Applying manometers appropriately: Liquid column manometers are generally used to measure pressures of what approximate magnitude (gauge)?

Introduction / Context:
Liquid column manometers indicate pressure by balancing a fluid column against the process pressure. They offer excellent resolution for small differentials and are common in laboratories and low-pressure process services, such as HVAC ducts and small furnace draught lines.

Given Data / Assumptions:

• Standard manometer fluids like water, oils, or mercury (with proper safety) are used.
• Readability and practicality decrease as required column height becomes excessive.
• Gauge pressures (relative to atmosphere) are considered.

Concept / Approach:
The height required is Δh = ΔP / (ρ * g). For large pressures, Δh becomes impractically tall unless high-density fluids like mercury are used. Even then, readability and safety issues arise. Hence, liquid manometers are best suited to relatively low pressures (often a few kilopascals to tens of kilopascals), roughly under a few kgf/cm2, with the conventional exam benchmark around 3 kgf/cm2 for practicality.

Step-by-Step Solution:

Verification / Alternative check:
Industrial practice uses gauges or transmitters for higher pressures; U-tube or inclined manometers are reserved for low differentials (e.g., mmWG, inWG).

Why Other Options Are Wrong:

• > 2 or < 10 indiscriminately: Too high for practical manometer heights and safe operation in many cases.
• < atmospheric only: Manometers measure positive or negative gauge pressures; not limited to vacuum.
• Exactly 1: Arbitrary; the tool is not restricted to a fixed value.

Common Pitfalls:
Choosing low-density liquids for higher pressures, resulting in unmanageably tall columns; always check Δh for your fluid.

Final Answer:
< 3 (gauge) kgf/cm2