DEVELOPMENT OF HUMIDITY CORRECTION ALGORITHM FOR SURFACE ACOUSTIC-WAVE SENSORS .1. WATER-ADSORPTION ISOTHERMS ON COATED SURFACE ACOUSTIC-WAVE SENSORS

The adsorption of water vapour onto surface acoustic wave sensors (SAWS) coated with polyvinylpyrrolidone or FPOL (a fluorinated polyether-polyol) was studied at 30, 40 and 50-degrees-C. Water adsorption isotherms were obtained which resemble the BDDT type III classification, characteristic of weak gas-solid interactions. At higher temperatures, and lower relative humidity particularly with FPOL, the isotherms demonstrated zero or negative response on exposure to water vapour (i.e., the frequency of the SAWS oscillator increased when exposed to humid air compared with dry air). Although this phenomenon is not yet understood, it may be due to swelling of the polymer, thus affecting its viscoelastic properties. The isotherms can be fitted to a quadratic equation, which is reasonable for a BDDT type III isotherm. It is clear that operation at 40 or 50-degrees-C will significantly reduce the effect of water vapour on the SAWS, although it is recognized that the adsorption of target gases may also be decreased.