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

No Questions Asked 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 completely in use for the control of water flows in mills and fields for centuries and today still represent 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, you have 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 includes a hollow float body with a built-in 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: Selection of reed contacts of a float switch
So how exactly 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. When the contact between the leaves is made, an ongoing can flow via the closed leaves and a switching signal will 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, all the time, 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 must be applied in order that the contact leaves close or open in order 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 right 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 each switching output
Guide tube length
Electrical connection (e.g. PVC cable outlet)
Process connection
Material (stainless, plastic, ?)
Note
As a leading provider of float-based measurement technology solutions, WIKA has a wide range of variants to meet up all your application-specific requirements. The available products can be found on the WIKA website. Your contact person will be pleased to advise you on selecting the appropriate product solution.

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