Pitot tube

Dictionary: Pi·tot tube (pē'tō, pē-tō') pronunciation

n.
A device, essentially a tube set parallel to the direction of fluid-stream movement and attached to a manometer, used to measure the total pressure of the fluid stream.

[After Henri Pitot (1695-1771), French physicist.]

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pitot tube

Instrument for measuring the velocity (speed) of a flowing fluid. Invented by Henri Pitot (1695 – 1771), it consists of a tube with a short, right-angled bend, which is placed vertically in a moving fluid with the mouth of the bent part directed upstream; the pressure, measured with an attached device, depends on the fluid flow and can be used to calculate the velocity. Pitot tubes are used to measure airspeed in wind tunnels and aboard aircraft in flight; they are also used to measure the flow of liquids (see flow meter).

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A device to measure the stagnation pressure due to isentropic deceleration of a flowing fluid. In its original form it was a glass tube bent at 90° and inserted in a stream flow, with its opening pointed upstream. Water rises in the tube a distance, h, above the surface, and if friction losses are negligible, the velocity of the stream, V, is approximately 2gh, where g is the acceleration of gravity. However, there is a significant measurement error if the probe is misaligned at an angle α with respect to the stream. For an open tube, the error is about 5% at α ≈ 10°.

The misalignment error of a pitot tube is greatly reduced if the probe is shielded, as in the Kiel-type probe. The Kiel probe is accurate up to α ≈ 45°.

The modern application is a pitot-static probe, which measures both the stagnation pressure, with a hole in the front, and the static pressure in the moving stream, with holes on the sides. A pressure transducer or manometer records the difference between these two pressures. Pitot-static tubes are generally unshielded and must be carefully aligned with the flow to carry out accurate measurements. See also Bernoulli's theorem.

When used with gases, estimate of the stream velocity is only valid for a low-speed or nearly incompressible flow, where the stream velocity is less than about 30% of the speed of sound of the fluid. At higher velocities, estimate of the stream velocity must be replaced with a Bernoulli-type theory, which accounts for gas density and temperature changes. If the gas stream flow is supersonic, or the stream velocity is greater than the speed of sound of the gas, a shock wave forms in front of the probe and the theory must be further corrected by complicated supersonic-flow algebraic relations. See also Compressible flow; Gas dynamics; Shock wave.

A disadvantage of pitot and pitot-static tubes is that they have substantial dynamic resistance to changing conditions and thus cannot accurately measure unsteady, accelerating, or fluctuating flows. See also Anemometer; Flow measurement.

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pitot tube

A device, used in conjunction with a suitable manometer or other pressure-reading instrument, for measuring the velocity of air in a duct or water in a pipe.

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An open-end tube that points directly into the air flowing over the aircraft structure, providing pressure for the pitot and, often, the static systems. See pitot pressure.

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Types of pitot tubes

Aircraft use pitot tubes to measure airspeed. The example in this photo combines a pitot tube with a static port and an angle-of-attack vane.

A pitot (pronounced /ˈpiːtoʊ/) 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 1700s^[1] and was modified to its modern form in the mid 1800s by French scientist Henry Darcy^[2]. It is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications.

Contents

1 Theory of Operation
2 Industry applications
3 See also
4 References
5 External links

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 of 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

$p_t = p_s + \left(\frac{\rho V^2}{2}\right)$

Solving that for velocity we get:

$V = \sqrt{\frac{2 (p_t - p_s)}{\rho}}$

Where $V$ is fluid velocity

and $p t$ is stagnation or total pressure

and $p s$ is static pressure

and $ρ$ is fluid density

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 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 Orient Airlines Flight 6231, AeroPeru Flight 603 (blocked static port), and of one X-31.^[3]

Industry applications

Pitot tube from a F/A-18

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 (alnor) for determining the velocity inside the ducted wind tunnel. One use of this technique is to determine the amount of cooling that is happening to a room.

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, air speed is typically measured in knots.

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (June 2008)

^ Pitot, Henri (1732). "Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des vaisseaux" (PDF). Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: pp. 363–376. http://gallica.bnf.fr/ark:/12148/bpt6k35294.image.f543.langFR. Retrieved 2009-06-19.
^ Darcy, Henry (1858). "Note relative à quelques modifications à introduire dans le tube de Pitot" (PDF). Annales des Ponts et Chaussées: pp. 351–359. http://gallica.bnf.fr/ark:/12148/bpt6k408489d.image.f354. Retrieved 2009-07-31.
^ Ice In or On Static System Cause of X-31 Crash

Kermode, A.C. (1996) [1972]. Mechanics of Flight. Barnard, R.H. (Ed.) and Philpott, D.R. (Ed.) (10th edition ed.). Prentice Hall. pp. 63–67. ISBN 0-582-23740-8.
Pratt, Jeremy M. (2005) [1997]. The Private Pilot's Licence Course: Principles of Flight, Aircraft General Knowledge, Flight Performance and Planning (3rd edition ed.). gen108-gen111. ISBN 1-874783-23-3.
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.

External links

Wikimedia Commons has media related to: Pitot tube

v • d • e Flight instruments

Pitot-static instruments	Altimeter · Airspeed indicator · Machmeter · Variometer

Gyroscopic instruments	Attitude indicator · Heading indicator · Horizontal situation indicator · Turn and bank indicator · Turn coordinator · Turn indicator

Navigation	Horizontal situation indicator · Course Deviation Indicator · Inertial navigation system · GPS · Magnetic compass · SIGI · Satellite navigation systems

Other	Air data inertial reference unit · EFIS · Glass cockpit · V speeds · Yaw string

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