 These two models intersect at points on the enthalpy-entropy and Mach number-entropy diagrams, which is meaningful for many applications.However, the entropy values for each model are not equal at the sonic state.The change in entropy is 0 at "M" = 1 for each model, but the previous statement means the change in entropy from the same arbitrary point to the sonic point is different for the Fanno and Rayleigh flow models.