Identifying non–variable-area (VA) flowmeters: Which of the following is <em>not</em> a variable area flow meter (areameter) used in process flow measurement?

Introduction / Context:
Variable-area (VA) meters, also known as areameters, indicate flow by allowing the flow passage area to change until the force balance is achieved. Recognizing which devices are VA aids correct selection and troubleshooting in instrumentation diagrams and specifications.

Given Data / Assumptions:

• VA meters include tapered tubes with floats, variable orifices, and piston-style mechanisms.
• Electromagnetic meters measure flow based on induced voltage proportional to velocity in a conductive fluid.
• All devices are considered under typical plant conditions.

Concept / Approach:
VA devices alter flow area with flowrate, e.g., a float rising in a tapered tube (rotameter) or a spring-loaded piston moving to change an orifice gap. Magnetic flow meters, by contrast, have a fixed cross-section and infer volumetric flow Q = velocity * area by measuring the induced voltage from a conductive fluid moving through a magnetic field. Thus, magnetic flow meters are not VA instruments.

Step-by-Step Solution:

Verification / Alternative check:
Instrument catalogs classify rotameters and piston-type meters as VA, while magnetic meters are velocity-based and require conductivity.

Why Other Options Are Wrong:

• Piston type: Classic spring-loaded VA design.
• Rotameter: Archetypal VA meter.
• Orifice and tapered plug VA meter: Area varies with plug position.
• Variable nozzle VA meter: Area changes with flow/position.

Common Pitfalls:
Assuming any device with a “taper” is VA; electromagnetic flow meters have no moving parts and fixed area.

Final Answer:
Magnetic flow meter (electromagnetic)