**9.4 Gas probing behaviour of PCFV coated Au and Al TPR and CRF RSAW sensors**

To compare the gas sensitivities of Au vs. Al sensors, pairs of devices of the same type according to Fig. 12 were PCFV coated in the same electro spray deposition method and probed with cooling agent, octane and tetrachloroethylene at different concentrations. Cooling agent and octane were among those gases that the sensors were intended to operate with in a specific application. After coating, the Au/Al pairs were selected to have nearly the same loss increase as a result of PCFV deposition to simulate identical mass loading. The gas probing results for 4 different loss increase values (thicknesses) are summarized in Table 9. As expected from the mass sensitivity data in Fig. 19, the Au devices demonstrate higher gas sensitivity than their Al counterparts. Another important conclusion evident from Table 9 is that soft polymer coating also requires an optimum film thickness for maximum gas sensitivity. In this case the optimum PCFV thickness is achieved when both the Au and Al devices are coated to about 6 dB loss increase, (numbers in bold in Table 9).


Table 9. Summary of the gas probing performance of Au vs. Al device pairs PCFV coated to nearly identical loss increase values in the same electro spray deposition process

## **10. Summary and conclusions**

This chapter has highlighted important practical aspects for the design and operation of chemical gas detection systems using STW and RSAW resonant devices. It has been shown that both acoustic wave modes provide excellent gas sensitivity and low detection limits, down to a few ppb when coated with solid, semisolid and soft polymer sensing layers. Furthermore, the RSAW and STW modes do not only compete but rather complement each other in different measurement tasks. The STW mode operates better with solid and semisolid sensing films featuring surface sorption and is better suited for high-resolution measurements at low gas concentrations (<1%) while the RSAW mode tolerates much better thick soft sensing layers with profound bulk sorption that operate better at high gas concentrations (>1%). Carefully designed RSAW sensors with Au metallization provide an excellent corrosion proof substitute of their Al counterparts when operated in highly reactive gas-phase environments, thereby greatly increasing system reliability and measurement reproducibility over time and a large number of measurement cycles. All gas sensors, regardless of acoustic wave mode, design, metallization and type of sensing polymer requires a careful thickness optimization to provide highest gas sensitivity, maximum dynamic range and lowest detection limit.
