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The air quality in the domestic area, workshops or industrial work places is of great interest, especially in respect to humidity and vapors. The simplest way to determine humidity is by resistive measurements using interdigital transducers (IDT). Combining these devices with polyelectrolytes, changes in conductivity due to variations of humidity can be monitored. A centralized surveillance of different control points can be accomplished by wirelessly, interrogative SAWs. A problem in measuring humidity by IDTs arises since cross sensitivities to solvent vapors are observed due to ion solvation effects. By varying the counterions from Li+ to Cs+ organic vapors are no longer able to compete with the hydration and selective moisture detection is achieved. Water insoluble sensor coatings which ensure long-term stability and resistance against flooding were synthesized by on-chip cross-linking on the surface of the applied microelectronic IDTs. Thus, a cross-linked Cs+ polyelectrolyte was designed as a selective sensor coating for humidity monitoring. One port wirelessly, interrogative SAW-devices have one transducer connected to an antenna and reflectors for identification (ID-tag). The interrogator transmits a burst signal and the sensor responds with a burst of signals depending on the number of reflectors. The ID-tags used have two reflectors which are approximately 4 and 1 5mm away from the transducer which respond in bursts at 3 and 9 ?s. On coating the area between the two reflectors with more hydrophilic or hydrophobic materials humidity or solvent vapors can be determined via the reflected amplitudes.
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