Flow metering classification in process plants: Among the following instruments, which one operates as an area meter (i.e., variable-area type) for measuring volumetric flow rate?

Introduction / Context:
Industrial flow measurement technologies are often grouped by their measuring principle: differential pressure, velocity, positive displacement, and variable area. An “areameter” is shorthand for a variable-area flowmeter in which the effective flow area changes with flow rate until the fluid forces balance. Identifying the rotameter as this type helps with correct selection, installation, and troubleshooting in chemical engineering plants.

Given Data / Assumptions:

• The term “areameter” refers to a variable-area device.
• We consider common instruments used in pipes carrying liquids or gases.
• Steady, single-phase flow is assumed.

Concept / Approach:
A rotameter consists of a tapered (usually vertical) tube and a freely moving float. As flow increases upward, the float rises to a position where the pressure drop across it balances the buoyant and gravitational forces, simultaneously increasing the annular area for flow. The scale is thus proportional to flow. This is the archetypal variable-area (areameter) device. By contrast, Venturimeters and orifice plates are differential-pressure devices with fixed geometry; pitot tubes measure local velocity head; and hot-wire anemometers infer velocity from heat transfer.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturers and standards refer to rotameters as variable-area flowmeters (VAFMs), commonly calibrated in L/min or Nm^3/h.

Why Other Options Are Wrong:

• Venturimeter / Orifice: Fixed-area primary elements; infer flow via differential pressure and Bernoulli.
• Pitot tube: Measures point velocity from stagnation and static pressure.
• Hot-wire anemometer: Heat-transfer based velocity probe, not a variable-area device.

Common Pitfalls:
Confusing “variable area” with “variable orifice” DP meters; in areameters the area changes to balance forces on a float, not to create a fixed DP across a restriction.

Final Answer:
Rotameter