We determined the superfluid density ${\mathrm{\rho }}_{\mathit{s}}$ from the speed of heat pulses, using a hydrodynamic model that contains coupling between the aerogel and the helium. To investigate the universality of the exponent of ${\mathrm{\rho }}_{\mathit{s}}$, measurements of ${\mathrm{\rho }}_{\mathit{s}}$ were taken from saturated vapor pressure to 29 bars. Fitting ${\mathrm{\rho }}_{\mathit{s}}$ with a pure power law in reduced temperature leads to a pressure-dependent exponent. Inclusion of a confluent singular term in the fits reveals a pressure-independent exponent ζ=0.755±0.003 which differs from ζ=0.672 for bulk helium.

• Received 14 March 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.695