Abstract: A drogue capable of being inserted into a water pipe and comprising a flexible bar in which are a pressure sensor, a battery pack, an electronics assembly and a magnetometer. The bar is surrounded by a bladder with an inlet valve at the front and an outlet valve at the rear. The drogue is attached to a suitable cable so that electrical signals can be conveyed to a computer to enable leaks within the water pipe to be identified and the position of the drogue identified. Also presented is a corresponding method which derives signals relating to volumetric flow for a series of locations in a conduit, in order to detect leakage and determine locations of such leakage, by measuring flow effects around a (moveable) large body and also measuring flow effects around a (moveable) small body at each location (the two bodies capable of being moved together thru the conduit at a set distance apart).