## 4 Results and Discussion  
The proposed leak detection method is based on the relative pressure change profile in the pipe; therefore it is unnecessary to calibrate the sensors. Raw data from the sensors in laboratory tests are illustrated in Figure 10. As can be seen from this figure four main phases of the experiment (pump start, stabilization, leak, pump off) are clearly visible from the output of the pressure sensors. These include the pressure before the pump was switched on, the increase in pressure when the pump was switched on, the drop in pressure due to the development of the leak and finally the drop in pressure due to the switching off of the pump (shortly after development of the leak). Figure 11: Pressure drop measured by the five FSR sensors due to the occurrence of the leak. Figure 10: Raw sensor output of laboratory experiment illustrating the pressure profiles for the five FSR sensors. It can be clearly seen from Figure 10 that the simulated burst can be detected from the sensors' raw output as a sudden pressure drop in the system. However, based on Figure 10 it is difficult to determine the exact position of the leak as all FSR sensors appear to respond similarly to the leak. Figure 11 shows a close-up of the pressure drop due to the burst. It can be seen from this figure that the pressure profiles of Sensors 4 and 5, which are downstream to the leak point, are different to the ones upstream of the leak. The profile for theses sensors exhibit a more gradual pressure drop profile than the ones before the leak. This difference can be used to determine the approximate location of the leak, _i.e._, it is somewhere between Sensors 3 and 4 in this case. In addition to the detection of a burst in the laboratory test, the FSR based pressure sensors successfully registered daily pressure variations in the pipes in the field trials. Figure 12 illustrates relative pressure data and temperature readings from a sensor node for a period of three days over which leak tests were carried out. During these tests various service valves were opened and closed as part of a valve training programme at the test site. Data presented in this figure are from a node attached to a 90 mm diameter MDPE pipe. As can be seen from Figure 12, the proposed non-intrusive pressure sensor clearly registered the daily pressure variations in the pipe. Valve trainings were carried out on three consecutive days and clearly show themselves by an abnormal pressure drop, further than normal daily relative pressure variations (0.4-0.95). In addition, it was noticed that the pipe wall temperature (blue line) dropped significantly at the same time and behaved differently to the temperature measurements in the soil. In contrast, on the 21 st July 2013 a pressure drop was recorded, with no corresponding temperature drop of the pipe wall. On this occasion, the pressure drop was not due to a leakage test, but simply a  
Figure 12: Sensor reading from one of the nodes attached to a 90 mm MDPE pipe for a period of five days. pressure change in the overall system (within daily pressure variation). The drop in temperature of the pipe wall for a leak is caused by a reduced residence time of the water in the pipe. Although the temperature sensors were never intended to be used for leak detection, Figure 12 shows the potential of using the temperature data in combination with the FSR data to differentiate leaks from other pressure changes in the system, especially when analyzing individual node locations and not taking the whole sensor network into account as shown in the laboratory tests. However, further tests are necessary to completely verify this hypothesis. Furthermore, the temperature measurements can also provide the operator with useful information to protect their assets in the winter.