Displacer Level Buoyancy Calculator
Calculate the buoyancy force on a cylindrical displacer and its apparent weight at 0 % and 100 % level, for both total level and interface service.
- V = π/4 × D² × L = 5.444 L
- Buoyancy = V × SG × 1000 kg/m³ (Archimedes)
- Apparent weight = weight − buoyancy
Displacer Buoyancy
Fb = V × ρ × g
Apparent weight = W − Fb
At 0 % the displacer is surrounded by the upper fluid (vapour or light liquid); at 100 % it is fully immersed in the lower liquid. The transmitter span corresponds to the change in buoyancy.
Worked example
A 76 mm × 1.2 m displacer (5.44 L) weighing 6 kg in water: apparent weight falls from 6.0 kg at 0 % to 0.56 kg at 100 %, a span of 5.44 kg (53.3 N).
Frequently Asked Questions
How does a displacer level transmitter work?
A heavy displacer hangs from a torque tube or spring. As liquid rises it loses apparent weight by the buoyancy force, which the transmitter measures.
Can a displacer measure interface?
Yes, if it stays fully submerged in the two liquids and the density difference is large enough.
How do I calibrate a displacer without filling the vessel?
Hang calibrated weights equal to the apparent weight at 0 % and 100 % (wet calibration is preferred where possible).
Learn more on the blog
Types of level transmitters
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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