## 3 Results
### Phase 2 Longer Term Experiments and Results  
For the phase 2 field trial, a durable, intrinsically safe prototype of the acoustic instrument was developed to be left in the harsh conditions of the CSO chamber over a period of six months. This low-cost instrument was used to determine the dependence of the acoustic signature on a range of flow and screen conditions. This instrument can be linked to the existing telemetry, i.e., Hawkeye [29] technology for water level measurement. These two technologies can be combined to broadcast the recorded water level and acoustic data wirelessly. The acoustic and water level data can then be compared to determine the dependence of the coefficients \(a_{0}\) and \(b_{0}\) on the flow conditions in the CSO chamber. Photographs of the screen condition studied in phase 2 experiments were obtained from regular manual inspection, which was carried out every two weeks. Figure 10 shows  
Figure 9: Results from phase 1 field trials on sites 1 and 2.  
schematically the setup for the installed screen sensor system and existing commercial Hawkeye ultrasonic water level metering system in the CSO chamber. These experiments enabled an evaluation of the reliability of the developed acoustic instrument to understand better its capabilities of operating in CSO chambers of various configurations under realistic operating conditions. The system used in phase 2 experiments consisted of an explosion-proof enclosure, which contained a control unit, power supply, microphone array and a speaker, as shown in Figures 11 and 12. In order to minimise the risk of electrically generated sparks, the data acquisition unit, speaker, power amplifier and a 12 V, 8 Ah battery pack (nickel metal hydride, NiMh) were placed in an ATEX-certified explosion-proof box (EExd) manufactured by ICE Europe (EJBSA certification number CESIATEX004U). The speaker (Visaton 4 Ohm, model no: FRS 8, part no: 431-8686.) and the microphone array (Knowles Acoustics, model no: SPM0208 HE5) remained outside the enclosure. These were connected to the electronic module in the explosion-proof enclosure via a 10-way weather-proof screened cable. The cable from the speaker and microphone module was made into the enclosure via an ATEX-certified EExd cable gland manufactured by Remora. In order to prevent the battery cell polarity reversal or reverse charging by another cell in the same battery, the guideline in British Standard (EN 60079-1:2007) was used. Shunt diodes (model: 1N4001S) were installed and connected to the battery cells to limit the reverse polarity voltage across each battery cell. Meanwhile, to prevent the inadvertent charging of a battery by other voltage sources in the enclosure, a pair of blocking diodes was used. The power supply lines were regulated electronically to 3.3 V for the control unit and microphone array and to 12 V for the power amplifier. These units were implemented on the same circuit board. In order to reduce the risk of a surge in the current in the external cable due to a malfunction in the electronics module, Zener barriers were placed on the control unit's board, limiting the voltage going to the speaker to 12 volts (c, manufacture no: BZX79-C12) and the voltage going to the microphone array to 5.6 volts (Fairchild, manufacture no: 1N5232B, Fairchild Semiconductor International, Inc., San Jose, CA, USA). The maximum current drawn on the speaker was limited to 250 mA. In addition, both voltage regulators had short circuit protection to prevent excessive currents from reaching the microphone array and the speaker in case of malfunction of the system. If a short circuit occurred, then the circuit would be instantaneously disconnected from the power supply. Figure 10: The installation of proposed acoustic sensor system for phase 2 experiments. The speaker was hermetically sealed in an airtight plastic cylinder as shown in Figure 12. The part of the speaker diaphragm exposed to the atmosphere was free from any electrical wires and connections that could cause a shortcut when exposed to debris or water.