The present invention relates to a beverage preparation system (10) for controlling a beverage preparation process by injection of liquid into a cartridge (20), the system comprising—a pressure adjustable pump (2) for delivering different pressures upon varying at least one of its drive characteristics such as the speed of the pump, —a sensor (3) for measuring an operating parameter of the pump (2) such as a current absorbed by the pump, —a flow sensor (1) for measuring a liquid flow parameter of liquid injected into the cartridge (20), —a control unit (6) designed to control an extraction pressure of the cartridge (20) by adjusting the drive characteristics of the pump (2) based on pre-established pressure interpolation data (12) depending on the measured liquid flow parameter and the measured operating parameter of the pump.