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Surface acoustic wave (SAW) sensors have been successfully exploited for the detection of both chemical and physical quantities. In this paper we report more recent results on SAW sensors for the detection of relative humidity (R.H.). Different chemical interactive materials (CIMs), of both organic and inorganic compounds, are tested and their performances in the R.H. range between 10 and 80% are reported and compared. All the devices analysed have been shown to operate in a reversible way, with a fairly good repeatibility and fast response times. The CIMs have been shown to produce a variety of different behaviours versus R.H. percentage, with respect to both the amplitude and linearity of the response. The response to R.H. has also been investigated for different orientations of both quartz and lithium niobate bare substrates. These data can be useful in order to investigate the effect of the R.H. of the surroundings on the performances of SAW based devices. In addition this test is of great importance in order to know how humidity influences the response of both chemical and physical sensors. The response of organic CIMs has also been tested toward hydroxylated molecules such as acetone and ethanol, and the relative calibration curves are reported.

The gas sensing properties of organic polypyrrole (PPS) film, deposited onto LiNbO(3) substrate by Langmuir-Blodgett (LB) technique, have been monitored by surface acoustic wave (SAW) delay lines and studied with respect to sensitivity, selectivity, response time, stability, repeatability, and aging. The SAW PPy elements demonstrate high sensitivity toward NH(3) gas with high selectivity against CH(4), CO, H(2), and O(2). The detectable threshold concentration has been estimated as 20 ppm NH(3) in air; the response time is in the 10s range, and the recovery time is about 15 min; the repeatability of the SAW response toward eight sequential NH(3) gas exposures is within 6%; the aging of the PPy film is within 4% over a month; and the effect of humidity on SAW NH(3) gas response is negligible for the typical conditions at room ambient air. Partially reversible SAW response recognizing NH(3) gas as one component of an interfering gases-mixture has been observed. Simultaneous chemoresponses of SAW phase and insertion loss have been performed in order to investigate the sensing mechanisms. By merging with electrical conductivity gas response, the dominant SAW sensing effects for NH(3 ) gas detection are defined as elastic loading.

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