  We present the first direct calibration of strong-line metallicity
diagnostics at significant cosmological distances using a sample at z=0.8 drawn
from the DEEP2 Galaxy Redshift Survey. Oxygen and neon abundances are derived
from measurements of electron temperature and density. We directly compare
various commonly used relations between gas-phase metallicity and strong line
ratios of O, Ne, and H at z=0.8 and z=0. There is no evolution with redshift at
high precision ($\Delta \log{\mathrm{O/H}} = -0.01\pm0.03$, $\Delta
\log{\mathrm{Ne/O}} = 0.01 \pm 0.01$). O, Ne, and H line ratios follow the same
locus at z=0.8 as at z=0 with $\lesssim$0.02 dex evolution and low scatter
($\lesssim$0.04 dex). This suggests little or no evolution in physical
conditions of HII regions at fixed oxygen abundance, in contrast to models
which invoke more extreme properties at high redshifts. We speculate that
offsets observed in the [N II]/H$\alpha$ versus [O III]/H$\beta$ diagram at
high redshift are therefore due to [NII] emission, likely as a result of
relatively high N/O abundance. If this is indeed the case, then nitrogen-based
metallicity diagnostics suffer from systematic errors at high redshift. Our
findings indicate that locally calibrated abundance diagnostics based on
alpha-capture elements can be reliably applied at z$\simeq$1 and possibly at
much higher redshifts. This constitutes the first firm basis for the widespread
use of empirical calibrations in high redshift metallicity studies.
