Pitot tube

A Pitot tube is a bent tube with an opening pointed directly upstream into a moving fluid. Fluid entering that opening is brought to rest, so the pressure sensed there is the stagnation pressure (also called total pressure) — the sum of the flow's static pressure and its dynamic pressure. A separate port, oriented perpendicular to the flow, senses static pressure alone. The instrument's output is the difference between the two.

Bernoulli's equation converts that pressure difference into a velocity. Setting stagnation pressure equal to static pressure plus ½ρv² and solving gives v = √(2(P_stagnation − P_static) / ρ), where ρ is fluid density. For example, a pressure difference of 800 Pa in air with density 1.2 kg/m³ yields v = √(1600 / 1.2) ≈ 36.5 m/s. Note that velocity depends on the square root of the pressure difference, so doubling the reading raises the measured speed by only about 41%.

The device is used wherever flow speed matters and intrusion is acceptable: aircraft airspeed indication, wind tunnel testing, HVAC duct balancing, and pipe flow surveys. Its accuracy depends on correct alignment with the flow and on knowing the fluid density, and a blocked or iced-over port is a well-known cause of erroneous airspeed readings. Unlike a Venturi meter, which infers flow rate from a constriction in the pipe, a Pitot tube measures local velocity at the point where it sits.

The MCAT covers Pitot tubes within fluid mechanics alongside Bernoulli's principle and the continuity equation, usually asking you to reason qualitatively about how pressure and velocity trade off. The FE Civil exam treats it quantitatively as a flow measurement device, expecting you to apply the velocity equation with values from the reference handbook.