# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 5 Sensing Technologies
### Acoustic  
Sound propagates differently depending on the propagation medium: in gases, the ultrasonic speed is a function of the temperature, pressure, humidity and the gas mixture properties. Taking advantage of these characteristics, acoustic waves can be used to determine whether a gas is present in the ambient or not. The analysis of the gases can be performed by the speed of the sound, by the attenuation of the signal, the acoustic impedance, or the combination these characteristics [28; 31; 99; 135]. Figure 5 shows the schematic used in the acoustic gas-sensing approaches. The most used and studied technique is based on the difference of sonic speed in different propagation medium, using the propagation time in a fixed distance to determine the composition of a given gas mixture. Equation (2) shows the dependency of the sound speed in relation to the gaseous propagation medium [100]. \[c=\sqrt{(kRT/M)}, \tag{2}\]  
where \(c\) is the speed of the sound, in meters per second; \(k\) is the average specific heat ratio; \(R\) is the gas constant; \(T\) is the temperature, in kelvin; and \(M\) is the average molecular weight [100]. The approach  
Figure 5: Ultrasonic speed-based gas sensor, which measures the target gas by a comparison of the sonic speed in the air and the sonic speed in the presence of the target gas. is efficient by measuring concentrations in binary gases, in other words, it is useful to measure gas concentrations in systems where there is only the possibility of having two types of gases. When more gases are added to the system, the detection can present a false positive to a given gas, due to the presence of another gas in the proportions that would generate the same sound speed in the propagation medium [101]. Moreover, the temperature variations can influence the sensing process; the calibration must take in consideration the working temperature of the system where the sensor will function, and analyze the possible moisture in these temperatures. A sensor using this technology combined with another sensing technique can refine the sensing accuracy, as well as the selectivity. Prototypes using this technology have been proposed by [PERSON] et al. [101], and [PERSON] et al. [100]. The attenuation of the acoustic signal is a reference to the scattered energy in a defined propagation distance. When a signal propagates through a gaseous medium, the equilibrium of the gases is broken and the molecules exchange energy through collisions, generating thermal energy. The energy generated is proportional to the energy consumed by the gaseous molecules creating the movement that leads to the collisions. It is said that the acoustic energy is absorbed by the system, and the level of absorption is proportional to the concentration of the gases that compose the propagation medium [28; 99; 102]. Prototypes of acoustic attenuation gas sensing were developed by [PERSON] et al., to sense CH\({}_{4}\), CO\({}_{2}\), N\({}_{2}\), ethylene (C\({}_{2}\)H\({}_{4}\)) and air [102]; and by [PERSON] and [PERSON] to sense SF\({}_{6}\)[99]. The attenuation of the signal is calculated using Equation (3), where Po and P are the acoustic pressure in two different points, \(\alpha\) is the attenuation coefficient and X is the distance from Po to P [99]. \[\mathrm{P}=\mathrm{P}_{\mathrm{o}}\;\mathrm{e}^{(-\alpha\mathrm{X})}, \tag{3}\]  
Acoustic impedance analysis is a less studied technique to sense gases, and no commercial uses of this technique have been identified. The gas is sensed through its density using Equation (4), where Z is the acoustic impedance, \(\rho\) is the gas density, and c is the sonic speed. This technique can generate detection errors, as a given gas mixture can be mistaken to another, as they can present the same density. It also could be used within another detection technique [133].