PITOT TUBES

GENERAL

It is the most common forms of flow rate measuring devices for the measurement of fluid 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.

OPERATING PRINCIPLE

When a solid body is kept centrally and stationary in a pipe line , the velocity of the fluid starts decreasing because of the presence of body till it is reduced to zero.

It is called stagnation point.

As the pressure is lost by the liquid, it gains a static head.

By measuring the difference between the pressure at normal flow line and at stagnation point, the fluid velocity can be found.

CONSTRUCTION

It consists of two co-axial cables.

The open end of the inner tube faces the incoming fluid.

The outer tube has a closed end but it contains a few holes on its walls.

There is a manometer in the inclined position to measure the pressure directly.

The impact opening of the pitot tube is 3.125 to 6.35 mm.

WORKING

With the arrangement shown in the construction, a situation is created where the flow velocity of the fluid is zero in one of the tubes that are interconnected through a manometer.

The outer tube senses the static pressure of the fluid as well as its velocity head.

The inner tube senses only the impact pressure.

The difference between the static pressure and the impact pressure is the flow rate.

+ =

static pressure(N/m2) Velocity(m/s)Impact pressure(N/m2)

Density of liquid

v=

Hence, velocity of the fluid can be found

The velocity of a fluid across a section is not uniform, but varies from zero at the pipe surface to the maximum at the centre of the pipe.

To find the mean velocity, the readings are taken fro, different positions along the cross section of the pipe by placing the pitot tube.

The accuracy of the pitot tube is .

V (mean) = Cv

Cv is the ratio of the mean velocity to the velocity at the centre.

Velocity correction factor

Advantages:

negligible loss of head

cheaper than orifice and venturi meters

Disadvantages:

low sensitivity

the small opening may get cogged if the fluid under measurement contains solid particles, which would give wrong results.

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