Float switches are simple, universally applicable and exceptionally reliable. It isn’t a coincidence that, today, float switches still represent probably the most commonly used principle for level monitoring. But so how exactly does a float switch really work?
Float switches, in a straightforward mechanical form, have been completely used for the control of water flows in mills and fields for years and years and today still represent probably the most commonly used technology. A hollow body (float), because of its low density and buoyancy, lifts or drops with the rising and, respectively, falling degree of the liquid. If one 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 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 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 apart from each other when a magnetic field is applied. In the case of 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 your 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, at all times, 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 to ensure that the contact leaves close or open so as 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 reaching the defined filling level.
So how exactly does one specify a float switch?
The following parameters should be defined:
Number 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 Ethical leading provider of float-based measurement technology solutions, WIKA includes a wide range of variants to meet up all your application-specific requirements. The available products are available on the WIKA website. Your contact person will undoubtedly be pleased to advise you on the selection of the appropriate product solution.