Automatic/dumpy level sensitivity: The radius of curvature of the arc of a bubble tube (spirit level) is generally kept approximately equal to which value to achieve suitable sensitivity in ordinary levelling?

Introduction / Context:

The sensitivity of a bubble tube depends on the radius of curvature of its internal arc. A longer radius means a flatter arc and a more sensitive bubble (greater displacement per unit tilt). Survey instrument design balances sensitivity with practicality to allow stable, readable bubbles during field work.

Given Data / Assumptions:

• Ordinary levelling (engineer’s/dumpy level) rather than high-precision geodetic levelling.
• Ambient field conditions (wind, vibration) require a practical bubble sensitivity.
• Graduations typically read to a few seconds of arc per division.

Concept / Approach:

Typical radii for general-purpose levels are about 10 m, yielding workable sensitivity without excessive bubble travel. More precise instruments may use longer radii (e.g., 25–50 m) for finer sensitivity, but the common teaching value for general use is 10 m.

Step-by-Step Solution:

Verification / Alternative check:

Instrument datasheets for standard dumpy levels cite bubble sensitivities consistent with ~10 m radii, whereas precise tilting levels reach higher radii values.

Why Other Options Are Wrong:

• 25–100 m: correspond to more sensitive, specialized levels.
• 5 m: would be too short, producing an overly stiff (insensitive) bubble.

Common Pitfalls:

• Assuming “more sensitive” is always better; too sensitive bubbles are hard to center outdoors.

Final Answer:

10 m