piezoelectric crystal such as gallium phosphate as pressure is transmitted to an edge of the quartz crystal its resonant frequency changes by correct selection of a direction of the face of the crystal the sensor may be made to be more sensitive to pressure or to temperature e g ac cut for pressure monitoring the crystal should be preferentially sensitive to pressure and relatively less sensitive to temperature e g at cut furthermore for monitoring of pressure changes with a relatively high frequency response e g monitoring of acoustic frequencies it is useful for the crystal to be generally relatively thin e g 0 2 2 0 mm and a typical size is on the order of 1 cm in diameter a return spring mechanism 214 may be provided to bias the crystal 210 and its holders towards the feed in tube 204 and thereby to tend to cause the diaphragm to return to a neutral position an electrical feed through 216 is provided to couple the sensor 200 to the sensor circuit not shown the sensor 200 may be coupled to the transmission line via an inductive ferrite ring 400 as illustrated in fig3 electrical leads 402 are provided through the electrical feed through 216 of the sensor module the leads 402 couple wire loops around the ferrite ring 400 in this embodiment the oscillator has the characteristics of an l c circuit and the ferrite ring essentially acts.