While several methods using ultrasonic, acoustic or RF waves have been proposed to measure the liquid level of a sealed container, they all need to install a transmitter and receiver pair inside the top of the tank. In this paper, we propose a simpler method which provides continuous measurement by installing a knocking device outside the tank. The vibratory frequencies generated by the knocking action are analyzed by the fast Fourier transform and the dominant frequency component which is related to the liquid level inside the tank is acquired. Our experimental results show that relationship between the vibratory frequency generated by the knocking action and the liquid level can be described by Bernoulli-Euler beam theory. Therefore, this acquired relationship can be used to estimate the liquid level based on the spectrum generated by knocking the tank surface.

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Fluid level detection for a sealed and pressurized mobile container is very useful for the provider to schedule the delivery of a new one before it runs out of the liquid. This study suggested using the frequencies of tone generated by knocking on the outside surface of the container to detect the liquid level inside. A detailed model based on Euler-Bernoulli beam theory has been proposed to study the feasibility of this method for a cylinder with complicated but practical structure. Household gas cylinders were used to validate the proposed model and the results show that experimental data agree well with the theoretical analysis. The results indicate that the proposed model can accurately explain the behavior of the vibratory frequencies under different liquid levels. An apparatus has been successfully implemented to automatically sense the near empty condition of the gas cylinder.

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