Float switches are simple, universally applicable and exceptionally reliable. It is not a coincidence that, today, float switches still represent the most frequently used principle for level monitoring. But how does a float switch actually work?
Float switches, in a straightforward mechanical form, have been in use for the control of water flows in mills and fields for centuries now 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 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, of course, are used for switching a power circuit and show a clearly more sophisticated design. In its simplest form, a float switch includes 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 located on its inside (see figure).
Dead : Collection 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. Once the contact between the leaves is made, an ongoing can flow via the closed leaves and a switching signal will undoubtedly 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.
Since the contact leaves are under a mechanical preload, a magnetic field must be applied in order that the contact leaves close or open to be able 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 / keep up with the desired switching signal on reaching the defined filling level.
So how exactly does one specify a float switch?
The following parameters ought to 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 leading provider of float-based measurement technology solutions, WIKA has a wide variety of variants to meet all your application-specific requirements. Shellacking are available on the WIKA website. Your contact person will be pleased to help you on selecting the appropriate product solution.