A novel sensor for measuring the acoustic pressure in buried plastic water pipes

Acoustic techniques are widely used to locate leaks in buried water pipes. However, difficulties are often encountered when attempting to detect a leak in a plastic pipe, as the leak noise signals attenuate very rapidly away from the leak. Identifying suitable sensors which can be easily deployed and are sufficiently sensitive has been problematic. Polyvinylidene fluoride (PVDF) wire ring sensors have been proposed and demonstrated successfully in laboratory conditions previously. Here it is proposed that the ring sensor is used in a modified configuration: a flexible hose instrumented with the ring sensor is connected to the pipe, via a fire hydrant or other standard access point. Some theoretical modelling has been carried out, which predicts that the acoustic pressure in the main pipe transmits well into the sidebranch, whilst the pressure in the main pipe is largely unaffected. This suggests that PVDF wire located on the sidebranch will effectively monitor the pressure in the main pipe. Moreover, if the sidebranch is sufficiently flexible, substantial sensitivity gains can be made using this configuration compared with locating the wire on the main pipe. Measurements made in the laboratory on a medium density polyethylene (MDPE) finite pipe with a polythene sidebranch connected to it confirm that the acoustic pressure in the main pipe can indeed be measured on the sidebranch. The expected sensitivity gains were not fully realized, and a number of different reasons for this are proposed.