The sensitivity of a bubble tube decreases when:
Correct Answer: D. The viscosity and surface tension of the liquid are increased
📚 Detailed Explanation: Bubble Tube Sensitivity Decreases When Viscosity and Surface Tension Are Both Increased
Why D (Both viscosity and surface tension are increased) is correct: This is the same principle as Q55 but asked from the reverse angle. Two factors that resist the movement of the bubble are (1) high liquid viscosity (internal friction) and (2) high surface tension (adhesion of bubble to tube). When either or both are increased, sensitivity decreases.
Summary: What Changes Sensitivity
| Factor | If Increased… | Effect on Sensitivity |
|---|---|---|
| Radius of curvature of tube (R) | Larger R | Increases sensitivity |
| Diameter of tube | Larger diameter | Increases sensitivity |
| Length of bubble | Longer bubble | Increases sensitivity |
| Viscosity of liquid | Higher viscosity | Decreases sensitivity |
| Surface tension | Higher surface tension | Decreases sensitivity |
| Both viscosity AND surface tension | Both increase | Decreases sensitivity (option D — correct) |
Q69 vs Q55 distinction: Q55 asked which specific single factor causes decrease — answer was “decrease in viscosity INCREASES sensitivity.” Q69 asks which combination DECREASES sensitivity — answer is both viscosity AND surface tension being increased.
- Sensitivity decreases when viscosity and surface tension of the bubble liquid both increase.
- Higher viscosity = more internal resistance → bubble sluggish and imprecise.
- Higher surface tension = stronger wall adhesion → bubble reluctant to settle at equilibrium.
