  There are limits of validity of classical constitutive laws such as Fourier
and Navier-Stokes equations, phenomena beyond those limits have been
experimentally found many decades ago. However, it is still not clear what
theory would be appropriate to model different non-classical phenomena under
different conditions considering either the low-temperature or composite
material structure. In this paper, a modeling problem of rarefied gases is
addressed. It covers the mass density dependence of material parameters, the
scaling properties of different theories and aspects of how to model an
experiment. In the following, two frameworks and their properties are
discussed. One of them is the kinetic theory based Rational Extended
Thermodynamics; the other one is the non-equilibrium thermodynamics with
internal variables and current multipliers. In order to compare these theories,
an experiment performed by Rhodes is analyzed in detail. It is shown that the
density dependence of material parameters has a severe impact on modeling
capabilities and can lead to very different results.
