  The diffusion of gas through porous material is important to understand the
physical processes underlying cometary activity. We study the diffusion of a
rarefied gas (Knudsen regime) through a packed bed of monodisperse spheres via
experiments and numerical modelling, providing an absolute value of the
diffusion coefficient and compare it to published analytical models. The
experiments are designed to be directly comparable to numerical simulations, by
using precision steel beads, simple geometries, and a trade-off of the sample
size between small boundary effects and efficient computation. For direct
comparison, the diffusion coefficient is determined in Direct Simulation Monte
Carlo (DSMC) simulations, yielding a good match with experiments. This model is
further-on used on a microscopic scale, which cannot be studied in experiments,
to determine the mean path of gas molecules and its distribution, and compare
it against an analytical model. Scaling with sample properties (particle size,
porosity) and gas properties (molecular mass, temperature) is consistent with
analytical models. As predicted by these, results are very sensitive on sample
porosity and we find that a tortuosity $q(\varepsilon)$ depending linearly on
the porosity $\varepsilon$ can well reconcile the analytical model with
experiments and simulations. Mean paths of molecules are close to those
described in the literature, but their distribution deviates from the
expectation for small path lengths. The provided diffusion coefficients and
scaling laws are directly applicable to thermophysical models of idealised
cometary material.
