The [[Efficiency|efficiency]] of a single device can be defined by comparing it to a theoretical isentropic ($s_i=s_e​$) device of the same type. The isentropic [[Turbines, Compressors, and Pumps|turbine, compressor, pump]], and [[Nozzles and Diffusers|nozzle]] will have the same inlet and outlet [[Pressure|pressure]], as well as the same inlet [[Temperature|temperature]] as the real device. In other words, the theoretical isentropic device, denoted with subscript $s$, will have $T_i=T_{i,s}$​, $P_i=P_{i,s}$​, and $P_e=P_{e,s}$. Turbine $\eta_T=\dfrac{w_T}{w_{T,s}}=\dfrac{h_i−h_e}{h_i−h_{e,s}}$ Pump/Compressor $\eta_{P/C}=\dfrac{w_{P/C,s}}{w_{P/C}}=\dfrac{h_i−h_{e,s}}{h_i−h_e}$ Nozzle $\eta_N=\dfrac{v_e^2}{v_{e,s}^2}$