Practical range of liquid manometers What is a typical upper limit for measurable differential pressure using a conventional liquid column manometer (e.g., mercury U-tube) expressed in psi?

Introduction / Context:
Liquid column manometers are simple, reliable devices for measuring differential pressure by balancing a pressure difference against the hydrostatic head of a fluid column. While they offer excellent accuracy for low to moderate differentials, practical considerations (column height, readability, density, and safety) limit their useful range.

Given Data / Assumptions:

• Typical fluids include water, manometer oils, or mercury (high density).
• “Conventional” implies standard lab or plant U-tube or inclined configurations, not specialized very tall installations.
• We are interested in a broadly cited upper bound.

Concept / Approach:
Differential pressure ΔP corresponds to ρ * g * Δh. To measure higher ΔP with reasonable Δh, a dense manometric fluid like mercury is used. About 1 inHg ≈ 0.491 psi, so 50 psi corresponds to roughly 102 inHg of height difference, which is already unwieldy. Hence, around 50 psi is commonly viewed as a practical upper limit for conventional manometers; beyond this, mechanical or electronic differential pressure transmitters are preferred due to size and safety considerations.

Step-by-Step Solution:

Verification / Alternative check:
Instrumentation handbooks treat manometers as best for low and moderate ΔP; industrial DP transmitters take over for higher ranges.

Why Other Options Are Wrong:

• 20–40 psi: Possible within manometer use, but 50 psi is the typical cited upper practical bound for conventional mercury columns.

Common Pitfalls:
Ignoring scale readability, safety of mercury, and mounting height; theoretical capability does not imply convenient operation.

Final Answer:
50