  After an extensive assessment of the effective collision strengths available
to model the He-like triplet of C V, N VI, O VII, Ne IX, Mg XI and Si XIII in
collisionally dominated plasmas, new accurate effective collision strengths are
reported for Ne IX. The uncertainty intervals of the density and temperature
diagnostics due to the atomic data errors are also determined for both
Maxwell-Boltzmann and $\kappa$ electron-energy distributions. It is shown that
these uncertainty bands limit the temperature range where the temperature
line-ratio diagnostic can be applied and its effectiveness to discern the
electron-energy distribution type. These findings are benchmarked with Chandra
and XMM-Newton spectra of stellar coronae and with tokamak measurements.
