  Diagnostic diagrams of electron density - excitation for a sample of 613
planetary nebulae are presented. The present extensive sample allows the
definition of new statistical limits for the distribution of planetary nebulae
in the log [Ha/[SII]] vs log [Ha/[NII]], log [Ha/[SII]] vs [SII] 6717/6731 and
log [Ha/[NII]] vs [SII] 6717/6731 planes. The diagrams provide a good
representation of the ranges of physical conditions, indicated by these
emission line ratios, present in planetary nebulae during different
evolutionary stages.
