Flow sensing assembly and method

A flow sensing assembly for detecting the flow of a fluid, comprising

• a sensor body having a flow passage therethrough,
• a resilient blade having an end which is fixed relative to the sensor body and an end which is freely movable, said blade extending as a cantilever from its fixed end to its free end into and across the flow passage transverse with respect to the direction of flow whereby fluid flowing through the flow passage causes the free end of the blade to deflect in a downstream direction from a no-flow position to a flow position, said free end of the blade returning to its no-flow position upon stoppage of flow,
• means defining an orifice of reduced diameter in the flow passage, said means comprising a tubular insert coaxially mounted inside said flow passage and separate from the sensor body whereby the insert can be machined separate from the sensor body before being mounted in the flow passage, said means having an inlet end for entry of fluid into the orifice and an outlet end for exit of fluid from the orifice, said blade being engageable with said outlet end when the blade is in its said no-flow position, and
• a detecting device for detecting deflection of the blade due to flow of fluid through the flow passage and for producing a signal indicating the occurrence of said fluid flow,
• wherein said blade is dimensioned to have a width at least as great as the diameter of said orifice whereby the blade completely covers the orifice when the blade is in its said no-flow position, and said outlet end has a flat smooth seat substantially sealingly engageable by the blade when in its said no-flow position whereby even a small quantity of fluid flowing past the seat is adapted to deflect the blade to its flow position.