An ultrasonic transducer is disclosed comprising a layer of piezoresonator material capable of transmitting from a top surface thereof an ultrasonic pulse into a tank of liquid and receiving at the top surface reflections of the transmitted pulse from the liquid; and a matching layer of pure crystalline Boron Nitride disposed on the top surface. The pure crystalline Boron Nitride layer is operative to match the acoustic impedances of the piezoresonator material and the tank liquid about the operational frequency passband of the ultrasonic pulse. The ultrasonic transducer may be disposed in an assembly for measuring a quantity of liquid in the tank. The assembly comprises a housing having a top surface for interfacing with the liquid of the container; the ultrasonic transducer disposed therein, the top and bottom surfaces of the piezoresonator layer covered with layers of conductive material; and a lead wire for each surface of the piezoresonator layer connected at one end to the conductive material layer thereof, the lead wires connectable at the other ends to a transducer driver/receiver circuit. The matching layer of pure crystalline Boron Nitride disposed on the top surface of the piezoresonator layer is configured as a window between the piezoresonator layer and the liquid at the top surface of the assembly. A surface of the matching layer at the liquid interface is covered with at least one metal layer.