## 2 Materials and Methods
### Measurement Protocol  
Before each measurement session, the sensors were flushed with clean air for 10 min. The specified time for the initial cleaning of the sensor array using dry air before each measurement session was based on calibration studies of the device. A period of 10 min was sufficient to stabilize the resistances of individual sensors. However, the shorter flushing time (2 min) before each measurement was due to the relatively low level of VOC air contamination. The initial flushing was necessary because during the periods when the meter was not in use between measurement sessions, ambient air could enter the sensor chamber. After each measurement session, the process of flushing the sensor array was also performed with clean air flow of 200 cm\({}^{3}\)/min. The first resistance measurement of  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Sensor Type** & **Purpose** &
\begin{tabular}{c} **Measurement** \\ **Range [ppm]** \\ \end{tabular} \\ \hline
1 & TGS2600- & Air pollution sensor & 1 \(\div\) 30 (for H\({}_{2}\)) \\
2 & TGS2602- & General air pollution sensor & 1 \(\div\) 30 (for ethanol) \\
3 & TGS2610- & Propane sensor & 500 \(\div\) 10,000 \\
4 & TGS2610- & Propane and butane sensor with carbon filter & 500 \(\div\) 10,000 \\
5 & TGS2611- & Methane and natural gas sensor & 500 \(\div\) 10,000 \\
6 & TGS2611- & Methane sensor with carbon filter & 500 \(\div\) 10,000 \\
7 & TGS2612- & Methane, propane and isobutane sensor & 1 \(\div\) 25\% (explosive) \\
8 & TGS2620- & Ethyl alcohol and solvent vapor sensor & 50 \(\div\) 5000 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the gas sensors used in the 8 \(\times\) MOS array.  
Figure 1: View of the sensor array arranged in a circular manner at an equal distance from the gas sample inlet. A DS18B20 temperature sensor and a HIF-4000 humidity sensor are located in the center (sensor numbering as shown in Table 1).  
individual sensors in the array was performed for a blank sample without added chemicals (sample bag background). Resistance measurements were then carried out for the reference gases, starting with the sample with the lowest concentration.  
Each reference gas measurement included a zero air flush phase for \(t^{0}=2\) min and a measurement phase for \(t^{samp}=5\) min. During the initial \(t^{samp}\) phase of the sample measurement, there was a rapid decrease in the resistance of the sensor, proportional to the concentration of impurities in the test sample. At the end of the \(t^{samp}\) period of exposure to a given sample, the sensor resistance values stabilized and the average resistance value of the \(R_{S}^{samp}\) period could be determined. An example of a graph of resistance changes for the sensors is shown in Figure 2.  
For each sensor, the relative resistance was determined according to the following formula:  
\[R_{w}={R_{S}}^{samp}/{R_{S}}^{0}, \tag{11}\]  
where \(R_{S}^{samp}\) is the stable value of sensor resistance for the sample under test and \(R_{S}^{0}\) is the baseline (sensor resistance for zero air). The baseline \(R_{S}^{0}\) was taken as the average value of the resistance of a given sensor from the final phase of flushing the sensors with zero air before the measurements.  
The phenomenon of drift was investigated during the initial flushing of the sensor array. Measurements were performed over a short period of time, and during this period, no drift in the sensor responses caused by aging of the sensor's detection elements was observed. The relatively low air pollution in the tested rooms also did not lead to excessive poisoning of the sensors.