Simultaneous measurement of CO/sub 2/ and humidity using a pair of SAW devices and cluster-analysis pattern recognition

We use a pair of polymer-coated 97-MHz ST-quartz surface acoustic wave (SAW) delay lines to simultaneously determine the concentrations of CO/sub 2/ and relative humidity (RH) in the gas phase at room temperature. The SAW response of a CO/sub 2/ permeable polyimide is rapid and reversible for CO/sub 2/ from 0.03 to 97% of one atm, with some interference from humidity; a layer of the hydrophilic polymer poly(N-vinylpyrrolidone) responds mainly to humidity. Visually-empirical region-of-influence (VERI) pattern recognition (PR) is used to select the optimum pair of films and to evaluate the robustness of the sensor pair to errors.