Abstract: This instrument contains a transducer and a sensor that are mounted on the exterior surface of a liquid container or tank. The transducer is tuned by electronic signals to the mechanical resonant frequency of the liquid container or tank. The sensor converts the mechanical vibrations into dynamic electrical signals using electronic components. A phase detector and feedback circuitry force the transducer to track the resonant frequency of the tank as liquid is removed from the tank. The signal output of the feedback control circuitry is converted to display the amount of liquid remaining in the tank. The resonant frequency of the liquid container is a measure of the total mass of the container. This includes the tare weight of the container plus the weight of the liquid in the tank. The tare weight of the tank is a constant. The stiffness of the tank can be considered a constant. The dampening coefficient changes do not adversely effect the accuracy requirements of the instrument.