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Significance Statement
Everybody knows that waves are diverged and attenuated as they propagate. However, we discovered a unique propagation phenomenon on a spherical surface. Once a surface acoustic wave (SAW) is exited on a sphere with an appropriate width, it keeps its width and continues to propagate around the equator hundreds of times.
We have applied this phenomenon to measure trace moisture. We used a quartz crystal sphere with 1mm diameter to excite the SAW. There is a sensing layer on the surface and it causes changes in propagation of the SAW according to H2O concentration in its environment. Even if such changes are very small, they are cumulated as a measurable range in delay time or in amplitude during multiple round trips.
To compensate temperature drift in real time, a dual-frequency burst analog detector was developed. By using a 150 MHz sensor with a water-vapor sensitive layer and a 240 MHz sensor as reference, the water-vapor mole fraction from 0.1 µmol･mol-1 to 5 µmol･mol-1 was successfully measured within several minutes.
Accordingly a ball SAW sensor has a potential to be a hygrometer with good sensitivity and fast response even at deep sub-micro mol･mol-1 humidity level.
Journal Reference
International Journal of Thermophysics, pp 1-13, 23 August 2015.
Takeda1, T. Oizumi1, T. Tsuji1, S. Akao1, K. Takayanagi2, N. Nakaso2, K. Yamanaka1.
[expand title=”Show Affiliations”]- Tohoku University, Sendai, Miyagi, Japan
- Toppan Printing Co. Ltd., Kitakatsushika-gun, Saitama, Japan
Abstract
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 μmol⋅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 μmol⋅mol−1 to 5μmol⋅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 0.01μmol⋅mol−1.
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