float and displacer devices are often similar in appearance they have differing theories of operation. Float devices operate on the buoyancy Principle, as liquid level changes 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.