Float switches are simple, universally applicable and exceptionally reliable. It isn’t a coincidence that, today, float switches still represent 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 in use for the control of water flows in mills and fields for centuries and today still represent probably the most commonly 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 Riveting 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, of course, are used for switching an electric circuit and show a clearly more sophisticated design. In Hoak , a float switch consists of 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 apart from each other when 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.
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, constantly, a normally closed and a normally open contact are simultaneously manufactured in every operating state.
Because the contact leaves are under a mechanical preload, a magnetic field must be applied to ensure that the contact leaves close or open as a way to generate the required switching signal (monostability). The adjusting collars fitted by the product 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.
How 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, plastic, ?)
Note
As a respected provider of float-based measurement technology solutions, WIKA has a wide range of variants to meet all your application-specific requirements. The available products are available on the WIKA website. Your contact person will be pleased to advise you on the selection of the appropriate product solution.