James Fesmire (NASA)
Several cryostat instruments for the testing of thermal insulation systems in a flat plate configuration have been developed and standardized for laboratory operation. The measurement principal is boiloff calorimetry for the determination of the effective thermal conductivity (ke) and heat flux (q) of a test specimen under a wide range of real-world conditions. The main instrument, Cryostat-500, is thermally guarded to provide absolute thermal performance data when properly calibrated with a known reference material. An adjustable edge guard ring enables calibration. Using liquid nitrogen as the energy meter, the cold boundary temperature can be adjusted to any temperature between 77 K and approximately 300 K by interposing a thermal resistance layer between the cold mass and the specimen being tested. The cold mass assembly, comprised of the heat measurement vessel and thermal guard vessel, is suspended from the vacuum chamber lid. A low thermal conductivity suspension system includes adjustable compliance rod assemblies for a given test specimen thickness and for amount of compression force. Different materials and varied test objectives require an appropriate combination of apparatus and method. Material type, thickness, density, flatness, compliance, outgassing, and temperature sensor placement are important considerations. Edge effects and calibration techniques for the flat plate apparatus are crucial in design and test operation. Over the full vacuum pressure range, the thermal performance capability is shown to be nearly four orders of magnitude. Compared to vertical cylindrical cryostats, the horizontal flat plate configuration provides key advantages for testing at ambient pressure conditions. The Cryostat-500 design details and laboratory standard test methods are discussed. Results for select thermal insulation materials including composites, foams, and aerogels are presented. Additional flat plate boiloff calorimeters including comparative instruments Cryostat-400 and Macroflash (Cup Cryostat) are also addressed.
James Fesmire (NASA)