Abstract: A control valve includes a tubular member and a truncated conical helix spring received in the tubular member. A diverging open end of the helix spring is fixed adjacent to a first open end of the tubular member. The helix spring has a truncated tapered open end opposed to the diverging open end. A plug member is fixed in the truncated tapered open end in order to close the truncated tapered open end. The truncated conical helix spring confines a truncated cone-shaped chamber and is formed of a plurality of turns which are wound in a watertight manner. The helix spring extends and forms a plurality of clearances among the turns under a predetermined axial tension force. The tubular member has two connecting outlet ports which are to be connected respectively to the ends of a transducer in order to measure the differential pressure and thereby measure the flow rate of the fluid which passes through the tubular member.