Float Switch (Level Switch)

A float switch is one of the simplest level-detection devices. A buoyant float attached to a lever arm (or a reed switch inside the float itself) rises and falls with the liquid surface. When the float reaches the trip point, it mechanically actuates a set of electrical contacts.

Side-mounted vertical float switches use a single float on a pivot. When the liquid rises above the set point, the float tips the lever and the contact state changes. Cable-style floats use a weight to set the switch point — as liquid rises, the float tilts to one side, closing or opening a mercury tilt switch or reed contact inside.

Most float switches are rated for general liquids. Caustic, viscous, or high-temperature fluids require special materials: polypropylene floats for acids, stainless steel for hot water or steam, PVDF for aggressive chemicals.

Wiring convention follows limit switch practice. NC wiring on a high-level float means the PLC sees a de-energised input as a fault (wire broken = alert), making it safer for critical overflow prevention. NO wiring is common for pump-start signals where the loss of signal simply stops the pump.

For continuous level measurement rather than point detection, a pressure transmitter or ultrasonic sensor is a better choice. Float switches excel at simple high/low setpoint logic where cost and simplicity matter more than precision.