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{{Short description|Level sensor}}
[[File:Pedestal pump.jpg|thumb|upright|A pedestal sump pump with a float switch]]
A '''float switch''' is a type of [[level sensor]], a device used to detect the [[Liquid level|level of liquid]] within a tank. The switch may be used to control a [[pump]], as an indicator, an alarm, or to control other devices.

One type of float switch uses a [[mercury switch]] inside a hinged float. Another common type is a float that raises a rod to actuate a [[Micro switch|microswitch]]. One pattern uses a [[reed switch]] mounted in a tube; a float, containing a magnet, surrounds the tube and is guided by it.  When the float raises the magnet to the reed switch, it closes.  Several reeds can be mounted in the tube for different level indications by one assembly.<ref name=BLG03> Bela G. Liptak (ed.), ''Instrument Engineers' Handbook, Fourth Edition, Volume One: Process Measurement and Analysis'', CRC Press, 2003,{{ISBN|1420064029}} page 477</ref>

<gallery caption="Opened float switch from a sump pump" mode=packed>
File:Float_switch_on.jpg|float switch on
File:Float_switch_off.jpg|float switch off
</gallery>

A very common application is in [[sump pump]]s and [[condensate pump]]s where the switch detects the rising level of liquid in the sump or tank and energizes an electrical pump which then pumps liquid out until the level of the liquid has been substantially reduced, at which point the pump is switched off again. Float switches are often adjustable and can include substantial [[hysteresis]]. That is, the switch's "turn on" point may be much higher than the "shut off" point. This minimizes the on-off cycling of the associated pump.<ref>Stephen Herman, ''Understanding Motor Controls'', Nelson Education, 2012, {{ISBN|1133713912}}, pp. 163-165</ref> 

Some float switches contain a two-stage switch. As liquid rises to the trigger point of the first stage, the associated pump is activated. If the liquid continues to rise (perhaps because the pump has failed or its discharge is blocked), the second stage will be triggered. This stage may switch off the source of the liquid being pumped, trigger an alarm, or both.<ref>{{Cite web |last=Kietzman |first=Shannon |date=26 September 2022 |title=What is a Level Switch? |url=http://www.aboutmechanics.com/what-is-a-level-switch.htm |access-date=2022-10-28 |website=About Mechanics |language=en-US}}</ref>

Where level must be sensed inside a pressurized vessel, often a magnet is used to couple the motion of the float to a switch located outside the pressurized volume. In some cases, a rod through a [[stuffing box]] can be used to operate a switch, but this creates high drag and has a potential for leakage. Successful float switch installations minimize the opportunity for accumulation of dirt on the float that would impede its motion. Float switch materials are selected to resist the deleterious effects of corrosive process liquids. In some systems, a properly selected and sized float can be used to sense the interface level between two liquids of different density.<ref name=BLG03/>

==See also==
* [[Float (liquid level)]]
* [[Fuel gauge]]
* [[Level sensor]]
* [[Sight glass]]

==References==
<references/>

{{commonscat|Float switches}}

{{Switches}}
{{Plumbing}}

{{DEFAULTSORT:Float Switch}}
[[Category:Fluid dynamics]]
[[Category:Heating, ventilation, and air conditioning]]
[[Category:Sensors]]
[[Category:Mechanisms (engineering)]]
[[Category:Pumps]]
[[Category:Switches]]

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