Flow measurement basics: which statement correctly describes a Venturi tube's measurement capability in industrial instrumentation?

Introduction / Context:
The Venturi tube is a classic differential-pressure flow element. By constricting the flow area, it creates a predictable pressure drop between the inlet and the throat that correlates with volumetric flow, after accounting for fluid properties and geometry. It is widely used for both liquids and gases due to its accuracy and relatively low permanent pressure loss compared to orifice plates.

Given Data / Assumptions:

• Venturi tubes operate under Bernoulli/continuity principles for incompressible or compressible flows with proper equations.
• Materials can vary: metals, composites, or lined constructions depending on media.
• Instrumentation taps measure differential pressure, which a transmitter converts to a flow signal.

Concept / Approach:
Flow rate (Q) relates to the square root of measured differential pressure (DeltaP) once geometry and discharge coefficients are known: Q ∝ sqrt(DeltaP). With proper temperature/pressure compensation for gases, Venturi meters work reliably across phases; their low pressure loss makes them favorable in large pipelines.

Step-by-Step Solution:
Recognize that Venturi tubes quantify rate, not mere presence, by differential pressure.Acknowledge that construction materials are application-dependent, not metal-only.Select the option stating they measure gas or liquid flow rates.

Verification / Alternative check:
Industry practice includes Venturi meters in waterworks, natural gas transmission, and chemical processing, confirming their cross-phase applicability.

Why Other Options Are Wrong:
A: They are standard instruments, not “extremely difficult.” B: Not limited to metal. C: They measure rate/quantity, not mere presence.

Common Pitfalls:
Neglecting compensation for gas density changes; misplacing pressure taps relative to the throat.

Final Answer:
It can be used to measure gas or liquid flow rates.