Levelling instruments – bubble tube filling: The liquid used to fill the spirit (bubble) level tube should have which properties?

Introduction / Context:
Bubble levels in dumpy/auto levels and theodolites depend on a sealed, slightly curved tube filled with a suitable liquid and an air bubble. The fluid must allow the bubble to respond quickly and reliably under field temperatures. The question checks the desirable properties and common fluids used.

Given Data / Assumptions:

• The bubble responds to instrument tilts by moving to the highest point.
• Field temperatures may be low; freezing must be avoided.
• Quick damping and mobility are needed for productivity and accuracy.

Concept / Approach:
A suitable bubble liquid typically has low viscosity (to let the bubble settle rapidly), a low freezing point (to avoid solidification in cold climates), and chemical compatibility with glass and seals. Alcohols (such as ethyl or methyl alcohol) and sometimes chloroform or ether-based mixtures have been traditionally used because they meet these requirements better than water or oils.

Step-by-Step Solution:

Verification / Alternative check:
Instrument manuals list denatured alcohol or specialized formulations with antifreeze characteristics to maintain sensitivity and speed across temperature ranges.

Why Other Options Are Wrong:

• Any single property alone is incomplete; optimal bubble fluids satisfy all three.

Common Pitfalls:
Using water (freezes easily and is too viscous at low temperatures); ignoring seal compatibility; allowing contamination that changes viscosity or creates bubbles/foaming.

Final Answer:
All of the above