SAW Sensors for the Room-Temperature Measurement of CO2 and Relative Humidity

A pair of polymer-coated 97-MHz SAW delay lines allows simultaneous determination of the concentrations of CO2 and H2O in the gas phase at room temperature. The SAW response of a 370-nm-thick, CO2-permeable polyimide prepared from hexafluoroisopropylidene diphthtalic anhydride and 2,4,5,6-tetramethylphenylenediamine is rapid and reversible for CO2 concentrations from 0.03 to 97% of 1 atm in a pure nitrogen carrier gas stream, but with significant interference from humidity. A 40-nm-thick layer of the hydrophilic material poly(N-vinylpyrrolidone) (PNVP) responds primarily to humidity; 10−60% relative humidity has been examined for a wide range of accompanying CO2 concentrations. Using visually empirical region-of-influence pattern recognition, the optimum two-sensor pair was selected from several possibilities. The effects of noise and gradual film sensitivity degradation (“aging”) upon the accuracy of determining CO2 and H2O concentrations were examined: we find that the polyimide- and PNVP-coated SAW device pair allows robust quantitation of H2O over the entire concentration range examined, but robust quantitation of CO2 is possible only at low humidity.