Cryogenic Thermal-Conductivity Measurements of Insulating Materials

Two methods of determining the thermal conductivities of cryogenic insulation systems over a wide range of variables are presented and discussed. Both methods are similar in that thermal-conductivity determinations are made calorimetrically by measuring the boiloff rate of the liquid cryogen in the test section. One method utilizes a modified National Bureau of Standards type of cryostat whose capabilities include variation of the external boundary temperature from 212 to −320 F, while the inner boundary temperature is dependent upon the choice of cryogen. Interstitial pressures can be varied from 10⁻⁷ torr to 1 atmos. A second method is provided by a flat-plate calorimeter which consists of two isothermal boundaries, one of which is a warm boundary whose temperature can be varied from −320 to 1500 F. The cold boundary consists of two cryogen reservoirs, the calorimeter reservoir and the guard reservoir, whose temperatures are dependent on the cryogen choice. The effect of residual-gas conduction on the thermal conductivity of the insulation can also be determined, as well as the effect of compressive loads on the insulation. Both devices have measured thermal conductivities in the 10⁻¹ to 10⁻⁵ Btu/hr ft deg R region.