Although float and displacer level sensor 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.
Displacer Level Sensor Theory
Both floats and displacers work well with clean liquids and are accurate and adaptable to wide variations in fluid densities. Once commissioned, however, the process fluid measured must maintain its density if repeatability is required, this is particularly true of displacers.
Float Switches are available with a glandless design and are capable of fail safe operation in extreme process conditions, unlike displacers, which if the torque tube fails can provide a leak path.
Displacers are affected by changes in product density since the displacement of the body (its weight loss) is equal to the weight of the fluid displaced. If the specific gravity changes, then the weight of the displaced material changes, thus changing the calibration.
This is especially problematic in interface measurements, where both liquids increase or decrease density, while the signal is proportional to the density difference. Because the displacer is emersed in the process fluid it will be vulnerable to
particulate deposition. This will change the displacer mass and the effective displacement causing a calibration shift. Glandless float systems provide more reliable readings under difficult conditions, however in situations of a coating media the moving parts may seize and the unit will no longer function.
Displacers and floats should only be used for relatively non-viscous, clean fluids and provide optimal performance in switch applications and over for short spans. Spans of up to 12m are possible, but they become prohibitively expensive. Cost of installation for displacers is high and many refineries are now replacing them due to the inaccuracies experienced under process density changes especially on interface duties.
High quality float switches still provide reliable and repeatable performance. Even with todays array of level technologies, if a 100% process seal is required under fail conditions for a Cryogenic application the only technique available, other than nucleonic’s, is a magnetically coupled float switch.