  One of the main problems in modeling the ionised outflows in Active Galactic
Nuclei is the unknown distance of the outflowing wind to the central source.
Only if the density is known this distance can be determined through the
ionisation parameter. Here we study density diagnostics based upon O V
transitions. O V is known to have metastable levels that are density dependent.
We study the population of those levels under photoionisation equilibrium
conditions and determine for which parameter range they can have a significant
population. We find that resonance line trapping plays an important role in
reducing the critical densities above which the metastable population becomes
important. We investigate the K-shell absorption lines from these metastable
levels. Provided that there is a sufficient population of the metastable
levels, the corresponding K-shell absorption lines are detectable and are well
separated from the main absorption line originating from the ground state. We
then present the Chandra LETGS spectrum of the Seyfert 1 galaxy Mrk 279 that
may show for the first time the presence of these metastable level absorption
lines. A firm identification is not yet possible due to both uncertainties in
the observed wavelength of the strongest line as well as uncertainties in the
predicted wavelength. If the line is indeed due to absorption from O V, then we
deduce a distance to the central source of one light week to a few light
months, depending upon the importance of additional heating processes.
