Abstract: A unidirectional fluidflow sensor system including: a sensing chamber for placement in a fluidflow to be monitored; the chamber including input port means facing toward the fluidflow and smaller, output port means spaced from the input port means and disposed transversely to the fluidflow; a sensing element in the chamber in the path between the first and second port means; and means, responsive to the element, for detecting a change in flow between the input and output port means.