<p>Proximity sensing is achieved by issuing a vortical sheet of fluid from an axial outlet passage of a vortex chamber and sensing the static pressure in a sensor tube disposed along the chamber axis. Pressure in the sensor tube is a function of object proximity to the outlet passage. Premature collapse of the vortical sheet is prevented by raising the pressure interiorly of the vortex sheet from a negative pressure. This is achieved by supplying a portion of the vortex chamber fluid to the sensor tube and hence to the interior of the vortex sheet.</p>
