We have obtained high-res, high S/N ratio CCD echelle spectra of 10 bright red giants in 3 GCs (47Tuc, NGC6752 and NGC6397) roughly spanning the range of metallicities of the galactic GC system; they reveal no evidence of star to star variation of [Fe/H] in these 3 GCs. A large set of high quality literature data (EWs from high-res CCD spectra) was re-analyzed in a self-consistent way to integrate our data and derive new [Fe/H] for more than 160 bright red giants in 24 GCs. This set was used to define a new [Fe/H] scale for GCs based on high quality, direct spectroscopic data, on updated model atmospheres from the grid of Kurucz (1992) and on a careful fine abundance analysis (using a common set of atomic and atmospheric parameters for all stars). Given the high internal homogeneity, our new scale supersedes the discrepancies of previous attempts. The internal uncertainty in [Fe/H] is very small: 0.06 dex (24 GCs) on average, that can be interpreted as the mean precision of cluster ranking. Compared to our system, metallicities on the widely used Zinn and Wests scale are about 0.10dex higher for [Fe/H]$>-1$, 0.23dex lower for $-1<[Fe/H]<-1.9$ and 0.11dex too high for [Fe/H]$<-1.9$: the non-linearity is significant at 3$sigma$ level. A quadratic transformation corrects older values to the new scale in the range of our calibrating GCs ($-2.24le[Fe/H]_{ZW}le-0.51$). A minor disagreement is found at low [Fe/H] between the metallicity scale based on field and cluster RR Lyraes (via a new calibration of the $Delta$S index) and our new GCs metallicities, that could be ascribed to non-linearity in the [Fe/H]$-Delta$S relationship. The impact of new metallicities on major astrophysical problems is exemplified through the Oosterhoff effect in the classical pair M3 and M15.
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