In differential pressure meters, what happens to pressure as velocity increases in a closed conduit?

• A. Pressure increases with velocity
• B. Pressure decreases with velocity
• C. Pressure remains constant as velocity changes
• D. Pressure fluctuates with velocity

Answer: (B)

In a closed conduit with steady, incompressible flow, Bernoulli’s principle says that along a streamline the sum of static pressure, dynamic pressure (½ρv²), and elevation head is constant. If the height doesn’t change, the elevation term is constant, so the sum of static pressure and dynamic pressure remains constant. As velocity increases through a restriction, the dynamic pressure term grows, and static pressure must fall to keep the total the same. Differential pressure meters take advantage of this by placing a constriction where velocity increases and static pressure drops; the pressure difference between the upstream and the constricted section is what’s measured to infer flow. Thus pressure decreases as velocity increases.