Rapid, Steady-State Measurement of the Effective Diffusion Coefficient of Gases in Closed-Cell Foams

A rapid steady-state technique was developed to measure the effective permeability and diffusion coefficients of closed-cell foam insulation. To test the new technique, N₂ data were first obtained by the long-term steady-state technique, and then reproduced ten times faster by the rapid steady-state technique. By using the new technique, reference values of effective diffusion coefficients of N₂, O₂, and Fluorocarbon 11 in closed-cell polyurethane foams were obtained at different temperatures. Data for Fluorocarbon 11 were obtained 30 times faster than data could be obtained by long-term steady-state tests. To estimate when steady-state has been achieved, the transient diffusion equation was solved, and the solution was given in the form of a chart. The time needed to achieve steady-state mass flux in a foam sample was found to depend strongly on the ratio of the partial pressures imposed on the surface of a tested sample. By use of the solution, the value of the foam effective diffusion coefficient can be obtained before steady-state conditions are achieved within the sample.