Flow measurement — principle of a rotameter A rotameter (variable area flowmeter) indicates flow rate primarily based on which operating principle?

Introduction / Context:
Among mechanical flowmeters, the rotameter is popular for simplicity and direct visual indication. It is commonly used for gases and liquids in laboratories and process skids.

Given Data / Assumptions:

• The device has a vertical, tapered, transparent tube and a float.
• Flow is upward; the float reaches an equilibrium position between buoyant, drag, and gravitational forces.

Concept / Approach:
As flow increases, the float rises to a larger cross-section of the tapered tube, increasing the annular area until the pressure drop and forces balance. Thus, the meter operates on a variable area principle, not on a fixed-area pressure drop or turbine rotation principle.

Step-by-Step Solution:
Identify unique geometry: tapered tube + moving float.Relate float height to flow via equilibrium of forces and area change.Conclude: measurement principle is variable area.

Verification / Alternative check:
Calibration marks on the tube correlate float height to standard volumetric flow under specified conditions.

Why Other Options Are Wrong:

• Turbine rotation: describes turbine meters, not rotameters.
• Nozzle pressure drop: that is an orifice/nozzle/venturi principle with fixed area.
• Stagnation pressure: used in pitot tubes, not rotameters.

Common Pitfalls:

• Assuming all differential-pressure meters are the same; rotameters vary area, not only measure Δp.

Final Answer:
Variable flow area created by a float in a tapered tube