## 4 System Power Consumption Analysis  
The selection of most of the components of the sensor detection nodes was based on the requirement for low power consumption. Details on the selection and power consumption of the main components involved in the sensor detection nodes are shown in Table 1.  
The monitoring system was divided into an intermittent operation mode and a continuous operation mode to reduce power consumption. The system was set to the intermittent operation mode when the methane concentration in the sewer gas environment did not exceed the alarm limit. The intermittent mode of operation was a low-power mode of operation intended to reduce the system's power consumption by reducing the active-state working time and increasing the sleep time. If the methane concentration in the sewer gas environment exceeded the alarm limit, the system was set to the continuous operation mode until the alarm state was eliminated, and was then switched to the intermittent operation mode. The intermittent mode was divided into four working states, as shown in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline
**Component** & **Device** & **Parameter** & **P\({}_{active}\)/mW** & **P\({}_{sleep}\)/mW** \\ \hline Ultrasonic sensor & 40C10T/R-2 & CH\({}_{4}\) concentration & 15 & 0 \\ CO\({}_{2}\) sensor & BSW-B200 & CO\({}_{2}\) concentration & 324 & 0 \\ Temperature and humidity sensor & SHT25 & Temperature and humidity & 0.9 & 0.0005 \\ Central processing unit & PIC16F1783 & - & 3.6 & 0.001 \\ Zigbee communication unit & CC2530 & - & 95.7 & 0.0015 \\ \hline \end{tabular}
\end{table} 
Table 1: Selection and power consumption of the main components of the sensor detection nodes. The components marked with \(\surd\) in Table 2 are in the active state; S1 indicates the sleep state, S2 indicates the working state of the ultrasonic sensor module, S3 indicates the joint working state of the methane sensor, carbon dioxide sensor, and temperature and humidity sensor, and S4 indicates the data communication state. The time duration is the continuous working time of an active component in a state, and the cycle time is the length of time that elapses until this state is restarted. Considering that the temperature and humidity of a sever vary slowly, it is not necessary to measure the temperature and humidity frequently. Since the carbon dioxide sensor consumed a large amount of power, the cycle time for S3 was set to be relatively long. We set the S3 state to be executed after every four repetitions of S1 and S2; thus, it could both compensate for temperature and humidity and reduce the system's power consumption. The relationship between time and power consumption in S3 is shown in Figure 4. Since the power consumption of the Zigbee module was high, the cycle time of S4 was set to 6 h (i.e., sending monitoring data every 6 h) if the sewer gas concentration was normal, and the Zigbee module was in the sleep state for the rest of the time. Table 3 shows the power consumption analysis of these four operating states. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Components/States** & **S1** & **S2** & **S3** & **S4** \\ \hline Ultrasonic sensor & - & \(\surd\) & \(\surd\) & - \\ CO\({}_{2}\) sensor & - & - & \(\surd\) & - \\ Temperature and & - & - & \(\surd\) & - \\ humidity sensor & - & - & \(\surd\) & - \\ Central processing unit & - & \(\surd\) & \(\surd\) & \(\surd\) \\ Zigbee communication unit & - & - & - & \(\surd\) \\ Time duration & 118 s & 2 s & 2 s & 2 s \\ Cycle time & 120 s & 120 s & 600 s (Repeat 4 times & 6 h \\  & & & for S1 to S2) & 6 h \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Four working states of the intermittent operation mode. Figure 4: Relationship between time and power consumption in the S1, S2, and S3 states. In Table 3, the total power of each state is the sum of the power consumption of the working components.