1.1 This guide establishes the approach and basic criteria for the laboratory measurement of the steady-state thermal transmission properties and heat flux of insulating materials under cryogenic conditions. Specimens may be flat or cylindrical, homogeneous or non-homogeneous at boundary conditions from 4K to 400 K and in environments from high vacuum to ambient pressure of residual gas. 1.2 The function of most cryogenic thermal insulation systems used in these applications is to maintain large to very large temperature differentials (delta-T) thereby providing high levels of thermal insulating performance. Cold boundary temperatures range from 4 K to 100 K or higher. Warm boundary temperatures range from 250 K up to 400 K. Temperature differentials of up to 300 K are typical. 1.3 The range of pressures is from 10-6 to 10+3 torr with different residual gases as required. Corresponding to the applications in cryogenic systems, three sub-ranges of vacuum are also defined: from 10-6 to 10-2 torr (high vacuum/free molecular regime), from 10-1 to 10 torr (soft vacuum, transition regime), from 100 to 1000 torr (no vacuum, continuum regime). 1.4 Full vacuum pressure range means nearly four orders of magnitude in thermal performance variations. The range of apparent thermal conductivity is from 0.01 to 100 mW/m-K, with concentrations in the range of 0.05 to 2 mW/m-K for high vacuum insulation systems and 10 to 25 mW/m-K for no vacuum systems. Soft vacuum systems are generally in between these two extremes. 1.5 The test devices (apparatus) designed for this purpose are listed in two categories: boil-off calorimetry (A through D) and electrical power (E and F). Both absolute and comparative methods are included. 1.6 These test methods set forth the general design requirements necessary to construct and operate a satisfactory apparatus. The methods cover a wide variety of apparatus constructions, test conditions, and operating conditions. Detailed designs conforming to this test method are not given but must be developed within the constraints of the general requirements. 1.7 These test methods are applicable to the measurement of a wide variety of specimens, ranging from opaque solids to porous or transparent materials, and a wide range of environmental conditions including measurements conducted at extremes of temperature and with various gases and pressures. 1.8 In order to ensure the level of precision and accuracy expected, persons applying this standard must possess a working knowledge of the requirements of thermal measurements and testing practice and of the practical application of heat transfer theory relating to thermal insulation materials and systems. Detailed operating procedures, including design schematics and electrical drawings, should be available for each apparatus to ensure that tests are in accordance with this test method. In addition, automated data collecting and handling systems connected to the apparatus must be verified as to their accuracy. This can be done by calibration and inputting data sets, which have known results associated with them, into computer programs. 1.9 It is impractical to establish all details of design and construction and to provide the procedures to cover all contingencies that can present difficulties to a person with incomplete technical knowledge concerning theory of heat flow, temperature measurements and general testing practices. The user may also find it necessary, when repairing or modifying the apparatus, to become a designer or builder, or both, on whom the demands for fundamental understanding and careful experimental technique are even greater. Standardization of this test method is not intended to restrict in any way the future development of new or improved apparatus or procedures. 1.10 These test methods do not specify all details necessary for the operation of the apparatus. Decisions on sampling, specimen selection, preconditioning, specimen mounting and positioning, the choice of test conditions, and the evaluation of test data shall follow applicable ASTM Test Methods, Guides, Practices or Product Specifications or governmental regulations. If no applicable standard exists, sound engineering judgment that reflects accepted heat transfer principles must be used and documented. 1.11 These test methods allow a wide range of apparatus design and design accuracy to be used in order to satisfy the requirements of specific measurement problems. Compliance with these test methods requires a statement of the uncertainty of each reported variable in the report. A discussion of the significant error factors involved is included.
The need for practical thermal conductivity data for low-temperature (cryogenic) applications continues to grow in areas such as oil & gas, electrical power, refrigerated transport, aerospace, aircraft, ground transportation, industrial processes, semi-conductor manufacturing, and many others. New materials, for example, aerogel blankets, aerogel bulk-fill, glass bubbles, polyimide foams, are now commercially available and are being applied for low-temperature insulation solutions and broad industrial application. New multilayer and composite insulation materials have also been developed for high levels of thermal performance, combined thermo-mechanical functionality, or other applications to meet the growing demands for energy efficiency and environmental responsibility in todays economy. The function of most of the thermal insulation systems used in these applications is to maintain large to very large temperature differentials (delta-T) thereby providing quite high levels of thermal insulating performance. This large delta-T functionality is combined with requirements for operating in a wide range of environments. The insulation materials used are typically of a low-density, porous nature. Therefore, the exposed environments together with the imposed temperature gradients can cause substantial variations in the thermophysical properties of the materials and even dramatic change within the make-up of the insulation system. Added to these two core factors, large delta-T and environment, are the well-recognized complexities of the highly mixed modes of heat transfer within such insulation systems: radiation, gas conduction, convection, and solid conduction. Accurate thermal characterization and standard thermal conductivity data are needed to meet the needs for specific engineering applications and global progress all areas related to energy efficiency. Materials include homogeneous, nonhomogeneous, organic, inorganic, reflective, blanket, and loose fill forms. Although much work has been done in the last 20 years, building on the advances of the 1950s and 1960s, the need for cryogenic insulation standards have become increasingly clear. But what is needed first is a standard for the test methods. The testing methods must reflect the way the materials are actually used: that is, for large delta-T and in a relevant environment. Furthermore, as the levels of heat flow are typically in the range of very low to low, the methods must be approached in such a way that the measurements are practical to achieve and consistent among labs. Existing methods would be complementary to the new methods, or vice-versa. The new methods will be used for testing of insulation materials under cryogenic-vacuum conditions or similar environments. The range of temperature is from 4 K to 400 K and temperature differentials of up to 300 K are typical. The range of pressures is from 10-6 to 10+3 torr with different residual gases as required. Reflecting the applications in cryogenic systems, three sub-ranges of vacuum are also defined: from 10-6 to 10-2 torr (high vacuum), from 10-1 to 10 torr (soft vacuum), from 100 to 1000 torr (no vacuum). The range of apparent thermal conductivity is from 0.01 to 100 mW/m-K, with concentrations in the range of 0.05 to 2 mW/m-K for high vacuum insulation systems and 10 to 25 mW/m-K for no vacuum systems. The definition of apparent thermal conductivity in this context must also be addressed: the proper terms can be recommended for inclusion as part of C168 and C1045 as necessary. There are two measurement principles currently used for cryogenic-vacuum thermal conductivity testing. These are boil-off (or evaporation) and electrical power. Boil-off testing gives a direct result from knowing the heat of vaporization of the cryogen. The cold boundary temperature is of course provided by the cryogen. Electrical power testing requires the use of precision temperature measurement and calibration. The cold boundary temperature is in this case provided by either a cryogen or a cryocooler. Both types of measurements could be included in one standard, but the strong differences in execution between them suggest that separate standards may be needed. In the past 15 years or so, extensive research testing of many different materials has been performed in collaboration with both research institutions and private companies. This work has produced a wealth of thermal conductivity data and testing technology for cryogenic insulation. Along these lines, a Task Group for new cryogenic insulation standards, has been started. The conclusion from the first two meetings is that a Guide to cryogenic insulation is needed first and foremost. The standard test methods can then follow as the needs and markets are further defined.
cryogenic, thermal insulation systems, vacuum, cold vacuum pressure, warm vacuum pressure, boil-off calorimetry, liquid nitrogen, cryogen, cold boundary temperature, warm boundary temperature, heat of vaporization, multilayer insulation, bulk-fill materials, aerogels, foams, powders
The title and scope are in draft form and are under development within this ASTM Committee.
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