Abstract: A compact bidirectional meter prover mechanism incorporates a straight calibration barrel having spaced detectors sensing passage of a fluid induced displacer piston. A four-way control valve is connected by flow conduits to the conduit containing the flow meter and is connected by a conduit system to respective extremities of the calibration barrel. Launch valves are interposed between the control valve and the calibration barrel and are selectively positioned to permit metering flow through the calibration barrel only after the control valve has been positioned for directional flow of fluid through the conduit system and the calibration barrel. Thus, the necessity for piston prerun is eliminated and minimum cycle time is permitted. The length of the calibration barrel is also minimized by elimination of piston prerun.