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 noninvasive 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.

Clamp-on ultrasonic transit-time difference measurements of liquid flowrate are widely used in industry for both flow metering and heat metering applications. However, the sensors used tend to be relatively large, hindering their use on small diameter pipes, and using more material in the transducer wedge than is strictly necessary. The accuracy of the technique depends on a number of factors, and particularly on the accuracy of the compression wave speed in the liquid that is used in the calculations to obtain flowrate or heat transfer rate from the liquid in the pipe. Many flow meters either assume a value for the wave speed or obtain it using thermocouple measurements of the pipe exterior with a look-up table or simple equation. An error in the liquid ultrasonic velocity relates directly to errors in the calculated flowrate. It is highly beneficial if the ultrasonic wave speed in the liquid can be accurately measured in real time for flowrate calculations, especially for temperature and pressure varying conditions. A new type of small clamp-on ultrasonic transducer is reported, using a 6mm wide PEEK wedge that contains two piezoelectric elements, one of which generates sound normal to the flow direction, yielding the measurement of ultrasonic wave speed in the liquid. The new transducers were tested on a small rig with a 15mm diameter copper pipe and a 70mm diameter stainless steel pipe, yielding accurate measurements of liquid ultrasonic velocity and flowrates.

Clamp-on ultrasonic transit time difference flow meters provide opportunities for metering where it is impractical or undesirable to cut into an existing pipeline to install an alternative flow meter. Up until now, it has been difficult to perform this type of measurement on thin-walled metal pipes, due to the difficulty of interpreting the guided wave modes in the combined pipe wall and internal fluid system, but a new method has been reported recently that utilises these guided wave modes for flow measurement. Through computational modelling, and construction and testing of different transducers, the design considerations for clamp-on transducers are highlighted and their impact on guided wave flow rate measurement is evaluated. The design features considered include a curved contact face to provide focusing of the ultrasound within the pipe and a scattering surface to reduce internal reflections. It is found that additional unwanted ultrasonic modes can be minimised by ultrasonic transducer wedge design features such as profiling the curvature of the transducer to conform to the pipe wall or creating a scattering edge to minimise internal wedge reflections. It is also observed that minimising these unwanted modes does not offer any advantage for the transit time difference measurement used in calculating flow.