# A Novel Particulate Matter 2.5 Sensor Based on Surface Acoustic Wave Technology
## 5 Results and Discussions  
In the experiment, the monodispersed PSL particles with the diameter of 2 \(\upmu\)m were adopted. Meanwhile, a light scattering aerosol monitor was parallel connected to the particle chamber to provide a reference. Although the light scattering method was proven inaccurate in practical application [12], in terms of the particles of known material and size, the light scattering method was accurate after calibration. In the experiments, an atomizer was used to disperse PSL particles and then mixed up uniformly by fans in the chamber. Figure 18 shows the frequency output of the SAW based PM\({}_{2.5}\)  
Figure 16: Experimental setup for evaluating the characteristics of the SAW based PM\({}_{2.5}\) sensor.  
Figure 17: Photograph of the experimental setup.  
sensor for PSL particle tests. The horizontal line areas are the intervals between sampling and the oblique line areas are the three sampling processes.  
To handle the results, the slope of the oblique line is used to represent the SAW sensor response. Figure 19 gives the performance comparison between the light scattering monitor and the SAW based PM\({}_{2.5}\) sensor for measuring PSL particles. The horizontal axis presents the sampling time. The left vertical axis presents the particle mass concentration (unit: \(\mathrm{\mu g/m^{3}}\)) measured by the light scattering monitor, and the right vertical axis presents the response of the SAW based PM\({}_{2.5}\) sensor (unit: Hz/min). Furthermore, the curve of the SAW sensor responses changing with the particle mass concentration is shown in Figure 20. The slope of the linear fitting equation is 93.96 (Hz/min)/(\(\mathrm{\mu g/m^{3}}\)). The results show that by detecting the PSL particles with a certain diameter of 2 \(\mathrm{\mu m}\), the response of the SAW based PM\({}_{2.5}\) sensor is linear, and in accordance with the response of the light scattering based PM\({}_{2.5}\) monitor. To get a better view of the sensor response, an optimal data processing approach needs to be developed.  
Figure 19: Sensor response comparison between the light scattering monitor and the SAW based PM\({}_{2.5}\) sensor for PSL particles sampling.  
Figure 20: SAW based PM\({}_{2.5}\) sensor response changes with the particle mass concentration.  
When more particles accumulate on the surface of SAW devices, the sensitivity of the SAW sensor will be worse. Therefore, to disperse the particles on the sensor, an \"inverse temperature gradient\" was established. In the experiment, to realize a stable temperature gradient, two Peltier elements were utilized. One was placed on the upper shell, and the other was pasted on the back of the lower shell. In the cleaning procedure, the upper Peltier element was set to low temperature, and the lower Peltier element was set to high temperature. The inverse temperature gradient was used to expel particles from the surface of the SAW device. Table 1 shows the cleaning effect of a SAW sensing device during two testing rounds, where IL represents the insertion loss and \(f_{0}\) represents the center frequency. The frequency responses were measured by a network analyzer. The second column shows the device performance after repeated sampling and the third column shows the device performance after cleaning. Obviously, this method was successfully applied in our experiments.