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A surface acoustic wave (SAW) with a shear horizontal (SH) polarized displacement can work as a liquid-phase sensor. In this paper, we describe the evaluation of KI-index of natural spring waters using an automatic liquid measurement system with the SH-SAW sensor. The KI-index is determined from the concentration of calcium and sodium in water. Using the self-calibrating ratiometric method and an artificial neural network, the KI-index is estimated without any selective coatings for calcium and sodium. Furthermore, the optimization method for measuring water samples is discussed. The results indicate that the low frequency SH-SAW sensor has possibility to reduce the measuring time and high sensitive detection.