Pitot tube
Overview
 
A pitot tube is a pressure measurement
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....

 instrument used to measure fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 flow velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

. The pitot tube was invented by the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy
Henry Darcy
Henry Philibert Gaspard Darcy was a French engineer who made several important contributions to hydraulics.-Biography:...

. It is widely used to determine the airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

 of an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 and to measure 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.
The basic pitot tube consists of a tube pointing directly into the fluid flow.
Encyclopedia
A pitot tube is a pressure measurement
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....

 instrument used to measure fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 flow velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

. The pitot tube was invented by the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy
Henry Darcy
Henry Philibert Gaspard Darcy was a French engineer who made several important contributions to hydraulics.-Biography:...

. It is widely used to determine the airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

 of an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 and to measure 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
Stagnation pressure
In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...

 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


Solving that for velocity we get:


Note: The above equation applies only to incompressible fluid.

where:
  • is fluid velocity;

  • is stagnation or total pressure;

  • is static pressure;

  • and is fluid density.


The value for the pressure drop or Δp due to Δh, the reading on the manometer:
Δp = g Δh


Where:
  • is the density of the fluid in the manometer

  • Δh is the manometer reading


The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure
Static pressure
In fluid mechanics the term static pressure has several uses:* In the design and operation of aircraft, static pressure is the air pressure in the aircraft’s static pressure system....

. 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
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....

 of the aircraft. The diaphragm arrangement is typically contained within the airspeed indicator
Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...

, 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.

Operation

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
Austral Líneas Aéreas Flight 2553
Austral Líneas Aéreas Flight 2553, better known as Austral 2553, was a domestic scheduled Posadas–Buenos Aires service operated with a McDonnell Douglas DC-9-32 that crashed on the lands of Estancia Magallanes, Nuevo Berlín, away from Fray Bentos, Uruguay, on . All 74 passengers and crew perished...

, Birgenair Flight 301
Birgenair Flight 301
Birgenair Flight 301 was a flight chartered by Turkish-managed Birgenair partner Alas Nacionales from Puerto Plata in the Dominican Republic to Frankfurt, Germany via Gander, Canada and Berlin, Germany...

 (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
Black and yellow mud dauber
Black and yellow mud dauber is a common name for the sphecid wasp species Sceliphron caementarium. They are solitary insects that build nests out of mud, in sheltered locations, frequently on man-made structure such as bridges, barns, open porches or under the eaves of houses...

 wasp), Northwest Airlines Flight 6231, AeroPeru Flight 603
AeroPeru Flight 603
Aeroperú Flight 603 was a scheduled flight from Jorge Chávez International Airport in Lima, Peru , to Comodoro Arturo Merino Benítez International Airport in Santiago, Chile , which crashed on October 2, 1996....

 (blocked static port), and of one X-31. It has also been suggested that pitot tube icing may have been a contributing factor for the crash of Air France Flight 447
Air France Flight 447
Air France Flight 447 was a scheduled airline flight from Rio de Janeiro-Galeão to Paris-Roissy involving an Airbus A330-200 aircraft that crashed into the Atlantic Ocean on 1 June 2009, killing all 216 passengers and 12 aircrew. The investigation is still ongoing, and the cause of the...

. although this claim has been contested.

Industry applications

In industry, the velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...

 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 being delivered 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, airspeed is typically measured in knots.

See also

  • Calibrated airspeed
    Calibrated airspeed
    Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...

  • Flow measurement
    Flow measurement
    Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...

  • Mach number
    Mach number
    Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

  • Piezometer
    Piezometer
    A piezometer is either a device used to measure static liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure of groundwater at a specific point...

  • Pitot-static system
    Pitot-static system
    A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments...

  • Position error
    Position error
    Position error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate...

  • True airspeed
    True airspeed
    True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...

  • Air data boom
    Air data boom
    An air data boom provides air pressure, temperature, and airflow direction data to data acquisition and air data computers for the computation of aircraft orientation, airspeed, altitude, and related information...


  • Atmospheric icing
    Atmospheric icing
    Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil...

  • Icing conditions
    Icing conditions
    In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine as carburetor icing. Inlet icing is another engine-related danger, often occurring in jet aircraft. These icing phenomena do not...

  • PIREP
  • AIRMET
    AIRMET
    An AIRMET, or Airmen's Meteorological Information, is a concise description of weather phenomena that are occurring or may occur along an air route that may affect aircraft safety...

  • SIGMET
    SIGMET
    SIGMET, or Significant Meteorological Information, is a weather advisory that contains meteorological information concerning the safety of all aircraft. There are two types of SIGMETs, convective and non-convective...

  • Anti-icing
  • Deicing
    Deicing
    For snow and ice control on roadways and similar facilities, see Snow removalDe-icing is defined as removal of snow, ice or frost from a surface...

  • Annubar
    Annubar
    An annubar is similar to a pitot tube used to measure the flow of gas or liquid in a pipe.The pitot tube measures the difference between the static pressure and the flowing pressure of the media in the pipe...



External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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