Relation between f_ion and [S iii]/[S ii] based on the Cloudy models. On the left-hand panel the value of f_ion is obtained directly from the model (as < n(H⁺) > / < n(H) >), while on the right-hand panel it was derived as in the observations (Eq. (6)) based on the integrated values of [Ο i]/Ηα metallicity and electron temperature T_e([N ii]). Central panel: f_ion = n(H⁺)/n(H) in individual zones of the models. The colors on all panels correspond to the age of the ionizing cluster, and the red curve is the empirical relation derived from the observational data (Eq. (15)). Spatially integrating across the entire line emitting region clearly reduces the scatter in this relation, and brings the model results into better agreement with the empirical relation.