A sensor system for sensing a plurality of process variables of a fluid at a down hole location, comprising: an acoustic transducer which includes an acoustic source and an acoustic sensor, wherein the acoustic transducer generates an acoustic signal, wherein the acoustic source is configured to transmit the acoustic signal to a down hole location, a variable resonator (110, 200, 220) disposed at the down hole location of a well pipe (107, 226) having a resonant frequency which varies in response to the plurality of process variables of the fluid, wherein the variable resonator (110, 200, 220) receives the acoustic signal generated by the acoustic transducer and begins to resonate as a function of a process variable of fluid at the down hole location and generates a reflected acoustic signal at the resonant frequency indicative of the plurality of process variables, and wherein the variable resonator includes an elongate element having an interior cavity and the interior cavity is coupled to a pressure fluid, wherein the process variable of the fluid comprises pressure; wherein the acoustic sensor of the acoustic transducer is disposed at a second location spaced apart from the variable resonator (110, 200, 220) and configured to receive the reflected acoustic signal transmitted from the variable resonator (110, 200, 220) and returned along the length of the well pipe; and a measurement circuitry coupled to the acoustic transducer and its acoustic sensor configured to provide a plurality of process variable outputs related to the plurality of process variables of the fluid in response to the received reflected acoustic signal of the variable resonator (110, 200, 220). an acoustic transducer which includes an acoustic source and an acoustic sensor, wherein the acoustic transducer generates an acoustic signal, wherein the acoustic source is configured to transmit the acoustic signal to a down hole location, a variable resonator (110, 200, 220) disposed at the down hole location of a well pipe (107, 226) having a resonant frequency which varies in response to the plurality of process variables of the fluid, wherein the variable resonator (110, 200, 220) receives the acoustic signal generated by the acoustic transducer and begins to resonate as a function of a process variable of fluid at the down hole location and generates a reflected acoustic signal at the resonant frequency indicative of the plurality of process variables, and wherein the variable resonator includes an elongate element having an interior cavity and the interior cavity is coupled to a pressure fluid, wherein the process variable of the fluid comprises pressure; wherein the acoustic sensor of the acoustic transducer is disposed at a second location spaced apart from the variable resonator (110, 200, 220) and configured to receive the reflected acoustic signal transmitted from the variable resonator (110, 200, 220) and returned along the length of the well pipe; and a measurement circuitry coupled to the acoustic transducer and its acoustic sensor configured to provide a plurality of process variable outputs related to the plurality of process variables of the fluid in response to the received reflected acoustic signal of the variable resonator (110, 200, 220).