Extremely Fast 1 μmol · mol−1 Water-Vapor Measurement by a 1 mm Diameter Spherical SAW Device

Abstract

When a surface acoustic wave (SAW) is excited on a spherical surface, a naturally collimated SAW propagates around the equator hundreds of times. The propagation characteristics such as the velocity and amplitude are affected by adsorbed and/or reacted molecules on the surface, and the changes are accumulated by multiple turns of propagation. This enables highly sensitive detection of adsorbed molecules including water vapor. In this paper, the development and testing of a 1 mm diameter spherical SAW sensor, which is capable of measuring water vapor at concentrations well below 1 µmol · mol −1 H 2 O in N 2 , are reported. The rise time from 10 % to 90 % of the spherical SAW sensor to a step change from dry N 2 to 1 µmol · mol −1 H 2 O in N 2 was approximately 15 s.

Development of ball surface acoustic wave gas chromatograph for environmental monitoring in spacecraft and its application on the ground

“…We have developed several types of ball SAW sensors, such as a hydrogen sensor coated with Pd-alloy film 10,[28][29][30] and a trace moisture analyzer coated with sol-gel silica film. 11,[31][32][33][34] In addition, since it operates at room temperature, it is power-saving and suitable for portable GC. In this study, we used a crystal quartz ball SAW sensor with a center frequency of 150 MHz and a diameter of 3.3 mm.…”

A Novel Method and an Equipment for Generating the Standard Moisture in Gas Flowing through a Pipe

“…There are various technologies for trace moisture measurement including aluminum oxide sensors [ 2 ], tunable laser diodes [ 3 ], and the cavity ring down spectroscopy [ 4 ]. Recently, the present authors developed a new technology called ball surface acoustic wave (SAW) moisture sensors [ 5 ]. It covers a wide range of moisture level, from a few ppbV to hundreds ppmV, and the most prominent characteristic is its quick response to a sudden variation in the moisture level [ 6 ].…”

A Thermodynamic Consideration on the Mechanism of Ultrasensitive Moisture Sensing by Amorphous Silica

“…In the following years, a variety of gas sensors were developed by depositing various sensitive layers on the ball SAW Ball Wave Inc., Sendai, Miyagi, Japan device: a hydrogen gas sensor [5], an odor sensor [6], and a portable gas chromatograph [7]. Then it was accidentally found [8] that a very small amount of moisture in the ambient gas affected the acoustic properties of a SAW device made of a singlecrystal quartz ball without any sensitive layers. It had been a mystery for a while how the surface of a single-crystal quartz was affected by the ambient moisture at room temperature, until a mechanism was eventually proposed that the surface of the particular quartz ball was damaged during the production process and a thin amorphous layer of silica was produced which captured water molecules resulting into the change of the acoustic properties.…”