Flow measuring device having a diffuser with plural passages for directing flow toward the impeller vanes

A device for measuring the flow rate includes an impeller rotatably mounted in a conduit about an axis, and a detector indicating the angular speed of the impeller. A diffuser has a plurality of passages for dividing the flow into reduced flows which are directed toward the vanes of the impeller to cause rotation. The diffuser forms peripheral passages which extend in planes parallel to the axis and inclined in a same direction about the axis. Each passage has a longitudinal inlet portion, parallel to the axis and a transverse outlet portion laterally facing a first axial portion of the impeller such that in operation, the outlet portions of the passages direct towards the vanes of the impeller corresponding flows or jets of fluid in non-radial directions inclined in a same direction around the axis.