Validation of the resulting values for T_e based on the Cloudy models. Crosses shows the values of T_e([N ii]) obtained from the volumetric version of Eq. (6), which accounts for variations in f_ion and the spatial extent of the [O i] and Hα emitting zones (Eq. (A.23)). Circles represent the T_e([N ii]) values yielded by Eq. (6) assuming the empirical calibration of f_ion (by Eq. (15)). The calculated values are compared with the“true” T_e([N ii]) from the Cloudy models. The symbol color encodes the age of the ionizing cluster. Both data sets show generally good agreement with the one-to-one relation, but the empirical calibration significantly underestimate T_e for old regions. We note that for the empirically calibrated results we show only the models with log([OI]/Hα) ≥ -3 and -1 ≥ log([SIII]/[SII]) ≤ −0.7 that correspond to the values obtained in our observations (see, e.g., Figs. 3 and 6). The prominent“tails” toward higher T_e for the empirically calibrated values correspond only to those regions where log([SIII]/[SII]) > 0.5. Only a few such regions were used for calibration of Eq. (15), and thus the calibration is less certain for Η ii regions with the highest ionization parameters.