  We derive the stellar and circumstellar parameters of the Galactic compact
planetary nebula Hen 2-90 with two models: the classical constant density
nebula model where the emission (especially of the forbiden lines) is formed in
a sphere or shell of constant density, and a model where the emission is formed
in a shell with a $r^{-2}$ density distribution. Depending on the density
$N_{\rm e,0}$ in the second model, the resulting range of valid values can
deviate from the classical model results. This seems to be especially true for
elemental abundances as is shown exemplary for S$^{+}$. We find that the
abundance of S$^{+}$ is larger for the models with density distribution than
for a constant density. A careful analysis of data with an appropriate input
model is advisable when determining stellar and circumstellar parameters of
planetary nebulae not only in our Galaxy.
