Abstract: The present invention includes a ferrous conduit mounted over an aperture in the pipeline, launch tube, or receiver through which the pigs pass. A non-magnetic sensing rod is reciprocally mounted within the conduit with the rod extending through the aperture and into the pipeline. That end of the non-magnetic sensing rod extending into the pipeline has attached thereto a head with a lower rounded surface for engaging the pigs. A non-magnetic end cap is secured to the upper end of the conduit and a non-magnetic magnet holder with a magnet disposed therein are mounted on the upper end of the rod. A magnetically operated switch is mounted adjacent the non-magnetic cap. The conduit shields the magnetic field of the magnet from the switch until a pig is detected. Upon the engagement of the head with a pig, the magnet on the upper end of the sensing rod moves above the conduit inside the non-magnetic cap and sends a magnetic flux to the magnetically operated switch.

Abstract: According to the invention, fluid is adapted to flow through a piston and cylinder disposed in the housing of a meter prover. The cylinder is telescopingly received within the piston. The fluid is permitted to flow into the piston and cylinder through an upstream inlet and leave through exit ports in the cylinder. A poppet valve is provided for closing the exit ports whereupon the piston becomes a fluid barrier and is adapted to move in synchrony with the fluid flow from the upstream fluid inlet. A single detector is provided which is activated at the start of the proving run and at the end of the proving run. A rack is mounted on the piston for turning a gear. The gear rotates a disc having a plurality of markings indicating the incremental displacement of the piston. A photopulser transmits a pulse to a counter for each marking detected upon rotation of the disc. Sufficient piston displacement is permitted to generate a minimum of 10,000 pulses.