Editing Pitot tube (section)

==Theory of operation==
The basic pitot tube consists of a tube pointing directly into the oncoming fluid flow. Pressure in the tube can be measured as the moving fluid cannot escape and stagnates. This pressure is the [[stagnation pressure]] of the fluid, also known as the total pressure or (particularly in aviation) the [[pitot pressure]].

The measured stagnation pressure cannot just by itself be used to determine the fluid flow velocity (airspeed in aviation) directly. However, with a measured static pressure as well it can be determined by the use of [[Bernoulli's equation]] which states:

:'''Stagnation pressure = [[static pressure]] + dynamic pressure'''

Which can also be written

:<math>p_t = p_s + \left(\frac{\rho u^2}{2}\right)</math>

Solving that for flow velocity gives

:<math>u = \sqrt{\frac{2 (p_t - p_s)}{\rho}}</math>

where
* <math>u</math> is the [[flow velocity]];
* <math>p_t</math> is the stagnation or total pressure;
* <math>p_s</math> is the static pressure;
* and <math>\rho</math> is the fluid density.

This equation applies only to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See [[Compressibility]].

The [[dynamic pressure]] is the difference between the stagnation pressure and the static pressure. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure.

In aircraft, the static pressure can be measured using [[static port]]s on the side of the fuselage.  The dynamic pressure measured can be used to determine the [[indicated airspeed]] of the aircraft.  The diaphragm arrangement described above can be contained within the [[airspeed indicator]], which can convert the dynamic pressure to an airspeed reading by means of mechanical levers.

Instead of separate pitot and static ports, a pitot-static tube (also called a [[Prandtl]] tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.<ref>[https://books.google.com/books?id=0SkDAAAAMBAJ&dq=Popular+Science+The+lads+who+fly+Britain%27s+1944&pg=PA116 "How Aircraft Instruments Work."] ''Popular Science'', March 1944, pp. 116.</ref>

If a liquid column [[Manometer#Liquid column|manometer]] is used to measure the pressure difference <math>\Delta p \equiv p_t - p_s</math>, 
:<math>\Delta h = \frac{\Delta p}{\rho_l g}</math>
where
* <math>\Delta h</math> is the height difference of the columns;
* <math>\rho_l</math> is the density of the liquid in the manometer;
* g is the [[standard gravity|standard acceleration due to gravity]].

Therefore,
:<math>u = \sqrt{\frac{2 \, \Delta h \, \rho_l g}{\rho}}</math>