SAW organophosphorus sensors coated with fluoropolyol isomers: role of humidity and temperature on the sensitivity

A surface acoustic wave (SAW) organophosphorus gas detector has been designed, fabricated and tested. The gas detector consists of a dual delay line fabricated on a single quartz substrate. Each delay line is connected into the feedback path of a radio-frequency amplifier, to realize a SAW oscillator. The propagation path of one delay line is coated, using spray technique, with fluoropolyol (FPOL). These polymers offer an interesting way to detect organophosphorus compounds like GB at low concentration levels. The absorption of vapors induces phase variations due to mass loading, and stress effects. These variations result in corresponding frequency shifts. Experimental results as a function of vapor concentration are presented. The influence of temperature and humidity are examined. Results showed on one hand that frequency variations are linear with GB gas concentrations from 0.5 ppm to 10 ppm and on the other hand that sensitivity to organophosphorus is twice in wet atmospheres (RH=60%) than in dry air. The sensitivity was also better at a working temperature close to the glass transition point (T_G) of the polymer. Above T_G, the modification of the polymer structure induced a great radiation of the acoustic energy in the coating