Mechanics of Fluids

it is difficult to measure the height h above the surface of a moving liquid.)
But for an enclosed stream of liquid, or for a gas, the Pitot tube indicates
simply the stagnation pressure and so the static pressure must be measured
separately.
Measurement of the static pressure may be made at the boundary of the
flow, as illustrated in Fig. 3.12a, provided that the axis of the piezometer is
perpendicular to the boundary and the connection is free from burrs, that the
boundary is smooth and that the streamlines adjacent to it are not curved.
A tube projecting into the flow (as at Fig. 3.12c) does not give a satisfactory
reading because the fluid is accelerating round the end of the tube. The Pitot
tube and that recording the static pressure may be connected to a suitable
differential manometer: piezometer tubes are shown in Fig. 3.12 only for the
sake of illustration.
The tubes recording static pressure and stagnation pressure are frequently
combined into one instrument known as a Pitot-static tube (Fig. 3.13).
The static tube surrounds the total head tube and two or more small holes
are drilled radially through the outer wall into the annular space. The position
of these static holes is important. Downstream of the nose N the flow
is accelerated somewhat with consequent reduction of static pressure; in
front of the supporting stem there is a reduction of velocity and increase
of pressure; the static holes should therefore be at the position where these
two opposing effects are counterbalanced and the reading corresponds to
the undisturbed static pressure. Standard proportions of Pitot-static tubes
have been determined that give very accurate results. If other proportions
Simple applications of Bernoulli’s equation 111
Fig. 3.11 Simple Pitot tube.
Fig. 3.12