Pitot Tube

Pitot tube measures velocity at a point in the fluid. Pitot tube consists of two tubes. Refer to Fig.(16). The tube which has opening(s) parallel to the flow measures the static pressure (\(P\)) of the fluid. The other tube which has opening perpendicular to the flow measures the sum of static and dynamic pressures (\(P+\rho v^2/2\)). The difference of these two pressures gives \(\rho v^2/2\). From which, velocity is estimated. 


Manometer connected between these two tubes directly gives the value of \(\rho v^2/2\). \[(\rho_m-\rho) g h = \frac{\rho v^2}{2} \qquad \Longrightarrow \quad v = \sqrt{\frac{2(\rho_m-\rho)gh}{\rho}}\] Instead of manometer, if there are pressure gauges, then the pressure difference between the two gauges gives \(\Delta P\). From which, \[v = \sqrt{\frac{2\;\Delta P}{\rho}}\] Pitot tube can be used to measure flow in open channels as well.