The Pitot tube is used to measure fluid velocity and consists of a narrow tube facing upstream and connected to a manometer. What principle does it employ?

• A. A differential pressure measurement between two static ports
• B. An electromagnetic flow measurement
• C. A stagnation pressure measurement
• D. A thermal expansion principle

Answer: (C)

The essential idea is that a Pitot tube determines velocity by measuring stagnation pressure. When the tube faces the flow, the moving fluid is brought to rest at the entrance, creating a stagnation (total) pressure p0 that is higher than the ambient static pressure. The manometer reads this stagnation pressure, and using Bernoulli’s relationship p0 = p + 1/2 ρ v^2, you can solve for velocity from the difference between p0 and the static pressure. This shows how kinetic energy of the flow is converted into pressure energy at the stagnation point to reveal the speed. While a Pitot-static arrangement can directly give a differential pressure to obtain dynamic pressure, the key principle in a basic Pitot tube is stagnation pressure measurement. Other methods like electromagnetic flow or thermal expansion are different technologies not used by this instrument.