A float switch is a type of

level sensor Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface. Substances that flow become essentially horizontal in their containers (or ...

, a device used to detect the level of liquid within a tank. The switch may be used to control a

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

, as an indicator, an alarm, or to control other devices. One type of float switch uses a

mercury switch A mercury switch is an electricity, electrical switch that opens and closes a electrical circuit, circuit when a small amount of the liquid metal mercury (element), mercury connects metal electrodes to close the circuit. There are several differ ...

inside a hinged float. Another common type is a float that raises a rod to actuate a

microswitch Comparison of some different switches A miniature snap-action switch, also trademarked and frequently known as a micro switch or microswitch, is an electric switch that is actuated by very little physical force, through the use of a tipping-poi ...

. One pattern uses a

reed switch The reed switch is an Electromechanics, electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at the Peter the Great St. Petersburg Polytechnic University#Soviet era, Petrogra ...

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. Bela G. Liptak (ed.), ''Instrument Engineers' Handbook, Fourth Edition, Volume One: Process Measurement and Analysis'', CRC Press, 2003, page 477 File:Float_switch_on.jpg, float switch on File:Float_switch_off.jpg, float switch off A very common application is in

sump pump A sump pump is a pump used to remove water that has accumulated in a water-collecting sump basin, commonly found in the basements of homes and other buildings, and in other locations where water must be removed, such as construction sites. Th ...

s and condensate pumps 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 Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

. 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. 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. 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 A stuffing box or gland package is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements. Components A stuffing box of a sailing boat ...

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.

References

Fluid dynamics Heating, ventilation, and air conditioning Sensors Mechanisms (engineering) Pumps Switches {{Tech-stub

See also

References