## 3 Calibration and Training the eNose for Ethanol Gas Detection
### Calibration of the eNose for Ethanol Detection  
The procedure used to calibrate the eNose for ethanol detection is based on two extreme measurement experiments: one with a low concentration of ethanol in the air and another with a high concentration of ethanol in the air. These two extreme concentration readings will be later used to train the classifier with the expectation that any intermediate concentration measured will also be classified as ethanol. At this point, it must be noted that the objective is not to measure the total gas concentration but to detect the presence of ethanol in the air. The measurement data gathered from the eNose during these two experiments is provided in the Supplementary Material of this article.  
The measurement experiments were conducted in a 5 \(\times\) 5 m room with a single window providing direct street access and a single door connecting to the rest of the building. Throughout the experiments, the window remained closed, while the door was left open to maintain typical room conditions. The two measurement experiments were executed in the same conditions, with the only difference being the evaporation rate of the volatile compound. The ethanol source was placed at the center of the room and the eNose was placed in one of the corners of the room.  
Figure 3 shows the evolution of the ohmic resistance of the sixteen gas sensors in the eNose sampled every 0.5 s. Figure 3a depicts the evolution of the sensing resistor of the gas sensors during the experiment with low ethanol concentration, and Figure 3b depicts the evolution of the sensing resistor of the gas sensors during the experiment with high ethanol concentration.  
During the initial 5 min of the experiments, there was no ethanol gas source, and the eNose measured air. The first 500 samples registered (highlighted in Figure 3 with blue and violet areas) are automatically labeled as Air class for the subsequent training of  
Figure 3: Resistance values for each of the MOX sensors in the eNose gathered during the two ethanol gas leak measurement experiments: (**a**) natural evaporation experiment (4.3 mg/min); (**b**) forced evaporation experiment (130 mg/min). The areas depict the data samples selected for the subsequent training of an LDA classifier.  
the LDA classifier. After 5 min, the eNose readings are stable. Then, ethanol evaporation started with a known evaporation rate: 4.3 mg/min for the low-concentration experiment and 130 mg/min for the high-concentration experiment. Figure 3 shows that, after a short delay, the resistance of all gas sensors starts decreasing. Note that in the case of the higher evaporation rate (Figure 3b), the sensors took longer to reach a stable value. However, the final resistance readings were significantly lower in the case of high concentration.  
Once the resistance values reached a stable minimum value, the recording continued for an additional 5 minutes. The last 500 samples (highlighted in Figure 3 with yellow and orange areas) are automatically labeled as Ethanol class for the subsequent training of the LDA classifier. Finally, after each experiment, the room was ventilated by opening the window for 15 min.