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A pitot tube ( ; 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 In aviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the sur ...
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 cannot escape and stagnates. This pressure is the
stagnation pressure In fluid dynamics, stagnation pressure, also referred to as total pressure, is what the pressure would be if all the kinetic energy of the fluid were to be converted into pressure in a reversable manner.; it is defined as the sum of the free-strea ...
of the fluid, also known as the total pressure or (particularly in aviation) the
pitot pressure Pitot pressure is the pressure that can be measured by a Pitot tube, with an open-ended tube facing into the oncoming fluid with the other end closed off. The stationary fluid can be connected to a pressure-measuring device, or used in various devi ...
. 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 + \left(\frac\right) Solving that for flow velocity gives :u = \sqrt where * u is the flow velocity; * p_t is the stagnation or total pressure; * p_s is the static pressure; * and \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. 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 ports 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 Ludwig Prandtl (4 February 1875 – 15 August 1953) was a German Fluid mechanics, fluid dynamicist, physicist and aerospace scientist. He was a pioneer in the development of rigorous systematic mathematical analyses which he used for underlyin ...
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. If a liquid column
manometer Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressu ...
is used to measure the pressure difference \Delta p \equiv p_t - p_s, :\Delta h = \frac where * \Delta h is the height difference of the columns; * \rho_l is the density of the liquid in the manometer; * g is the standard acceleration due to gravity. Therefore, :u = \sqrt


Aircraft and accidents

A pitot-static system is a
system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...
of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's
airspeed In aviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the sur ...
, Mach number,
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as airspeed, is potentially safety-critical. Several commercial airline incidents and accidents have been traced to a failure of the pitot-static system. Examples include Austral Líneas Aéreas Flight 2553, Northwest Airlines Flight 6231, Birgenair Flight 301 and one of the two X-31s. The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of
Air France Flight 447 Air France Flight 447 was a scheduled international passenger flight from Rio de Janeiro/Galeão International Airport, Rio de Janeiro, Brazil, to Charles de Gaulle Airport, Paris, France. On 1 June 2009, inconsistent airspeed indications and mi ...
into the
Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...
. In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s. Birgenair Flight 301 had a fatal pitot tube failure which investigators suspected was due to insects creating a nest inside the pitot tube; the prime suspect is the black and yellow mud dauber wasp. Aeroperú Flight 603 had a fatal pitot-static system failure due to the cleaning crew leaving the static port blocked with tape.


Other applications

In industry, the flow velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential
pressure gauge Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of press ...
for determining the flow velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space. The fluid flow rate in a duct can then be estimated from: :Volume flow rate (cubic feet per minute) = duct area (square feet) × flow velocity (feet per minute) :Volume flow rate (cubic meters per second) = duct area (square meters) × flow velocity (meters per second) In aviation, airspeed is typically measured in
knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, including List of hitch knots, hitches, List of bend knots, bends, List of loop knots, loop knots, ...
s. In weather stations with high wind speeds, the pitot tube is modified to create a special type of anemometer called pitot tube static anemometer. In many modern carburetors a Pitot tube at the intake is fed to the fuel float chamber as an alternative to feeding ambient air pressure there to better control air/fuel ratio.


See also


References


Notes


Bibliography

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External links


3D animation of the Pitot Tube Differential Pressure Flow Measuring Principle

How 18th Century Technology Could Down an Airliner
(wired.com) {{DEFAULTSORT:Pitot Tube Flow meters Aircraft instruments Speed sensors Measuring instruments French inventions Navigational flight instruments