A pitot // tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas velocities in industrial applications. The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.
Theory of operation
The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. 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 itself be used to determine the fluid velocity (airspeed in aviation). However, Bernoulli's equation states:
- Stagnation pressure = static pressure + dynamic pressure
Which can also be written
Solving that for velocity we get:
NOTE: The above 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.
- is fluid velocity;
- is stagnation or total pressure;
- is static pressure;
- and is fluid density.
The value for the pressure drop – or due to , the reading on the manometer:
- is the density of the fluid in the manometer
- is the manometer reading
The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The static pressure is generally measured using the static ports on the side of the fuselage. 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, which can then be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement is typically contained within the airspeed indicator, which converts 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.
Pitot tubes on aircraft commonly have heating elements called pitot heat to prevent the tube from becoming clogged with ice. The failure of these systems can have catastrophic consequences, as in the case of Austral Líneas Aéreas Flight 2553, Birgenair Flight 301 (investigators suspected that some kind of insect could have created a nest inside the pitot tube: the prime suspect is a species called the black and yellow mud dauber wasp), Northwest Airlines Flight 6231, Aeroperú Flight 603 (blocked static port), and of one X-31. The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean. In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s.
In industry, the 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 for determining the 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) × velocity (feet per minute)
- Volume flow rate (cubic meters per second) = duct area (square meters) × velocity (meters per second)
In aviation, airspeed is typically measured in knots.
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- Darcy, Henry (1858). "Note relative à quelques modifications à introduire dans le tube de Pitot" (PDF). Annales des Ponts et Chaussées: 351–359. Retrieved 2009-07-31.
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- Tietjens, O.G. (1934). Applied Hudro- and Aeromechanics, based on lectures of L. Prandtl, Ph.D. Dove Publications, Inc. pp. 226–239. ISBN 0-486-60375-X.
- Saleh, J.M. (2002). Fluid Flow Handbook. McGraw-Hill Professional.
|Wikimedia Commons has media related to Pitot tube.|
- 3D animation of the Pitot Tube Differential Pressure Flow Measuring Principle
- How 18th Century Technology Could Down an Airliner (wired.com)