Static Pressure The static pressure is measured by a piezometer opening or a pressure tap. The piezometer opening in the side of the conduit should be normal to and flush with the surface. A piezometer ring is a manifold into which are connected several sidewall static taps. Its advantages are: (1) it gives average pressure; (2) it reduces the possibility of completely plugging all the static openings. The specifications for pressure tap holes are given by Perry. ^[1a]

Local Velocities, Pitot Tubes Consider a Pitot tube of the type shown in Fig. 2-6a. Apply the Bernoulli equation to the points A and B:

Figure 2-6: ( a) Pitot tube, ( b) venturimeter, ( c) sharp-edged orifice, ( d) round-edged orifice.

In the above equation, Z _A ? Z _B, u _A = 0, W = 0, F = 0. Therefore

or

The constant C is introduced in the above equation to account for the fluctuations in the velocity which cause errors in the measurement. For the incompressible fluids, ? _A = ? _A = ?, and the above equation becomes