Surface wave liquid sensor

A liquid sensor comprising comb-shaped electrodes, applied to a piezoelectric substrate, as interdigital transducers for generating an electroacoustic wave from a transmitting transducer to a receiving transducer, where the transmitting transducer generates a surface shear wave and a bulk shear wave, wherein:

• the number of finger pairs of the interdigital transducers and the material out of which the electrodes are made are so chosen that the surface shear wave generated by the transmitting transducer and the bulk shear wave generated by the transducer have different frequencies,
• for the interdigital transducers, two fingers of an electrode are arranged next to one another in each case and form a pair, a pair of one electrode forming a finger quartet with a neighboring pair of the second electrode, and
• the number N of finger quartets is calculated according to the following formulae:
• ( Δ ⁢ ⁢ f f ) m = ( 2 · π ) 2 2 · ( γ · h · ρ m λ ) · 1 c 66 2 · [ v c 2 - v m 2 ] ( Δ ⁢ ⁢ f f ) N = ( Δ ⁢ ⁢ f f ) m ⁢ ⁢ and ⁢ ⁢ N > 2 · ( f / Δ ⁢ ⁢ f ) m
• where Δf is the frequency difference between the surface shear wave and the bulk shear wave, f is the excitation frequency of the transmitting transducer, γ is the ratio width(width+distance) of the electrode fingers, h is the height of the electrodes, ρm is the density of the electrode material, λ is the wavelength, c66 is the shear rigidity coefficient of the substrate material as a function of the cutting angle, vc is the speed of the bulk shear wave as a function of the cutting angle, and vm is the acoustic propagation speed in the electrode material.