Abstract: A tank contents gauge is disclosed having two pressure sensors to sense the liquid pressure at two tank positions at known vertical separation, a port to sense the ambient pressure in the tank, and a transducer to which the three pressure readings are transmitted one at a time by a switching mechanism. If the transducer is a differential transducer, its second side is connected to the port to allow (1) comparison of the pressure readings with the ambient pressure reading and (2) determination of transducer offset to adjust the calculations to give accurate readings. The readings are analysed by a data processing system which can provide volume and weight readings by internal look-up calibration tables. The system may also control the switching mechanism. Specific gravity values can be calculated from the pressure readings and the known vertical separation; the values can be stored in the system for use when the tank contents approaches, or falls below, the level of the upper sensor.