To overcome the limitations on the maximum measured angle, seven – holes probes with a spherical or conical head are used (Figure 2.31). The angular resolution reaches 70° – r – 80°. The typical diameter of the measuring head is about 3 mm.
The process of calibration of such complex probes, as well as five-holes probes, consists in putting the probe in a known and uniform stream (known in terms of speed, direction of velocity, density, temperature, pressure) and in rotating the probe in a few thousand (e. g. 2000) different angles with respect to the stream and storing the readings of the various pressure taps. A map of calibration is obtained to find magnitude and direction of velocity on the basis of the pressures detected by the seven taps.