A Passive Wireless Tag With Digital Readout Unit for Wide Range Humidity Measurement

Moisture measurement in ppm level is always challenging and costly. Among different technologies available for moisture measurement, the thin film capacitive sensor-based dew point meter offers a low-cost solution. Still, the cost of the dew point meter is high and it suffers from high response time. There is a need of wireless moisture measurement in ppm level that is not available with the commercial meter according to our knowledge. This paper deals with development of a lowcost passive wireless tag for capacitive type sensors. The tag can be energized by any 13.56-MHz commercial tag reader from a distance of 1 cm (approx). The tag is capable of transmitting sensor data at 2.4-GHz ISM band up to a distance of 12 m. An efficient algorithm has been developed to reduce the power consumption. The prototype system has been tested with two custom designed capacitive sensors to measure humidity over a wide range. Both sensors, having a highly stable hygroscopic film of Al
₂
O
₃
, are fabricated in the lab. Finally, a low-cost (approximately U.S. $250) digital hygrometer to measure trace moisture has been developed. The performances of the digital hygrometer have been compared with the commercial dew point meter, and accuracy is found to be nearly 1% in the range of 6.5-127-ppm moisture. The system can be employed for contactless measurement with any capacitive sensor in the range of 50-3200 pF.