This page shows a schematic drawing of a pitot tube. **Pitot
tubes** are used on aircraft as speedometers. The actual tube on
the aircraft is around 10 inches (25 centimeters) long with a 1/2
inch (1 centimeter) diameter. Several small holes are drilled around
the outside of the tube and a center hole is drilled down the
axis of the tube. The outside holes are connected to one side
of a device called a **pressure transducer**. The center hole in the tube
is kept separate from the outside holes and is connected to
the other side of the transducer. The transducer measures
the difference in pressure in the two groups of tubes
by measuring the strain in a thin element using
an electronic strain gauge. The pitot tube is mounted on the aircraft
so that the center tube is always pointed in the direction of
travel and the outside holes are perpendicular to the center
tube. (On some airplanes the pitot tube is put on a longer boom
sticking out of the nose of the plane or the wing.)

__Difference in Static and Total Pressure__

Since the outside holes are perpendicular to the direction
of travel, these tubes are pressurized by the local
random
component of the air velocity. The pressure in these tubes is
the **static pressure (ps)** discussed in Bernoulli's
equation. The center tube, however, is pointed in the direction of
travel and is pressurized by both the random and the ordered air
velocity. The pressure in this tube is the **total pressure (pt)**
discussed in Bernoulli's equation. The pressure transducer measures
the difference in total and static pressure.

measurement = pt - ps

__Solve for Velocity__

With the difference in pressures measured and knowing the local value of air density (r) from pressure and temperature measurements, we can use Bernoulli's equation to give us the velocity. Bernoulli's equation states that the static pressure plus one half the density times the velocity (V) squared is equal to the total pressure.

ps + .5 * r * V ^2 = pt

Solving for V:

V ^2 = [2 * {pt - ps}] / r

There are, however, some practical limitations:

- If the velocity is low, the difference in pressures is very small and hard to accurately measure with the transducer. Errors in the instrument could be greater than the measurement! So pitot tubes don't work very well for very low velocities.
- If the velocity is very high (supersonic), we've violated the assumptions of Bernoulli's equation and the measurement is wrong again. At the front of the tube, a shock wave appears that will change the total pressure. There are corrections for the shock wave that can be applied to allow us to use pitot tubes for high speed aircraft.

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*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *