Float switch: The facts and how does it actually work?

Float switches are simple, universally applicable and exceptionally reliable. It is not a coincidence that, today, float switches still represent the most frequently used principle for level monitoring. But how does a float switch really work?
Float switches, in a simple mechanical form, have been used for the control of water flows in mills and fields for centuries now still represent probably the most frequently used technology. A hollow body (float), because of its low density and buoyancy, lifts or drops with the rising and, respectively, falling level of the liquid. If one uses this movement with a mechanical lever, e.g. as a simple flap control for an irrigation channel, one has implemented a mechanical float switch.
Modern float switches, of course, are used for switching a power circuit and show a clearly more sophisticated design. In its simplest form, a float switch consists of a hollow float body with an integral magnet, helpful information tube to guide the float, adjusting collars to limit the travel of the float on the tube and a reed contact located on its inside (see figure).
Figure: Collection of reed contacts of a float switch
How does the float switch function?
Reed contacts (see figure) of a float switch feature contact leaves within the hermetically sealed glass body, which move together or apart from each other whenever a magnetic field is applied. In the case of a float switch with a reed connection with a normally open function, on applying a magnetic field, the leaves are brought into contact. When Backlash between your leaves is made, an ongoing can flow via the closed leaves and a switching signal will undoubtedly be detected.
Regarding a float switch with normally closed switching function, the contact or circuit is interrupted on applying a magnetic field. If one selects a change-over contact, the glass capsule will contain three contact leaves, with which, always, a normally closed and a normally open contact are simultaneously manufactured in every operating state.
Because the contact leaves are under a mechanical preload, a magnetic field should be applied in order that the contact leaves close or open to be able to generate the desired switching signal (monostability). The adjusting collars fitted by the manufacturer serve as a limitation for the float body in the correct position, to make sure / maintain the desired switching signal on achieving the defined filling level.
So how exactly does one specify a float switch?
The following parameters ought to be defined:
Amount of switch contacts / switching outputs
Position and function of every switching output
Guide tube length
Electrical connection (e.g. PVC cable outlet)
Process connection
Material (stainless, plastic, ?)
Note
As a respected provider of float-based measurement technology solutions, WIKA includes a wide variety of variants to meet all your application-specific requirements. The available products can be found on the WIKA website. Your contact person will undoubtedly be pleased to help you on the selection of the appropriate product solution.

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