Feng, Y. Campell, M.L. Krzywinski, J. Ortiz, E. Raubenheimer, T.O. Rowen, M. Schafer, D.W. TUP026 FEL undulator simulation detector linac 2015 2015-12 Thermodynamic simulations using a finite difference method were carried out to investigate the measurement uncertainties in gas-based X-ray FEL diagnostic monitors under high repetition rate operations such as planned for the future LCLS-II soft and hard X-ray FEL's. For monitors using relatively high gas pressures for obtaining sufficient signals, the absorbed thermal power becomes non-negligible as repetition rate increases while keeping pulse energy constant. The fluctuations in the absorbed power were shown to induce significant measurements uncertainties, especially in the single-pulse mode. The magnitude of this thermal effect depends nonlinearly on the absorbed power and can be minimized by using a more efficient detection scheme in which the gas pressure can be set sufficiently low