When an application requires the end position of a movement sensed, or the sensing of a part when it comes into a zone, or when the movement of a part as it crosses an intermediary point, or when a part moves away from a zone, it is classified as a Position Sensing Application.
The best practice is to use a reed switch or a reed sensor on the fixed surface, with the magnet on the moving surface. Both normally open, normally closed or latching devices are suitable for position sensing. Provided the sensing circuit is low in the range of milli-amperes, this combination achieves error free operation for millions of operations and is also cost effective.
As the moving part with the magnet approaches the stationary part, the reed sensor or the reed switch gets actuated. This distance depends on its Operate AT band. When the moving part moves away, the reed sensor or the reed switch opens, and this distance depends on its Release AT band. The opening distance and and closing distance is highly repeatable. More information on actuating positional actuation of reed switches is available here.
Reed Switch based Vibration Switches, Inclination Switches, Pressure Switches, Float Switches and Flow Switches use these basic position sensing principles to function. The variations in the type of sensing for vibration, inclination, pressure or flow, are achieved by changes in the mechanical design of the assembly with regards to how the magnet moves to and from the reed switch.
As Reed Switches are hermetically sealed, they are immune to dust and moisture, and can safely be used in liquid level sensing applications even in explosive liquids such as fuel. Apart from tank full or empty levels that can be sensed, the actual volume of a liquid present in a tank can also be determined by combining reed switches with a simple resistance matrix to generate a potentiometeric two wire output.
Although position sensing for such applications can also be be achieved by using active devices like Hall effect sensors, Reed Switches and Reed Sensors, are passive devices that do not drain the batteries, they have the imperative advantage of using no power at all for operation, and have no leakage current across the contacts. More information on the advantages of Reed based products over Hall-effect devices is available.