Development of quartz crystal oscillators for under-liquid sensing

Abstract A brief history of crystal sensing techniques is presented. Simple resonator equivalent circuits are outlined and expanded upon for cases of crystal immersion in liquids. Conditions for oscillation and choice of appropriate oscillator are discussed. Several well-known recently published sensor oscillator circuits are briefly reviewed and some suggestions for improvements are made. Details are given of two new oscillator circuits developed and employed by the author. Uniquely, these new circuits allow total immersion of the exposed crystal plate in hitherto impossible media, such as viscous liquids of viscosity up to 40 cP and in electrolytes and buffers of several millimolar ionic strength. Finally, suggestions for gain and stability control of oscillators in practical ‘under-liquid’ sensing situations are made in relation to the future potential of these techniques.