Flow transducer

A flow transducer to measure the rate of flow of a fluid in a conduit, said transducer including a body to be inserted in said conduit and having a sleeve serving as a fixed barrier to flow in the conduit, a piston slidable within said sleeve and having first and second fluid reaction surfaces, one of which is subjected to fluid pressure acting in one direction on one side of said sleeve and the other of which is subject to fluid pressure acting in the opposite direction on the opposite side of said sleeve, a first and second set of apertures formed in said sleeve and spaced apart in the direction of movement of the piston, a port in said piston and movable to overlie one or other of said sets of apertures and thereby define an orifice for fluid flow from one side of said sleeve to the other, biasing means acting between said piston and said body to bias said piston to a predetermined position in which said port is axially located between said apertures and flow past said sleeve is prevented, and a signal generating means associated with said piston to provide a signal indicative of the position thereof relative to said predetermined position, said biasing means comprising a coil spring located within a cavity in said body, said cavity including a pair of axially spaced abutment surfaces co-operating with said coil spring to inhibit axial movement of said spring beyond said abutment surfaces, said piston extending through said spring and being engageable with opposite ends thereof whereby axial movement of said piston in either direction from said predetermined position causes one end of said spring to engage an adjacent abutment surface and the opposite end of said spring to be engaged by said piston to compress the spring and causing said signalling means to provide a signal indicative of the rate of flow of fluid through said conduit.