  The calculations of the ratios of the Helium-like ion X-ray lines from C V to
Si XIII are revisited in order to apply the results to density, temperature and
ionization process diagnostics of data from high-resolution spectroscopy of the
new generation of X-ray satellites: Chandra and XMM-Newton.
  Comparing to earlier computations, Porquet & Dubau (2000), the best
experimental values are used for radiative transition probabilities. The
influence of an external radiation field (photo-excitation), the contribution
from unresolved dielectronic satellite lines and the optical depth are taken
into account.
  These diagnostics could be applied to collision-dominated plasmas (e.g.,
stellar coronae), photo-ionized plasmas (e.g., ``Warm Absorber'' in AGNs), and
transient plasmas (e.g., SNRs).
