Introduction |

The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by converting the kinetic energy of the flow into potential energy. The conversion takes place at the Cross-section of a Typical Pitot Static TubeConverting the resulting differential pressure measurement into a fluid velocity depends on the particular fluid flow regime the Pitot tube is measuring. Specifically, one must determine whether the fluid regime is incompressible, subsonic compressible, or supersonic. |

Incompressible Flow |

A flow can be considered incompressible if its velocity is less than 30% of its sonic velocity. For such a fluid, the Bernoulli equation describes the relationship between the velocity and pressure along a streamline,
Evaluated at two different points along a streamline, the Bernoulli equation yields, If z and point _{2}2 is a stagnation point, i.e., v = 0, the above equation reduces to,
_{2}The velocity of the flow can hence be obtained, or more specifically, |

Subsonic Compressible Flow |

For flow velocities greater than 30% of the sonic velocity, the fluid must be treated as compressible. In compressible flow theory, one must work with the When a Pitot tube is exposed to a subsonic compressible flow (0.3 < If we assume that the flow decelerated and compressed from the free-stream state isentropically, the velocity-pressure relationship for the Pitot tube is, where g is the ratio of specific heat at constant pressure to the specific heat at constant volume, If the free-stream density r where is the speed of sound (i.e. sonic velocity), |

Supersonic Compressible Flow |

For supersonic flow ( The flow velocity is an implicit function of the Pitot tube pressures, Note that this formula is valid only for Reynolds numbers |