Although float and displacer devices are often similar in appearance they have differing
theories of operation.
Float devices operate on the buoyancy principles, as liquid level change a (predominately)
sealed container will, providing its density is lower than that of the liquid, move
correspondingly.
Displacers work on the Archimedes principle, when a body is immersed in a fluid it loses
weight equal to that of the fluid displaced. By detection of the apparent weight of the
immersed displacer, a level measurement can be inferred.
When the cross sectional area of the displacer and the density of the liquid is constant, then
a unit change in level will result in a reproducible unit change in displacer weight