A tank contents gauge comprises pressure sensors 11 for sensing the pressure at two positions in the tank at known vertical separation, a port 24 for sensing the ambient pressure in the tank, which may be atmospheric pressure, and a transducer 19 to which three pressure readings are transmitted one at a time via a switching mechanism 21,22. Where the transducer is a differential transducer the second side thereof is connected to the port 24 so that the pressure readings are compared in each case with the ambient pressure reading, and the transducer offset can be determined and catered for in calculations made from the readings.
A data processing system 25 with a memory 26 is provided for analysing the readings and making the required calculations of liquid depth and, by using internal look-up calibration tables, those of volume and weight. The data processing system may also serve to control the switching mechanism 21,22. The specific gravity of the contents can be calculated from the pressure readings and the known vertical separation and stored for use when the contents approaches, or falls below, the upper sensor 11.