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Moisture measurement in parts per million (ppm) in electronic gases and in liquids for different industrial applications is challenging and a costly affair. There are numerous critical applications of ppm moisture measurement systems such as transformer oil, sulfur hexafluoride gas (SF 6 ) in switchgear and protection, and the electronic industry for integrated circuit (IC) fabrication. Conventionally chilled mirror hygrometer and laser spectroscopy are used but some low-cost solutions using the thin-film capacitive sensors are also available in the market. This paper presents the fabrication of an inexpensive miniature moisture sensor using a commercial low-cost surface acoustic wave (SAW) resonator of 433.92 MHz frequency. To detect moisture, a hydrophilic metal oxide film is deposited on the interdigital electrodes (IDTs) of the SAW device. The film thickness is optimized to provide a sharp resonating peak under dry humidity conditions. Experiments were conducted to determine the frequency shift in the peak with the variation of moisture in nitrogen gas from 6 to 460 ppm using a vector network analyzer. The maximum sensitivity was 24 Hz/ppm with correlation coefficient of linearity equal to 0.99. An effort was also made to develop a prototype meter to measure moisture in ppm. Important features of the sensor are linear response, inexpensive fabrication, and appreciable sensitivity in the ppm range.

instrumentation science and technology

Determination of the relative humidity at the parts-per-million (ppm) level in gases by a nanoporous alumina thin-film on a surface acoustic wave (SAW) resonator