DISCLOSED IS A SYSTEM FOR MEASURING THE DENSITY OF A LIQUID IN RESPONSE TO CHANGES IN THE RESONANT FREQUENCY OF A PIEZOELECTRIC CRYSTAL. FIRST AND SECON PIEZOELECTRIC CRYSTALS ARE POSITIONED IN PROXIMITY TO EACH OTHER AND APPROXIMATELY AT THE SAME DEPTH BELOW THE LIQUID SURFACE. ONE OF THE CRYSTALS IS LOADED BY THE LIQUID BEING MONITORED WHILE THE SECOND CRYSTAL IS POSITIONED IN A BODY OF REFERENCE FLUID THAT IS COUPLED WITH THE LIQUID BEING MONITORED SO THAT THE DIFFERENCE IN RESONANT FREQUENCIES OF THE CRYSTALS IS INDICATIVE ONLY OF THE DENSITY OF THE MONITORED LIQUID. THE DIFFERENCE IN THE RESONANT FREQUENCIES OF THE TWO CRYSTALS IS DERIVED BY DRIVING THE CRYSTALS WITH OSCILLATORS, THE OUTPUTS OF WHICH FEED A MIXER DRIVES A FREQUENCY METER.