Level measurement for cryogenic liquid in a pressurised tank: Liquid argon level in a pressurised storage tank (about 3 kg/cm²) should be measured using which primary method?

Introduction / Context:
Measuring level in cryogenic, pressurised service (e.g., liquid argon) requires accuracy, safety, and reliability. The instrument must tolerate low temperatures, avoid icing hazards, and work on a closed, pressurised tank. This drives the choice toward indirect, sealed methods rather than direct sighting devices.

Given Data / Assumptions:

• Service: liquid argon, cryogenic, tank pressure ≈ 3 kg/cm².
• Closed vessel without open venting to atmosphere at the measurement point.
• Industrial-grade instrumentation options.

Concept / Approach:
Differential pressure (DP) transmitters infer level from hydrostatic head: ΔP = ρ * g * h (after compensating for any vapour-space pressure via a high-pressure tap). With proper wet legs or remote seals rated for cryogenic use, DP level is standard and robust. Gauge glasses are unsafe and impractical (frosting, breakage hazard) on cryogenic pressured service. External floats can suffer from icing, leakage paths, and poor accuracy under pressure. Thus, DP is preferred in practice for cryogenic pressurised tanks.

Step-by-Step Solution:

Verification / Alternative check:
Vendor application notes for cryogenic storage (LIN, LOX, LAr) consistently recommend DP with remote seals or weigh systems; sight glasses are avoided for safety.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring vapour-space pressure compensation; DP must reference top/bottom taps correctly to cancel vessel pressure and read only the head of liquid.

Final Answer:
Differential pressure (DP) level measurement