Float Level
Provides ON/OFF level indication (or, with multiple reeds, multi-step level steps) by mechanically converting fluid level to a magnet position, then electrically detecting the magnet with a hermetically-sealed reed switch.
A buoyant float moves with the liquid surface. Its position can operate a switch or drive an indicator or transmitter. In a magnetic reed-and-resistor-chain design, the output changes in steps as successive contacts close; closely spaced steps approximate continuous level measurement but do not make it infinitely fine.
In plain terms
Like a flag at half-mast — the level rises, the float rides up, and the reed switch closes the moment the magnet passes its sealed glass.
Why designers use it
- Providing a mechanically understandable level indication.
- Operating a suitable level switch or transmitter.
- Using buoyancy where the fluid and vessel allow free, reliable float movement.
Best for
- Sump pumps
- Fuel tanks
- Washing machines
Key specifications
- Fluid suitability: Density, chemistry, temperature and pressure
- Output: Switch state or transmitter resolution
- Installation: Orientation, travel and obstructions
When not to use it
- On viscous or sticky fluids — the float drags or freezes; pull non-contact sensing.
- In wave-prone tanks (vehicle fuel) — sloshing makes the reed chatter; an electronic damping or capacitive sensor is steadier.
Common mistakes
- Ignoring minimum fluid density or chemical compatibility.
- Allowing deposits, turbulence or obstructions to impede the float.
- Assuming the device is inherently fail-safe; sticking and contact failures must be considered.
- Ignoring the specified mounting direction and electrical contact rating.
Where you will find it
- A Liberty Pumps 257 sump pump uses a tethered float-and-mercury-free reed-switch float to start its motor when the basin reaches 9 inches: the reed's hermetic seal handles the dirty groundwater that would corrode an open contact within months.
- A Bosch Aquastar tankless water-heater's overpressure relief uses a Gems Sensors LS-3 reed-switch float to trip a high-temp shutdown if the drain line backs up: even after years in scaling water, the magnetic-only path keeps the safety circuit alive.
- A Ford F-Series automotive fuel tank uses a magnet-on-float arm sweeping a printed-resistor card with multi-tap reed sense: the resistance reading at the dashboard gauge maps the float angle to fuel quantity, the same architecture in service since the 1960s.
A short history
A float uses buoyancy to follow the surface of a liquid. Its motion can operate a switch or be transferred to an indicator or transmitter to report the level. The float must suit the liquid and installation; turbulence can disturb the reading.
Good to know
- A float must be less dense than the liquid to follow its surface.
- A moving float can become stuck, so simple construction does not guarantee safe failure behaviour.