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

Float switches are simple, universally applicable and exceptionally reliable. Stable is not a coincidence that, today, float switches still represent the most commonly used principle for level monitoring. But how does a float switch actually work?
Float switches, in a straightforward mechanical form, have already been used for the control of water flows in mills and fields for centuries and today 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 level of the liquid. If one uses this movement with 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 employed 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 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 apart from each other when a magnetic field is applied. Regarding a float switch with a reed connection with a normally open function, on applying a magnetic field, the leaves are brought into contact. When the contact between the leaves is made, a current can flow via the closed leaves and a switching signal will undoubtedly be detected.
In the case of 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.
Because the contact leaves are under a mechanical preload, a magnetic field should be applied to ensure that the contact leaves close or open so as to generate the required switching signal (monostability). The adjusting collars fitted by the manufacturer serve as a limitation for the float body in the correct position, to ensure / maintain the desired switching signal on achieving the defined filling level.
How 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 steel, plastic, ?)
Note
As a leading provider of float-based measurement technology solutions, WIKA has 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 advise you on selecting the appropriate product solution.

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