Measuring very low (near-vacuum) pressures in fluid mechanics Among the following instruments, which is appropriate for measuring very low pressures (small positive/negative gauge values) in typical laboratory practice?

Introduction / Context:
Very low pressures (small departures from atmospheric) are commonly measured in labs using liquid columns because they can resolve tiny pressure differences with suitable geometry (e.g., inclined tubes). Selecting the proper instrument depends on whether absolute, gauge, or differential pressure is needed.

Given Data / Assumptions:

• Non-volatile manometric fluids (e.g., water, mercury, alcohol) available.
• Small gauge pressures or slight vacuums to be measured.
• Steady conditions without rapid oscillations.

Concept / Approach:

A manometer translates pressure to a hydrostatic head difference h via p = ρ g h. For very small pressures, an inclined manometer increases the scale resolution. Piezometers only measure positive gauge pressure and cannot measure vacuum; barometers read atmospheric (absolute) pressure; differential manometers compare two points but are not specialized for near-zero absolute detection without careful setup; Bourdon gauges lack the fine resolution of an inclined liquid manometer for very low ranges.

Step-by-Step Solution:

Verification / Alternative check:

Instrument catalogs list micromanometers and inclined manometers specifically for low-pressure measurements, confirming suitability.

Why Other Options Are Wrong:

(a) Barometer measures absolute atmospheric pressure only. (b) Piezometer cannot register negative gauge pressure. (d) Differential manometers are comparative devices and not inherently optimized for very small single-point gauge pressures. (e) Bourdon gauges generally lack sufficient sensitivity at very low ranges without special design.

Common Pitfalls:

Using too small a-diameter tube causing capillarity errors; choosing a dense manometric fluid that reduces head sensitivity for tiny Δp.

Final Answer:

manometers