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A surface acoustic wave (SAW) gas sensor coated with poly-N-vinylpyrrolidone (PNVP) film for monitoring low concentration ammonia was developed. By miniaturization of SAW chip and employment of polymer as sensing material, the sensitivity of SAW gas sensor can be improved. A dual-device configuration was applied to construct SAW gas sensor for eliminating detection deviation from environment such as temperature and humidity. Frequency shifts of SAW sensor were measured to monitor the presence of different concentration ammonia. The as-developed SAW gas sensor presented good repeatability and response to low concentration ammonia detection, even though the ammonia was below 10 ppm. The sensitivity of PNVP-coated SAW sensor for ammonia detection was 6.91 Hz/ppm, and the frequency shift of the as-developed SAW sensor to monitor 8 ppm ammonia was 27.5 Hz.