The evolution of the entrainment coefficient K of the rotating fluid in a rotor–stator cavity with an inward through flow and pre-rotation is studied according to the flow parameters. Measurements are obtained in water for a turbulent Batchelor type of flow with two separated boundary layers on the rotating and stationary disks by means of a laser Doppler anemometer, and the results are compared to those performed using pressure transducers. We show that the entrainment coefficient K depends on a local flow rate coefficient Cqr according to a 5/7 power law whose coefficients depend on the boundary conditions. A theoretical analysis confirms this behaviour of K.