Abstract: The peak amplitude values of two different frequency pressure changes produced by the displacement device of a volume sensing arrangement, are obtained and one divided by the other to produce a value indicative of the volume of the unfilled portion of a storage vessel. The contents of the vessel are derived by subtracting the unfilled volume from the vessel capacity. In the embodiments wherein a single pressure sensor is used, the signal is simultaneously filtered with a low pass filter and a band pass filter in order to obtain static and dynamic pressure data. In some embodiments the single sensor is disposed in a chamber in which the displacement means produces two different frequency pressure variations. The chamber is communicated with the storage vessel by a pipe the length and diameter of which are selected to prevent the pressure variations produced by one of the frequencies from being transmitted therethrough.