The disclosed portable self-powered flow measuring device has a non-metallic housing through which a fluid flows. An annular turbine spins within the housing, and has a set of inwardly-extending vanes set to reduce the pressure of the flowing fluid by more than 10%. A set of magnets that are spaced circumferentially about the turbine. Coils are mounted to the housing with axes that are radial to the axis of the central waterway. A set of circumferentially spaced reinforcing arms, one for each coil, are positioned outside the coils, the circumferential width of each reinforcing arm near the axial position of the turbine being no more than the radius of the coil.