Abstract: An in-situ sensor system and method which measures the density of a liquid accurately with pressure sensors by measuring pressure in the liquid at two separate positions defined by a fixed distance. The fixed distance is known accurately to at least the nearest 0.0001 inch by measuring temperature simultaneously with pressure measurements. The temperature is used to correct for temperature-related changes in length of the material to which the pressure sensors are fixed. Because pressure sensors and temperature sensors can be relatively small, the sensor system is small, portable, and can be easily and safely inserted into or removed from tanks through a small opening. A data acquisition system acquires data with a remote telemetry system, provides the data to users through a communications network, and provides density, height, volume, and leak rate of the liquids using one or more data management systems.