Tank are simple, universally applicable and exceptionally reliable. It is not a coincidence that, today, float switches still represent probably the most frequently used principle for level monitoring. But how does a float switch really work?
Float switches, in a simple mechanical form, have already been in use for the control of water flows in mills and fields for years and years and today still represent probably the most frequently 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, of course, are used for switching a power 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 steer the float, adjusting collars to limit the travel of the float on the tube and a reed contact located 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 whenever 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, a current 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 manufactured in every operating state.
Since the contact leaves are under a mechanical preload, a magnetic field should 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 product manufacturer serve as a limitation for the float body in the right position, to ensure / 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:
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 Master of float-based measurement technology solutions, WIKA has a wide variety 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.