## 3 Results
### Gas Sensors  
To evaluate the sensitivity of our prototype to hazardous gases, we conducted tests for both CO and NO\({}_{2}\) individually in a chemical laboratory (Center for Life Science Automation, Rostock, Germany). We co-locate a calibrator (BW alert, IL, USA) at similar position and distance to the gas sources and calibrated both devices before the test. The calibration of the prototype lasts about 10 s to read the environmental humidity, pressure, and temperature and calibrate the gas sensor. The red LED and beeper indicate the process. When the sensor sticks to zero, the alert is stopped and the device is ready for measurement. After the calibration, a container is filled with gas. The values are measured and monitored in real-time (Table 4). Regarding the calibrator values as ground truth, the accuracy of our prototype in majority of the tests is above 95% (error less than 5%).  
The NO\({}_{2}\) test was performed by a technician wearing the prototype. The set up, configuration, sampling rate, and location of the prototype are the same comply with the regulations stated in Section 2.4. We have asked the subject to fold up the sleeve to avoid covering the sensor. As the part of chemical tests at center for life science automation, the digestion acid of vessels in microwave is performed. These vessels contain high concentration of NO\({}_{2}\) which are measured by opening the lead. This is a realistic performance of chemical technicians which more often is implemented. Thus, we decided to do this experiment, testing the air pollutant sensor of the prototype exposed to hazardous gases. The reason was to carefully check the characteristics of sensor (response time, recovery time, resolution, and spin). This also can be an application of the prototype in safety monitoring of workplaces for chemical laboratories. Following the laboratory's safety conditions, seven vessels of microwave digestion are opened one after another (Figure 10). As this was a realistic experiment with hazardous air pollutants in chemical laboratory, at the day of experiment the number of sampling vessels prepared by the technician were seven. This does not indicate any specific reason.  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline Prototype & 5 & 12 & 25 & 50 & 90 & 185 & 205 & 265 & 290 \\ \hline Calibrator & 4 & 10 & 24 & 52 & 95 & 192 & 204 & 271 & 298 \\ \hline \end{tabular}
\end{table}
Table 4: CO test. Comparison of the prototype performance with a CO-calibrator in ppm.  
Figure 9: (**Left**) general device test. A typical daily routine (75 min) while recording sound level, NO\({}_{2}\), air humidity, and temperature. (**Right**) UV Test. The radiation is measured outside for 1200 s (20 min).  
In some tests with high gas concentration, the response time is instant, while the recovery time is increased. The maximum gas concentration was recorded for the vessels 1, 2, 4, and 6 with 52.8 ppm. This is the saturation boundary of the device. As the minimum, the device measures 8.9 ppm for vessel 3. Vessels 6 and 7 yield 21.7 and 37.6 ppm, respectively. These concentrations are extremely dangerous and yield alerts from the prototype. Figure 10 also shows the rising (response) and recovery times of sensor, which are defined as the time required for the sensor to react to an exposure and to reach back the zero-level after removal of the gas, respectively. The response time of the sensor did not exceed 2 s. The recovery time of the sensor node varies from 5 s to 30 s.