Pitot tube

A pitot tube ( PEE-toh; also pitot probe) measures fluid flow velocity. It was invented by French engineer Henri Pitot during his work with aqueducts and published in 1732, and modified to its modern form in 1858 by Henry Darcy. It is widely used to determine the airspeed of aircraft; the water speed of boats; and the flow velocity of liquids, air, and gases in industry.

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 stagnates, as it cannot escape. 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

p

t

=

p

s

+

(

ρ

u

2

2

)

{\displaystyle p_{t}=p_{s}+\left({\frac {\rho u^{2}}{2}}\right)}

Solving that for flow velocity gives

u

=

2

(

p

t

−

p

s

)

ρ

{\displaystyle u={\sqrt {\frac {2(p_{t}-p_{s})}{\rho }}}}

where

u

{\displaystyle u}

is the flow velocity;

p

t

{\displaystyle p_{t}}

is the stagnation or total pressure;

p

s

{\displaystyle p_{s}}

is the static pressure;

and

ρ

{\displaystyle \rho }

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.