Level measurement in a pressurized vessel: which device directly determines the liquid level while cancelling the effect of vessel pressure?

Introduction / Context:
Measuring level inside a closed or pressurized vessel requires an approach that eliminates the influence of the gas space pressure above the liquid. The most common, robust method is to use a differential pressure arrangement that measures the hydrostatic head between the bottom tap and the vapor space reference, so the vessel pressure cancels out.

Given Data / Assumptions:

• Vessel is closed and under positive pressure.
• Liquid is reasonably clean and has known density.
• Objective: continuous level indication unaffected by varying headspace pressure.

Concept / Approach:
Hydrostatic level head is ρ * g * h. In a pressurized vessel, the measured pressure at the bottom is P_vessel + ρ * g * h. A differential pressure device compares bottom pressure to the vapor space pressure (high vs. low side). The common-mode term P_vessel cancels, leaving only ρ * g * h proportional to the level. This principle is embodied in DP manometers or modern DP transmitters with wet or dry legs.

Step-by-Step Solution:
Identify the interfering variable: vessel top pressure P_vessel.Use two pressure points: bottom tap and vapor space reference.Compute differential: (P_vessel + ρ g h) − P_vessel = ρ g h → convert to level.

Verification / Alternative check:
Field practice: DP transmitters with remote seals are standard for pressurized reactors, boilers’ drums, and storage spheres. Compensation for temperature and density may be applied if required.

Why Other Options Are Wrong:
Bubbler system: Typically suited to open tanks or where adding purge gas is acceptable; in pressure vessels it is less practical and complicates sealing.Diaphragm box system: More of a local mechanical indicator, not inherently cancelling vessel pressure without a DP reference.Air-trap system: Not a standard solution for pressurized enclosures.Ultrasonic (open-air top): Requires free air space and is not intended for sealed, pressurized conditions without special arrangements.

Common Pitfalls:
Forgetting density compensation; improper impulse line routing (wet/ dry leg errors); allowing condensate accumulation on the reference side leading to calibration drift.

Final Answer:
Differential pressure manometer