Chapter 5 : Applications of Equations of Motion and Mechanical Energy
Lecture 16 :


Pitot Tube for Flow Measurement

Construction: The principle of flow measurement by Pitot tube was adopted first by a French Scientist Henri Pitot in 1732 for measuring velocities in the river. A right angled glass tube, large enough for capillary effects to be negligible, is used for the purpose. One end of the tube faces the flow while the other end is open to the atmosphere as shown in Fig. 16.3a.

Working:

  • The liquid flows up the tube and when equilibrium is attained, the liquid reaches a height above the free surface of the water stream.

  • Since the static pressure, under this situation, is equal to the hydrostatic pressure due to its depth below the free surface, the difference in level between the liquid in the glass tube and the free surface becomes the measure of dynamic pressure. Therefore, we can write, neglecting friction,

where p0, p and V are the stagnation pressure, static pressure and velocity respectively at point A (Fig. 16.3a).

  • Such a tube is known as a Pitot tube and provides one of the most accurate means of measuring the fluid velocity.

Fig 16.3  Simple Pitot Tube  (a) tube for measuring the Stagnation Pressure
                                     (b) Static and Stagnation tubes together

  • For an open stream of liquid with a free surface, this single tube is suffcient to determine the velocity. But for a fluid flowing through a closed duct, the Pitot tube measures only the stagnation pressure and so the static pressure must be measured separately.

  • Measurement of static pressure in this case is made at the boundary of the wall (Fig. 16.3b). The axis of the tube measuring the static pressure must be perpendicular to the boundary and free from burrs, so that the boundary is smooth and hence the streamlines adjacent to it are not curved. This is done to sense the static pressure only without any part of the dynamic pressure.

  • A Pitot tube is also inserted as shown (Fig. 16.3b) to sense the stagnation pressure. The ends of the Pitot tube, measuring the stagnation pressure, and the piezometric tube, measuring the static pressure, may be connected to a suitable differential manometer for the determination of flow velocity and hence the flow rate.

Pitot Static Tube

  • The tubes recording static pressure and the stagnation pressure (Fig. 16.3b) are usually combined into one instrument known as Pitot static tube (Fig. 16.4).

Fig 16.4  Pitot Static Tube

  • The tube for sensing the static pressure is known as static tube which surrounds the pitot tube that measures the stagnation pressure.

  • Two or more holes are drilled radially through the outer wall of the static tube into annular space. The position of these static holes is important. Downstream of the nose N, the flow is accelerated somewhat with consequent reduction in static pressure. But in front of the supporting stem, there is a reduction in velocity and increase in pressure.

  • The static holes should therefore be at the position where the two opposing effects are counterbalanced and the reading corresponds to the undisturbed static pressure. Finally the flow velocity is given by
(16.9)

 

where ∆p is the difference between stagnation and static pressures.

  • The factor C takes care of the non-idealities, due to friction, in converting the dynamic head into pressure head and depends, to a large extent, on the geometry of the pitot tube. The value of C is usually determined from calibration test of the pitot tube.