Pitot Tube:

The pitot tube, described in Figure, is another primary flow element used to generates a differential pressure for flow detection.  Within its simplest form, it consists of a tube along with an opening at the end. A small hole in the end is positioned such in which it faces the flowing fluid. The velocity of the fluid at the opening of the tube decreases to zero. This gives for the high pressure input to a differential pressure detector.  A pressure tap gives the low pressure input.

Figure: Pitot Tube

The pitot tube actually measures fluid velocity instead of fluid flow rate.  Therefore, volumetric flow rate can be obtained using Equation 1.

v' = KAV                                                                    (1)

where

v' = volumetric flow rate (ft3/sec.)

A = area of flow cross-section (ft2)

V = velocity of flowing fluid (ft/sec.)

K = flow coefficient (normally about 0.8)

Pitot tubes must be calibrated for every specific application, as there is no standardization. These categories of instrument could be used even while the fluid is not enclosed within a pipe or duct.