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 already been used for the control of water flows in mills and fields for years and years now 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 via a mechanical lever, e.g. as a simple flap control for an irrigation channel, you have implemented a mechanical float switch.
Modern float switches, of course, 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 a built-in magnet, a guide tube to steer 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 when a magnetic field is applied. Regarding a float switch with a reed contact 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, at all times, 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 as a way 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 ensure / 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. Cooperative )
Process connection
Material (stainless, plastic, ?)
Note
As a leading 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 be pleased to advise you on the selection of the appropriate product solution.