Clamp-on measurements of fluid flow in small diameter metal pipes using ultrasonic guided waves

Clamp-on ultrasonic transit time difference is used extensively to calculate the volumetric flow rate of a fluid through a pipe. The operating principle is that waves travelling along a path that is generally against the flow direction take longer to travel the same path than waves travelling along the same path in the opposite direction. The transit time difference between the waves travelling in opposite directions can be used to calculate the flow rate through the pipe, by applying suitable mathematical correction factors. The approach is non-disruptive and non-invasive and can be retrospectively fitted to pipes and easily relocated to different positions. When ultrasonic clamp-on transducers are attached to pipes with diameters of less than 30mm and a wall thickness of less than a few millimetres, the resulting guided waves can appear confusing, and produce very different signals to those observed on larger diameter pipes. The experimentally observed behaviour of these guided waves in fluid-filled, small diameter pipes is analysed, modelled and explained. Experiments are performed in copper pipes of sizes that are commonly used in buildings, and accurate measurements of water flow rates are taken down to a few millilitres per second . This technique presents new possibilities for smart metering of water supplies, where the positioning of the small clamp-on transducers is not sensitive to variations in water temperature, and low power electronics can be used.