The plate orifice (Figure 2.36) is used in cases where the high pressure drop introduced by the device can be tolerated. The duct is obstructed by a plate with a circular hole, the static pressure is measured at the tube wall immediately upstream and immediately downstream of the orifice.
Since the stream separates from the walls of the duct both upstream and downstream of the orifice, the outflow is strongly swirling and cannot be considered isentropic. Furthermore the stream continues to accelerate downstream of the orifice because of the formation of a vena contracta and thus the static pressure is lower than that corresponding to the assumed uniform flow through the orifice. Of course, due to the strong deviation from the assumption of isentropic motion assumed in the formulation of Equation (2.8), the discharge coefficient, C, is not very close to 1: it varies from 0.6 to 0.8 for area ratios, A2/Ab ranging between 0.2 and 0.75 and for Reynolds numbers, based on the diameter of the orifice and the speed in the orifice, in excess of 5 x 104.
The device is inexpensive, it can easily be adapted to different ranges by simply changing the perforated plate, and it is therefore suitable for laboratory activities.