A valve assembly (1) included in a system for measuring differential pressures in a fluid system and where the valve assembly (1) includes a valve body (7) with an inlet (13) and an outlet (14) for connection to the fluid system, ducts (32, 33) for communication with a differential pressure sensor (12) for registration the differential pressure, a cavity (22) including a calibration cone (10) which is movable in the cavity (22) between a measuring position and a position for zero point calibration/flushing via the valve assembly (1). In instances when the measuring position of the device does not exists, the calibration cone (10) separates the differential pressure sensor (12) from the fluid system, i.e., in its initial position, and, at the same time, the valve assembly (1), in this position, is automatic flushed so as to get rid of any enclosed air in the valve assembly (1).