Abstract: A fluid flow meter that includes a flow tube having a fluid flow input and a fluid flow output, the tube also having a peripheral wall that defines a generally circular flow cross-section. A round measuring orifice is provided in the tube and between the input and the output. A contoured plug is slidably mounted in the orifice on an axial slide, the plug being normally biased toward the inlet so as to close the orifice. The plug is of a curvilinear tapering cross-section, the curvature of which is such as to produce a linear relationship between differential pressure across an annulus formed by the coincidence of the plug and the orifice and between the flow through the annulus. A fluid flow indicator serves to indicate the fluid flow through the flow tube.