CHAPTER 14 - MEASUREMENTS IN PRESSURE CONDUITS

10. Pitot Tube Velocity Measurements

The straight upstream tube shown on figure 14-9a, which is connected perpendicular and flush to the inside wall of the pipe so it does not sense any velocity force, is a called a piezometer. Water rises in the piezometer to an elevation that only balances the pressure head in the conduit. A simple pitot tube is shown downstream on figure 14-9a. This open tube has a right-angle bend that is inserted into conduit flow with its horizontal leg pointed upstream and parallel to velocity. Water runs into the tube and rises into the vertical stem until its weight balances both the force of the pipeline pressure head, hp, sometimes called static head, and the force of approach velocity that has been converted to velocity head, hv, by stagnation at the tip. In this form, the pitot tube is sometimes called a total head tube because the water rises above the tip a height equal to the sum, Ht, of pressure head in the conduit plus velocity head.

 Figure 14-9 -- Pitot tubes and manometer.

For a velocity measurement, the pressure head is subtracted from the total head, Ht, resulting in velocity head, hv, or V 2/2g. Solving for the velocity of flow, V, results in:

where:

V = velocity
g = gravity constant