Description of the different parts

The magnet located inside the float (in devices using reed switch)

Selecting a magnet for a level switch application must take into account the characteristics of the liquid in which it will be immersed, of the temperature at which it will be subjected, of its corrosion resistance, of the magnetic field required to operate the switch and its distance to the reed switches. Sintered magnets are shock and vibration sensitive, “bonded” magnets have a low temperature resistance due to the resins used to agglomerate, and Neodymium –Iron-Boron magnets contain 60-75% iron (amount is dependent on grade) and are therefore prone to corrosion. Their price is extremely variable depending on the materials and manufacturing process, and therefore it is the sum of all these parameters that will decide if a type of magnet will be used rather than another in a specific application.

Remanence (Br) is the flux density of a magnetic material in closed circuit, which remains after the removal of the magnetising  field. Remanence is measured in Gauss, Tesla or mT. (1 Tesla = 10,000 Gauss).
Flux Density (B) is a measure of magnetic field strength of the magnet in an ‘open circuit’ condition. The actual flux density measured on the pole face of a magnet will depend on the material, the grade, the relationship of its pole area to its magnetic length and any additional pole pieces that create a further magnetic circuit. Flux density is measured in Gauss, Tesla or mT.
Coercive Force (Hc) is the strength of the demagnetizing field needed to reduce the flux density of the magnet to zero. Coercive force is measured in Oersted or kA/m.
Maximum Energy Product (BHmax) indicates the peak energy that a magnet can deliver when operating at a working point on the demagnetization curve. Maximum Energy Product is measured in Mega-Gauss-Oersted or kJm³.