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Pitot tube

A Pitot tube measures the speed in a flow by bringing the fluid to rest in a small opening and comparing the stagnation pressure with the static pressure nearby. The difference between them is the dynamic pressure, which Bernoulli links directly to the speed.

p_0 = p + \tfrac12\rho v^2

stagnation pressure

v = \sqrt{\dfrac{2\Delta p}{\rho}}

speed from the pressure difference in a Pitot tube

Symbols

$p_0$	stagnation pressure	Pa
$\Delta p$	pressure difference (dynamic pressure)	Pa

Example

A Pitot tube in air ( $\rho = 1.2$ kg/m³) measures $\Delta p = 250$ Pa. $v = \sqrt{2\cdot 250/1.2} \approx 20.4$ m/s.

The stagnation pressure is always the highest pressure along a streamline, since all the kinetic energy there has converted to pressure.

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Part of Fluid Mechanics: Continuity and Bernoulli.