  How well do we know the physical/chemical properties of PNe? 1D (CLOUDY) and
3D (MOCASSIN) photoionisation codes are used in this contribution to model the
PNe K 4-47 and NGC 7009 as an attempt to question whether or not the high Te
(higher than 21,000K) of the K 4-47's core and the N overabundance of the outer
knots in NGC 7009 are real.
  These are very basic parameters, obtained for Galactic PNe, e.g. nearby
objects, even though with large uncertainties. Based on the comparison of the
modelling with, mainly, optical images and long-slit spectroscopic data, it is
suggested here that K 4-47 high Te can be explained if its core is composed of
a very dense and small inner region --that matches the radio measurements-- and
a lower density outer core --matching the optical observations. This approach
can account for the strong auroral emission lines [OIII]4363A and [NII]5755A
observed, and so for the high temperatures. This teaches us that the assumption
of a homogeneous distribution of the gas is completely wrong for the core of
such PN.
  In the case of NGC 7009 a simple 3D model that reproduces the observed
geometry of this nebula is constructed. The aim of this modelling was to
explore the possibility that the enhanced [NII] emission observed in the outer
knots may be due to ionisation effects instead to a local N overabundance. Here
it is discussed the model that can best reproduce the observations employing a
homogeneous set of abundances throughout the nebula, not only for nitrogen but
also for all the other elements considered.
