Flow measurement – identify the device that is not a differential-pressure meter Which of the following is <em>not</em> a differential-pressure (DP) type flow meter?

Introduction / Context:
Industrial flow measurement devices come in many forms. Differential-pressure (DP) meters infer flow from a pressure drop created by a restriction. Variable-area meters (rotameters) infer flow from the equilibrium position of a float in a tapered tube. Recognizing the measurement principle is foundational in instrumentation selection and sizing.

Given Data / Assumptions:

• DP meters: pressure drop across a primary element is related to flow (Bernoulli + discharge coefficient).
• Variable-area meter: float rises until drag force balances weight minus buoyancy.
• Ideal fluids and standard installation practices are assumed for principle identification.

Concept / Approach:
Venturimeters, flow nozzles, and orificemeters are textbook DP meters. Each creates a constriction; the static pressure difference between upstream and throat/taps correlates with volumetric or mass flow after calibration. A rotameter, however, uses a tapered transparent tube and a float. The area available to flow increases with float height; the float's position (not a DP reading) is directly indicated and correlated to flow rate. Thus, the rotameter is not a DP meter.

Step-by-Step Solution:

Verification / Alternative check:
Standards (ISO/ASME) classify Venturi, nozzle, orifice under DP primary devices, while rotameters are listed under variable-area (VA) meters.

Why Other Options Are Wrong:

• Venturimeter / Flow nozzle / Orificemeter: All operate on differential pressure across a constriction.

Common Pitfalls:
Assuming every meter with a pressure effect is a DP meter; in rotameters the indicated variable is position, not measured pressure drop.

Final Answer:
Rotameter