Electron temperature derived from [O iii] λ4363/5007 versus electron temperature from [N ii] λ5755/6583. Dark dots mark the Galactic disk PNe observed in this work (Vela-Norma); blue open squares – Galactic bulge PNe from Chiappini et al. (2009); green open triangles – Galactic inner-disk PNe from the same reference. The numbers in the top right give the numbers of objects presented in the plot for each sample.