Generally the gas metallicity in distant galaxies can only be inferred by using a few prominent emission lines. Various theoretical models have been used to predict the relationship between emission line fluxes and metallicity, suggesting that some line ratios can be used as diagnostics of the gas metallicity in galaxies. However, accurate empirical calibrations of these emission line flux ratios from real galaxy spectra spanning a wide metallicity range are still lacking. In this paper we provide such empirical calibrations by using the combination of two sets of spectroscopic data: one consisting of low-metallicity galaxies with a measurement of [OIII]4363, and the other one consisting of galaxies in the SDSS DR4 whose metallicity has been determined from various strong emission lines in their spectra. This combined data set constitutes the largest sample of galaxies with information on the gas metallicity available so far and spanning the widest metallicity range. Our empirical diagrams show that the line ratio [OIII]/[OII] is a useful tool to break the degeneracy in the R_23 parameter when no information on the [NII] line is available. The line ratio [NeIII]/[OII] also results to be a useful metallicity indicator for high-z galaxies. Finally, we compare these empirical relations with photoionization models. We find that the empirical R_23-metallicity sequence is strongly discrepant with respect to the trend expected by models with constant ionization parameter, which is interpreted as a consequence of a strong metallicity dependence of the average ionization parameter in galaxies. This result should warn about the use of theoretical models with constant ionization parameter to infer metallicities from observed line ratios. (abbreviated)
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