Ratio of observed to modelled column densities, N_obs/N_model, of OH⁺ (red), H₂O⁺ (green) and H₃O⁺ (blue), as a function of the FUV field for a slab with A_V = 1, n_H = 100 cm^-3, and ζ_cr = 3 × 10^-16 s^-1. The widths of the red and green bands represent the errors around the measured column densities of OH⁺ and H₂O⁺. The blue area marks the only range of FUV field values possible for the models to be compatible with the derived 3σ upper limit to the H₃O⁺ column density. A horizontal black dotted line is depicted at N_obs/N_model = 1.