A novel acoustic sensor for multi-component analysis of gases and liquids

The relative changes in the phase velocity (Δv_n/v_n) of Lamb and SH modes, propagating in ST-cut quartz plates, along and perpendicular to the x direction respectively, to changes in the density, elastic moduli: temperature and thickness of the plate as well as mass, elastic, electric loading of the surface have been investigated for the different modes. It is shown how some of these changes do not affect the modes at all, others affect the modes of equal amounts, and finally some of them modify the velocity of the modes of amounts that depend on the mode family (SH or Lamb) and order n. This dependence on the order mode is smooth for the SH-one and non-smooth for the Lamb counterpart. On the basis of these properties a novel single channel sensor for multi-component analysis has been developed and tested using humid air as an example. Using different modes of one or of the two families, the sensor consists of one or two pairs of interdigital transducers (IDTs) and operates as an array of sensors with different response and calibration curves at the different frequencies.