Deep Sub-micro $\hbox {mol}{\cdot }\hbox {mol}^{-1}$ Water-Vapor Measurement by Dual-Ball SAW Sensors for Temperature Compensation

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 $\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$ 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 $\hbox {ppm}{\cdot }^{\circ }\hbox {C}^{-1}$ 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 $21.0\, ^{\circ }\hbox {C} \pm 1.0\,^ {\circ }\hbox {C}$ 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 $\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$ to $5\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$ 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 $0.01\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$.