If a hole is made at the wall and is
connected to any pressure measuring device, it will then
sense the static pressure at the wall. This type of hole
at the wall is known as a wall tap.
The fact that a wall tap actually senses
the static pressure can be appreciated by noticing that
there is no component of velocity along the axis of the
hole.
In most circumstances, for example, in
case of parallel flows, the static pressure at a cross-section
remains the same. The wall tap under this situation registers
the static pressure at that cross-section.
In practice, instead of a single wall
tap, a number of taps along the periphery of the wall are
made and are mutually connected by flexible tubes (Fig.
16.1b) in order to register the static pressure more accurately.