Abstract: A system for continuously monitoring fills of liquid such as fuel oil in a tank, and the condition of the liquid. A dual in-line air bell structure senses fluid pressure at upper and lower locations inside the tank, the locations being spaced a known distance apart from one another in a normal direction relative to the tank. Processing circuitry responsive to the fluid pressures obtained via the air bell structure, determines quantities of liquid contained in the tank at certain times and produces corresponding outputs such as gallons of liquid delivered to the tank at each of a number of delivery times, and total gallons of liquid in the tank as the liquid is consumed. The air bell structure is formed with upper and lower air bell sections joined end-to-end along a common axis. Each of the air bell sections includes at least one aperture in its wall to define a cross-section through the air bell section.