Abstract
We present a comparison among electron density estimates for planetary nebulae based on different emission-line ratios. We have considered the density indicators [O II] lambda 3729/ lambda 3726, [S II] lambda 6716/ lambda 6731, [Cl III] lambda 5517/ lambda 5537, [Ar IV] lambda 4711/ lambda 4740, C III] lambda 1906/ lambda 1909 and [N I] lambda 5202/ lambda 5199. The observational data were extracted from the catalogue of Kaler et al. (1997). We verified that statistically N[e][ArIV] < N[e][ClIII] > N[e][NI] ~ N[e][OII] < N[e][SII], being these density sensors ordered by the ionization potentials of the parent ions. These differences can be attributed to errors on the atomic parameters used, to the adoption of a same electron temperature for different ions and, what seems to be more probable, to internal variations of density in the studied nebulae.
