:: INTRO2U ::

:: ENGINEERING FUNDAMENTALS::

INSTRUMENTATION

MECHANICAL

SIVIL

ELECTRICAL

Introduction : Consists of 2 hollow tubes that sense 2 pressure simultaneously, impact and static The impact unit consists of a tube with one end bent at right angles toward the flow direction (facing upstream) The static tube's end is closed, but a small slot is located in the side of the unit (facing downstream) The dP is proportional to the square of the velocity
To install - determine the location of maximum velocity with pipe traverses.

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 which is the dynamic pressure q.

measurement = q = 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

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

where sqrt denotes the square root function. There are, however, some practical limitations to this equation:

1. 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.
2. 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.

Advantages :  Low cost for installation and operation (essentially no pressure drop) Standard differential readout device for all sizes they create practically no pressure drop inexpensive can be removed for maintenance without stopping line flow.

Disadvantages :  Single point measurement - Point velocity measured is assumed to represent full pipeline average which limits accuracy unless flow profile is closely controlled. For normal pipeline velocities, the indicated differential is very low which make its accuracy poor and its range ability very limited Susceptible to plugging by foreign material.