What principle does a displacer level instrument use?

• A. It forces liquid to appear lighter or heavier, displacing weight as the liquid level changes
• B. It measures electrical resistance
• C. It uses a magnetic field to sense level
• D. It relies on optical level sensing

Answer: (A)

Displacer level instruments rely on buoyancy. As the liquid level changes, more or less of the displacer is submerged, so the buoyant force on it changes. This change in upward force alters the net load on the sensing element (often a spring or a pressure capsule), and that variation is converted into an electrical signal that indicates the level. This is Archimedes’ principle in action: the buoyant force equals the weight of the liquid displaced. The other methods—electrical resistance, magnetic sensing, or optical sensing—are different approaches used by other types of level instruments, not by the displacer type.