Abstract: A non-contact lid sensor is provided for use with an oil field thief hatch and a latch. The sensor is mounted to the latch and comprises a reed switch. The latch pin is removed and embedded with a magnet with the magnet's north-south poles parallel to the pin's length so that the magnet triggers the reed switch. The reed switch is oriented so that it is perpendicular to the lower latch pin and the face of the sensor. The magnet and reed switch are aligned so that when the lid is closed the reed switch never sees the 0 Gauss line of the magnet. When the latch is in a closed position the reed switch allows the current to pass through the sensor and signal that the hatch is closed. The sensor system also allows for a Hall effect sensor to be used.

Abstract: A liquid level sensor is provided for use with a container. The sensor protrudes through an opening in the container. The sensor includes a float linked by means of a linkage to a first magnet axially rotatable on a first axis. The first magnet has a magnetic moment with a nonzero component at a right angle to the axis. A divider separates the first magnet from a second magnet having a magnetization and axially rotatable on a second axis. The second magnet has a nonzero magnetic moment at a right angle to the second axis. The first and second magnets are juxtaposed in magnetic linkage so that the second magnet is urged to follow the first magnet in rotation. The divider plugs the opening in the container. A magnetic field sensor is positioned to sense axial magnetic field strength at a location offset from the second axis.