In the day-to-day routine, the sensor nodes (marked with the letter ‘S’ in Fig. 6) periodically measure the methane concentration in the environment, alternating with the ultra low-power sleep mode: measurement mode–sleep mode–measurement mode. In the measurement mode, all components of the sensor node are functioning except for the ZigBee transceiver. Gas measurement occurs as follows: in the measurement mode the voltage on the sensor sensitive layer, which depends on the gas concentration,is measured using the built-in ADC of the microcontroller. The obtained value is compared with two preset thresholds specified for the sensor node. For methane, the lower specified threshold is
set to 0.15–0.5% vol.; if the concentration rises above this threshold,the WGSN will send an alert about the presence of methane in the
environment. The second limit is set to 0.5–1%, vol. In fact, the lower
limit of methane explosion is 5% vol. [9]. If the second threshold is
crossed, an appropriate alarm is transmitted over the data channel.
These specified limits could be changed by reprogramming.