Temperature Effect and Compensation

The surrounding temperature affects the performance of a SAW device in two ways. Temperature changes the crystalline structure of the piezoelectric substrate that in turn affects the propagation property of the SAW. Temperature changes also affect the sorption and desorption property of the coating film. The effect that affects the property of the piezoelectric crystal can be reduced through installation of a reference SAW sensor that is isolated from the sample vapor. Temperature fluctuation affects not only the SAW sensors for the detection but also the reference sensor. Therefore, the temperature effect on the sensors could be compensated for by negating the similar effect experienced by the reference sensor. 

Temperature changes the sorption coefficient of the polymer coating film. The polymer has stronger sorption ability to chemicals at lower temperatures. Consequently, the sensors theoretically should have better detection sensitivity at lower temperatures than those at higher temperatures. However, at low temperatures, much more energy and time are required for effective total release of sorbed chemicals before the next analysis cycle. It is also possible that the sorbed chemicals cannot be totally released at low temperatures, which means that sensors malfunction. In addition, the polymer may become very rigid at extremely cold temperatures and lose its adsorption ability, which also causes the detector to fail. 

For both stationary and handheld detectors, it is possible and desirable to maintain a constant temperature on the sensors during the analysis. Heating the sensor cavity to a temperature above the ambient temperature reduces the effect of ambient temperature fluctuations that are known to influence detection characteristics. Unfortunately, higher-than-ambient temperatures will tend to worsen the sensors LOD. Therefore, a compromise setting must be chosen to provide the best detection results. 

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