Technical Foundation
What Is a Float Switche?
A float switch is a level-sensing device using a buoyant float, connected mechanically or via an internal mercury-free tilt mechanism to an electrical switch, that changes state (opens or closes a circuit) as liquid level rises or falls past the float's position. Vertical float switches slide up and down a rigid stem or rod mounted in the tank, giving a precise, repeatable trip point at a fixed elevation; tethered (pendant) float switches hang freely on a cable and trip when the float itself tilts past a certain angle as it rises or sinks with the liquid surface, offering flexible positioning but a less precise trip point than a vertical stem-mounted design.
Switch action is specified as normally open (NO, closes on rising level) or normally closed (NC, opens on rising level), and function follows from wiring: a single float wired to start a pump on rising level and stop it on falling level performs simple on/off control, while paired floats set at different heights provide independent high-level alarm and pump control functions. Wetted material (polypropylene, PVDF, stainless steel) must resist the specific liquid's chemistry, and cable length must reach from the tank-top mounting point to the switch's control location without a field splice in the liquid-contact zone.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Mounting Style | Vertical (stem-mounted) vs. tethered (cable-hung) | Vertical gives a precise, fixed trip point; tethered allows flexible positioning but a less precise, angle-dependent trip |
| Switch Action | Normally open (NO) or normally closed (NC) | Determines whether the switch closes or opens as level rises — must match the pump control or alarm circuit's logic |
| Wetted Material | Polypropylene, PVDF, stainless steel, Buna-N cable jacket | Must resist chemical attack from the specific liquid — a standard poly float will degrade in aggressive chemical service |
| Cable Length & Jacket | Length from mounting point to control panel; jacket material rated for submersion | Must reach the control location without a splice submerged in liquid, and the jacket must resist the fluid and any oil/fuel contact |
| Electrical Rating | Contact rating in amps at rated AC/DC voltage | Must match or exceed the pump motor's starting current if switching the pump directly, or use a control relay/pilot duty rating otherwise |
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Applications
Where Float Switches Get Used
Float switches are specified wherever simple, reliable liquid level detection is needed to control a pump or trigger a level alarm without electronic level instrumentation.
Sump & Sewage Pump Control
Vertical and tethered float switches starting and stopping sump and effluent pumps automatically based on pit water level.
Wastewater & Lift Station Control
Multi-float sets providing lead/lag pump alternation and high-level alarm functions in municipal and industrial lift stations.
Water Storage Tank Level Control
Float switches controlling fill pumps and preventing overflow or dry-running of storage and process water tanks.
Chemical Tank Level Monitoring
Chemically compatible float switches (PVDF, PTFE-coated) providing level control in corrosive chemical storage tanks.
Agricultural & Irrigation Systems
Float switches controlling irrigation pump operation based on reservoir or tank level.
Bilge & Marine Applications
Sealed float switches automatically activating bilge pumps in boats and marine equipment.
FAQ
Float Switches Questions, Answered
What's the difference between a vertical and a tethered float switch?
A vertical float switch rides up and down a fixed rod or stem mounted in the tank, giving a precise, repeatable trip point at a specific height that's easy to adjust by sliding a stop collar. A tethered (pendant) float switch hangs on a length of cable and trips based on the angle it tilts to as it floats up or sinks with the liquid — simpler to install (no rigid mounting needed) but the trip point is less precise and depends on the float settling at the right angle.
How do I choose normally open vs. normally closed?
It depends on the control logic you need: a normally open (NO) float switch, wired to start a pump, closes as level rises to activate the pump — the common configuration for sump pump control. A normally closed (NC) switch, wired the opposite way, is used when you need the circuit to be active by default and break on rising level, such as some high-level alarm configurations. Check your pump controller or relay logic to determine which action your circuit needs.
Can I use one float switch for both pump-on and pump-off control?
A single float switch only provides one trip point, so it turns the pump on when level reaches that point but often also turns it off nearly immediately as level drops slightly, causing short-cycling. For clean on/off differential control (start at a high point, stop at a distinctly lower point), most installations use two floats set at different heights, or a single wide-differential float switch specifically designed for that range.
What float switch material should I use for chemical or wastewater service?
Standard polypropylene floats handle general water and mild service well. For aggressive chemicals, PVDF (Kynar) offers much broader chemical resistance. For wastewater and sewage applications with solids and grease, look for a float and cable jacket specifically rated for sewage/effluent service, since standard cable jackets can degrade from prolonged contact with fats, oils and grease.
Why does my float switch fail to trip reliably?
Common causes include the float or cable becoming fouled with sediment, grease, or debris buildup that changes its buoyancy or restricts its movement range, a tethered float's cable being too short or improperly weighted so it doesn't hang and tilt correctly, or turbulence in the tank/sump causing the float to bounce and give erratic switching. Periodic cleaning and confirming the float still moves freely through its full range resolves most reliability issues.
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