Diagnostic diagrams of the [Cii]/[Oi]63 ratio plotted against the ([Cii]+[Oi]63)/F_TIR ratio for NGC 891, similar to Fig. 12. We superimpose our adjusted observations onto a grid of constant hydrogen nuclei density, log n (black dashed lines), and FUV radiation field strength, log G₀ (red solid lines), determined from the PDR model of Kaufman et al. (1999, 2006). Each data point represents one pixel, with colours as described in Fig. 11. We present our unadjusted observations (upper panel) and the observations including the adjustments applied to the [Cii] and F_TIR emission as described in Sect. 4.1, and compare our two approaches to remove the fraction of the [Cii] emission arising from ionised gas: the -based (middle panel) and -based corrections (lower panel). See Fig. 17 for the corresponding maps.