  Adequate modelling of the multiphase interstellar medium requires optically
thin radiative cooling, comprising an inherent thermal instability. The size of
the occurring condensation and evaporation interfaces is determined by the
so-called Field-length, which gives the dimension at which the instability is
significantly damped by thermal conduction. Our aim is to study the relevance
of conduction scale effects in the numerical modelling of a bistable medium and
check the applicability of conventional and alternative adaptive mesh
techniques. The low physical value of the thermal conduction within the ISM
defines a multiscale problem, hence promoting the use of adaptive meshes. We
here introduce a new refinement strategy that applies the Field condition by
Koyama & Inutsuka as a refinement criterion. The described method is very
similar to the Jeans criterion for gravitational instability by Truelove and
efficiently allows to trace the unstable gas situated at the thermal
interfaces. We present test computations that demonstrate the greater accuracy
of the newly proposed refinement criterion in comparison to refinement based on
the local density gradient. Apart from its usefulness as a refinement trigger,
we do not find evidence in favour of the Field criterion as a prerequisite for
numerical stability.
