Abstract: In an operating system for a centrifuge rotor having valve means being operable by means of so called operating liquid for opening and closing peripheral outlets in the rotor, an injector device for supplying operating liquid to the rotor is used which comprises a chamber divided by a movable wall into an air side and a liquid side. The liquid side of the injector device is maintained in hydraulic contact with an injection inlet to the centrifuge rotor before an opening procedure is to be started, which is effected by opening a time controlled air valve on the air side of the injector device. The air valve is further connected to a compressed air source through a buffer vessel so that a controlled air pressure is used for the injection of operating liquid to the rotor.