## 2 System Design
### Sensors  
The ideal sensor for pipeline monitoring should be non-invasive to the pipe, low in power consumption and easy to install. Furthermore, they should be able to gather useful information without extensive data processing or high sampling rates. One of the key parameters in pipeline monitoring is the internal pressure of the pipe. Leaks or blockages can potentially alter the normal pressure in the pipe and hence monitoring the pressure can potentially help to identify these. Temperature measurements of a pipe and its surroundings can also provide useful data in pipeline monitoring. A slow leak might not have a major effect on the internal pressure of the pipe, but it can potentially change its surrounding temperature profile. Common pressure sensors used to measure the pressure inside water pipes require pressure tappings or special valves that allow access to the sensor to the inside of the pipe. This can have a negative effect on the structure of the pipe and is not suitable for certain types of pipe (e.g., old PVC pipes, older more brittle cast iron, glass reinforced or asbestos cement pipes). A novel, relative pressure sensing method based on force sensitive resistors (FSR) is used for pressure measurements in the proposed UWSN for pipeline monitoring [25]. This system operates based on the principle of a changing diameter of the pipe caused by an internal pressure change. Figure 6 shows a schematic of the sensor node assembly. In the proposed sensor node assembly the FSR sensor is attached to the outside of a pipe with a clip whose Young's modulus should be greater than that of the pipe. The clip is initially tightened to fix the FSR sensor to the pipe. All pipes expand to some extent when they are pressurised. Pressure in the pipe causes a contact force between the pipe and the clip. This contact force is then measured by the FSR sensor and relative internal pressure changes can be calculated from this force. This contact force is also dependent on the pipe diameter, pipe thickness, pipe material, clip material, clip diameter and clip thickness. Equation (1) can be used to calculate the contact pressure between the pipe and clip \(P_{c}\). where \(p\)(Pa) is the internal pressure of the pipe, \(r_{p}\)(m) is the radius of the pipe, \(r_{f}\)(m) is the radius of the pipe. Figure 6: Schematic of the pressure sensor assembly FSR sensor. clip, \(E_{p}\)(Pa) is the Young's modulus of the pipe, \(E_{p}\)(Pa) is the Young's modulus of the clip, \(t_{p}\)(m) and \(t_{s}\)(m) are the thickness of the pipe and clip respectively. \[P_{c}=\frac{P\cdot p_{p}^{2}\cdot E_{j}\cdot t_{j}}{\left(r_{p}^{2}\cdot E_{j} \cdot t_{j}\right)+\left(r_{j}^{2}\cdot t_{p}\cdot E_{p}\right)} \tag{1}\]  
The contact force on the sensor \(F_{c}\)(\(N\)) can then be calculated from Equation (2), where \(A_{s}\)(m\({}^{2}\)) is the area of the sensor and \(K\) is a constant between 0 and 1 which indicates the fraction of the contact pressure which is applied to the sensor. \[F_{c}=K\cdot P_{c}\cdot A_{s} \tag{2}\]  
From Equations (1) and (2) it can be concluded that a change in pressure will result in a change in the force which is applied on the sensor. This will result in a change in resistance of the FSR. This change in resistance is then measured by the sensor nodes' analogue to digital converter through a voltage divider. The performance of the proposed pressure sensor assembly was compared to a commercial pressure sensor in order to validate its suitability and reliability in plastic pipes; which are currently the most common type of newly installed pipes in water distribution [25]. In these tests a 152 mm diameter PVC pipe was pressurised from 0 to 4 bars and the pressure was measured simultaneously using both a direct pressure sensor and the FSR based relative pressure assembly. Results from this study showed a high correlation between data from the commercial sensor and the proposed FSR based sensor (an R-squared value of 0.9905 was obtained for these tests). More details of these tests can be found in [26].