## 3 Results
Figure 8: Breathing rate sensor worn on the user’s chest. Figure 9: Serial plotter showing the raw data (blue trace), filtered data (red trace), and peak detector output (green trace): wide (**a**) and detailed (**b**) views, highlighting the inspiration and expiration phases. Reference volume vs. time trend acquired by a spirometer (**c**). Furthermore, a detailed view of the acquired waveforms is shown in Figure 9b, where the characteristic time instants of the respiratory signal are highlighted, with the corresponding time duration. Through these data, the developed algorithm calculates the instantaneous TE and TI, and then the corresponding average values, the IER, and the flow rate in a time window of 30 s.  
In addition, the time trend of air volume inhaled and exhaled was acquired by a portable spirometer (model SP10, manufactured by Contec Medical Systems Co., Ltd., Qinhuangdao, China) and compared with the signal collected simultaneously and provided by the chest band (Figure 9c). As is evident, the two signals follow the same trend, showing peaks and troughs in the same instants, thus indicating the correct operation of the developed system in detecting the breathing movements. Next, the developed piezoresistive chest band was characterized in different operative conditions, namely, when the user was sitting, standing, and walking. Twenty tests were carried out for each operating condition, in which the RR measurements provided by the piezoresistive band were compared with those obtained from the counting of breaths over a 30 s observation interval (T\({}_{\text{W}}\)). To carry out the measurements, a mobile application (Clickr, developed by RDQ-Smart Productivity Tools, Czech Republic) was used to track the inspirations, and assigned a timestamp to every counted event and ensured that they belonged to the considered 30 s time window. Moreover, the beginning of the count and the time window considered by the chest band were synchronized. The mean breathing frequency over the 30 s time window was calculated by multiplying the number of counted events (Equation (3)) by two. Given the reduced frequency characterizing the breathing, the proposed methodology can be considered to be reliable enough for a comparison with the measurements acquired by the chest band. In Figure 10, the correlation diagrams of the measurements provided by the chest band and those obtained by breath counting are reported for one of the six patients, specifically, an adult male (age 30 years, weight 81 kg), as an example. A summary table is also presented later with the performance of the chest band for the six patients. The tests were repeated in different physiological states (rest and under stress) to cover a larger RR range; in the graphs, the multiplicity of each data point is reported. Figure 10: _Cont._  
Pearson correlation coefficients were calculated between the RR measurements obtained with the developed chest band and those measured by counting breaths (Equation (11)):  
\[\mathrm{r}=\frac{\mathrm{n}(\sum\mathrm{RR}_{\mathrm{Band}}\mathrm{RR}_{\mathrm{ Man}})\ -\ (\sum\mathrm{RR}_{\mathrm{Band}})(\sum\mathrm{RR}_{\mathrm{Man}})}{\sqrt{\left[ \mathrm{n}\sum\mathrm{RR}^{2}_{\mathrm{Band}}-(\sum\mathrm{RR}_{\mathrm{Band} })^{2}\right]\left[\mathrm{n}\sum\mathrm{RR}^{2}_{\mathrm{Man}}-(\sum\mathrm{ RR}_{\mathrm{Man}})^{2}\right]}} \tag{11}\]  
Furthermore, the Bland-Altman graphs of the measurements obtained by the piezoresistive chest band and those obtained by counting breaths are depicted in Figure 11 for different operative conditions (i.e., seated, standing, and walking). This graph allows evaluation of the agreement between two quantitative measures using the mean difference to determine the agreement limit [55]. Figure 11: _Cont._  
Figure 10: Correlation diagrams of RR measurements obtained by the developed piezoresistive chest band and counting the number of breaths, considering three different operating conditions: seated user (**a**), standing user (**b**), and user walking at 1 ms\({}^{-1}\) (**c**).