temperatures, of heat flux through evacuated insulations (Note 1) within the approximate range from 0.3 to 30 W/m . Heat flux values obtained using this method apply strictly only to the particular specimens as tested.

1.2 This shall be a primary test method for measuring heat flux through evacuated insulations (Note 2), since calibration of the apparatus depends on measurement standards traceable to the National Institute of Standards and Technology (NIST) for length, force, temperature, time, etc. Traceable standards are not yet available for heat flux through standard evacuated reference specimens or transfer standards.

1.3 Specimens to be tested using this method shall be flat and may be either a circular or a rectangular configuration, as appropriate for the particular apparatus being used (Note 3). Contoured specimens or those of other shapes must be tested by other methods which are outside the scope of this standard. Specimen sizes and thicknesses shall conform to the limitations specified in Section 7.

1.4 Environmental and other parameters that can be varied in the application of this method are ( ) the hot and cold boundary temperatures, ( ) the boundary temperature at the exposed edge of the specimen, ( ) the mechanical compressive pressure to be imposed on the specimen, and ( ) the species and partial pressure of the gas occupying the interlayer cavities of the specimen and the test chamber (Note 4). Hot boundary temperature can be varied within the approximate range from 250 to 670 K, while cold boundary temperature can be varied from approximately 20 to 300 K (Note 5). Selection of boundary temperatures to be imposed at the hot and cold surfaces and at the edge of the specimen shall be subject to the limitations specified in Section 5. Mechanical compressive pressure values to be imposed using this method can vary in the approximate range from 5 to 10 kPa (Note 6).

1.5 Stating that test results were obtained using this specific method requires that all of the variables must be controlled, measured, and recorded as specified herein.

1.6 Details of construction of the calorimeter cannot be covered entirely by this specification since some technical knowledge is required regarding the compatibility of materials with the fluids used, temperature extremes that will be encountered, practical limitations in achieving and controlling the mechanical compressive pressure, and other contingencies. However, existing types of construction and measuring techniques were considered as a guide for this specification and are presented herein as requirements with the realization that developments and improvements can always be made.

1.7 SI units are to be regarded as standard in this test method. Conversion factors for use to obtain imperial equivalents are presented in Table A1.1.

1.8 This standard does not purport to address all of the safety problems, 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. Specific precautionary statements are given in Section 6.