A fluid-metering device for a hose or pipe, comprises: a body 10 having an inlet 12 and an outlet 11； an impeller 20 drivable by a flow of fluid through the device； a barrel 40, having an inlet 42 and an outlet 43, slidably mounted in the body 10 such that the barrel 40 is slidable between an open position wherein the barrel inlet 42 and outlet 43 are aligned with the body inlet 12 and outlet 11 such that a fluid can flow through the device and drive the impeller 20, and a closed position wherein the barrel inlet 42 and outlet 43 are offset from the body inlet 12 and outlet 11 such that a fluid cannot flow through the device, the barrel 40 being biased towards the closed position by biasing member 18； and latching means 50 arranged to hold the barrel 40 in the open position, wherein a predetermined number of rotations of the impeller 20 will release the barrel 40. A depressible member, button 17, allows the device to be reset. The device comprises a dog clutch 55 and epicyclic gearing. The impeller blades extend radially inwardly.
