An apparatus for use in measuring fluid flow across a boundary is disclosed including a conduit, supply means for control fluid and two fluid flow sensors. The conduit is positioned to extend across the boundary so that its open ends are located on opposite sides of the boundary. The supply means introduces control fluid at an intermediate point so that control fluid flows through the conduit in opposite directions and discharges through both open ends. The fluid flow sensors are mounted so that each may sense variations in control fluid flow exiting at the ends of the conduit. These sensors generate output signals responsive to fluid flow across the boundary.