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Liquid level detector for a sealed gas tank based on spectral analysis

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.