In an inclined manometer used to measure very small pressure heads accurately, the inclination to the horizontal is chosen to minimize surface-tension (capillary) reading errors while still providing adequate sensitivity. What is a commonly adopted minimum inclination angle to avoid surface tension effects dominating the reading?

Introduction / Context:
An inclined manometer stretches a small vertical pressure head into a longer length along the tube, improving readability for low pressures. However, if the inclination is too small, meniscus and surface tension (capillary) effects can distort readings. A practical minimum angle is therefore recommended.

Given Data / Assumptions:

• Typical laboratory inclined manometer with clean glass tube and standard manometric liquids (water or light oils; sometimes mercury in special cases).
• Objective is to limit capillary and meniscus errors relative to the measured length while preserving sensitivity.

Concept / Approach:
For a given vertical head h, the length read along the tube is L = h / sinθ. As θ decreases, L increases (better resolution). But extremely small θ magnifies meniscus deformation relative to graduations, and small-angle mounting becomes impractical. Empirical practice balances these factors at a small, but not too small, θ.

Step-by-Step Reasoning:
Resolution improves as θ decreases because L increases.Capillary effects and parallax become relatively larger below a few degrees.Practical designs therefore keep θ in the small-angle range but not below about 5°.Choice θ ≈ 5° is a widely used minimum to mitigate surface tension while keeping sensitivity acceptable.

Verification / Alternative Check:
Many laboratory manuals suggest θ between about 5° and 15° for water-based measurements to balance readability against meniscus error, confirming 5° as a sensible lower bound.

Why Other Options Are Wrong:
2°, 3°, 4°: too small; meniscus and capillary effects become relatively significant, risking larger percentage error.12°: acceptable but not the minimum commonly adopted to avoid capillary dominance.

Common Pitfalls:

• Ignoring the need to read at the same meniscus side consistently.
• Using dirty or rough tubes which amplify wetting effects.
• Not temperature-stabilizing the manometric fluid; viscosity changes alter damping and reading stability.

Final Answer:
5°