Abstract: An apparatus for detecting fluid flow by obtaining a differential pressure across an obstruction in a fluid conveying channel. The obstruction is hingably connected to the housing of the channel. The connection is of a width substantially less than the periphery of the channel so as to reduce the collection of contaminants which decrease the accuracy of the apparatus. Because the orifice defined by the area between the obstruction and the periphery of the channel is variable according to the fluid flow, the pressure differential versus flowrate relationship is different from the square-law relationship of a fixed orifice, and provides a relatively high pressure differential at low flowrates and a relatively low pressure differential at high flowrates.