In applications where can vary substantially from one location to another, and must be measured close to each other. This is accomplished using a pitot static tube (Figure 2.3). A probe with a spherical head in which five holes are present at right angles to it is used in the measurement of three dimensional velocity field (Figure 2.4). This is sometimes called a five-hole probe and can be used as follows: The probe is rotated along two independent axes till pressure becomes equal to and equal to , i.e., the manometer shows a null reading for the pairs of holes and . Under these conditions is the stagnation pressure of the flow and the probe axis is parallel to the flow direction. In applications where the probe cannot be rotated, it is common to calibrate and as functions of the magnitude of , local flow velocity and two angles. Calibration charts for prescribed probe dimensions and hole locations are available and need not be repeatedly generated. A simpler route is to employ the potential flow solution for flow over the surface of the sphere, namely

where is angular position of the point under consideration from the flow axis and is the stagnation pressure.