Deep Sub-micro mol·mol-1 Water-Vapor Measurement by Dual-Ball SAW Sensors for Temperature Compensation

• Published in: International journal of thermophysics 2015-08, Vol.36 (12), p.3440-3452
• Main Authors: Takeda, N., Oizumi, T., Tsuji, T., Akao, S., Takayanagi, K., Nakaso, N., Yamanaka, K.
• Format: Article
• Language: English
• Subjects: Classical Mechanics; Condensed Matter Physics; Industrial Chemistry/Chemical Engineering; Physical Chemistry; Physics; Physics and Astronomy
• ISSN: 0195-928X; 1572-9567
• DOI: 10.1007/s10765-015-1967-3

A collimated surface acoustic wave (SAW) circles around the equator of a sphere hundreds of times. Because of the long distance travel of the collimated SAW, a small change in the SAW propagation caused by the environment of the sphere can be accumulated as a measurable range in amplitude and/or in delay time. So, a spherical SAW device enables highly sensitive water-vapor measurements. In this paper, deep sub μ mol · mol - 1 water-vapor detection by 1 mm diameter quartz crystal ball SAW sensors is described. To measure such a low water-vapor concentration in real time, it is necessary to compensate the temperature dependence of the ball SAW sensor, which is about 20 ppm · ∘ C - 1 in delay time change. A dual-frequency burst analog detector was developed for the temperature compensation in real time. By using a harmonic SAW sensor, which was excited by 80 MHz and 240 MHz at the same time, it was confirmed that the delay time drift for a temperature range of 21.0 ∘ C ± 1.0 ∘ C became less than 0.05 ppm in delay time change. By using dual-ball SAW sensors (which included a 150 MHz sensor with a water-vapor sensitive layer and a 240 MHz sensor as a reference), water-vapor concentrations from 0.1 μ mol · mol - 1 to 5 μ mol · mol - 1 were successfully measured. It appears that the delay time change is proportional to the square root of the water-vapor concentration. The detection limit determined by the electrical noise of the system was estimated at 0.01 μ mol · mol - 1 .