Abstract: A method of calibrating a choke system includes connecting to the positioner an electronic controller which is programmed to output a control signal representing the desired position of the plug, connecting to the electronic controller a flowmeter which is configured to provide a flow signal representing the rate of flow of fluid from the pump to the inlet and a pressure sensor which is configured to provide a pressure signal representing the fluid pressure at the inlet, connecting the inlet to a reservoir of fluid via a pump which is operable to pump fluid from the reservoir into the inlet at varying flow rates, and using gain scheduling to determine optimum values of proportional gain and integral gain required for control of the choke using a proportional differential and integral controller at a plurality of different rates of flow of fluid along the central flow passage.