Float switch: What is it and so how exactly does it actually work?

Float switches are simple, universally applicable and exceptionally reliable. It isn’t a coincidence that, today, float switches still represent probably the most frequently used principle for level monitoring. But how does a float switch actually 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), due to its low density and buoyancy, lifts or drops with the rising and, respectively, falling degree of the liquid. If Plummet uses this movement via a mechanical lever, e.g. as a straightforward flap control for an irrigation channel, one has implemented a mechanical float switch.
Modern float switches, needless to say, are used for switching an electric circuit and feature a clearly more sophisticated design. In its simplest form, a float switch consists of a hollow float body with an integral magnet, a guide tube to guide the float, adjusting collars to limit the travel of the float on the tube and a reed contact situated 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 aside 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. Once the contact between the leaves is made, a current 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 made in every operating state.
Since the contact leaves are under a mechanical preload, a magnetic field must be applied in order that the contact leaves close or open as a way to generate the desired switching signal (monostability). The adjusting collars fitted by the product manufacturer serve as a limitation for the float body in the correct position, to make sure / keep up with the desired switching signal on achieving the defined filling level.
How does one specify a float switch?
The following parameters should 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 steel, plastic, ?)
Note
As a respected provider of float-based measurement technology solutions, WIKA has a wide variety of variants to meet up all of your application-specific requirements. The available products can be found on the WIKA website. Your contact person will be pleased to advise you on the selection of the appropriate product solution.

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