Which principle describes how a Venturi flow meter measures flow?

• A. Differential pressure across throat
• B. Temperature difference
• C. Electric current
• D. Vibration frequency

Answer: (A)

Venturi flow meters measure flow by using the pressure change created as fluid is forced through a constricted throat. When the flow enters the narrow section, the fluid speeds up, and the static pressure drops. This creates a differential pressure between the upstream area and the throat that can be measured with pressure taps.

The relationship comes from continuity and Bernoulli’s principle. Continuity tells us that the flow rate must be the same, so the product of area and velocity is constant (A1 V1 = A2 V2). Bernoulli’s principle relates pressure and velocity along a streamline (P1 + ½ ρ V1^2 = P2 + ½ ρ V2^2). Combining these, the pressure difference ΔP = P1 − P2 depends on how fast the fluid is moving, which in turn depends on the flow rate. By calibrating ΔP, the device gives that flow rate.

This approach is not based on temperature differences, electric current, or vibration frequency, which is why those options don’t describe how a Venturi meter works.