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.

外夹式超声波时差法（Clamp-on ultrasonic transit time difference）被广泛应用于计算管道内流体的体积流量。其工作原理为：沿与流体流向相反路径传播的超声波，相较于沿同一路径顺流传播的超声波，所需的传播时长更长。通过引入合适的数学修正因子，可利用双向传播超声波的传播时差计算管道内的流体流量。该检测方法无接触、无创，可事后加装于现有管道，且可轻松搬迁至不同安装位置。当将外夹式超声波换能器（ultrasonic clamp-on transducers）安装于直径小于30mm、壁厚不足数毫米的管道时，所产生的导波（guided waves）信号往往难以辨识，且与大直径管道中观测到的信号差异显著。本研究对充液小直径管道内此类导波的实验观测特性展开了分析、建模与阐释。实验采用建筑领域常用规格的铜管开展，可精准测量低至数毫升每秒的水流速率。该技术为供水智能计量提供了全新的应用可能：小型外夹式换能器的安装位置不受水温变化影响，且可采用低功耗电子器件。