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 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
the pitot pressure. The measured stagnation pressure cannot itself
be used to determine the fluid velocity. 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.
where: is fluid velocity to be measured in m/s;
is stagnation or total pressure in pascals; is static pressure in pascals;
and is fluid density in . 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 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 is used to measure the pressure difference – , or ,
where: is the height difference of the columns in
meters. is the density of the liquid in the manometer;
g is the acceleration of gravity in
Therefore
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, Birgenair Flight 301, Northwest Airlines Flight 6231, Aeroperú Flight 603,
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.
Industry applications
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. In weather stations with high wind speeds,
the pitot tube is modified to create a special type of anemometer called pitot tube static
anemometer. See also
References Notes
Bibliography
External links 3D animation of the Pitot Tube Differential
Pressure Flow Measuring Principle How 18th Century Technology Could Down an
Airliner