The cobra probe

The previous probes have significant distances between the holes and therefore are not suitable for measurements in boundary layers. The cobra probe eliminates this problem because it consists of two tubes mounted side by side with entrances inclined to ±45° or ±60° (Figure 2.27). To reduce the response time, tapered tubes can be used so that the connecting tubes to the manometer have a larger diameter than the probe. These characteristics make it the best probe for the null reading method.

Sensitivities of the cone and wedge probes as functions of the Mach number

Often a squared third central tube is provided for the measurement of p0, but in this case the distance between the holes increases. It should not be used to calculate the dynamic pressure and static pressure because the pressure difference between each of the side holes and the central one is only a relatively small fraction of the dynamic pressure. The probe is also used in supersonic flows and has a better sensitivity than the wedge for M > 2.