Values of the density indicator [S ii] λ6731/6717 as a function of [O ii] λ3726/3729. Points with error bars: observed values in the sample of PNe by Wang et al. (2004). Continuous lines: lines of equal electron density at a temperature of 10⁴ K from n_e = 20 cm^-3 (upper right) to n_e = 10⁵ cm^-3 (lower left). Red: using the [S ii] transition probabilities by Tayal & Zatsarinny (2010). Blue: using the [S ii] transition probabilities from Mendoza & Zeippen (1982).