SIMBAD references

We present a comparison of electron density estimates for planetary nebulae based on different emission-line ratios. We have considered the density indicators [OII]λ3729/λ3726, [SII]λ6716/λ6731, [ClIII]λ5517/λ5537, [ArIV]λ4711/λ4740, CIII]λ1906/λ1909 and [NI]λ5202/λ5199. The observational data were extracted from the literature. We have found systematic deviations from the density homogeneous models, in the sense that: N_e(NI)≲N_e(OII)<N_e(SII, CIII, ClIII or ArIV) and N_e(SII)≃N_e(CIII)≃N_e(ClIII)≃N_e(ArIV). We argue that the lower [OII] density estimates are likely due to errors in the atomic parameters used.