How does a displacer level transmitter work?

Displacer level transmitters work on Archimedes' principle - a cylindrical displacer partially submerged in liquid experiences buoyancy force proportional to liquid level. As level rises, buoyancy increases, reducing apparent weight sensed by the torque tube or range spring. The change in force is converted to 4-20 mA output. Displacers provide accurate interface measurement (oil-water), work in high pressure/temperature, and are common in refinery level applications but require density compensation.