The invention relates to a sensor (1) for determining a liquid level (NE) for an aircraft tank (100), the determining sensor (1) comprising a closure device (2) for closing a port (101) of the tank (100) and a measuring device (3), removably mounted on the closure device (2), comprising a liquid line (20) configured to convey liquid from the port (100) of the tank (100), and a member (21) for automatically sealing the liquid line (20) if the measuring device (3) is not mounted on the closure device (2), the measuring device (3) comprising at least one pressure measuring member (30) configured to measure a pressure difference between the liquid pressure (P1) in the liquid line (20) and a reference pressure (P2) in order to deduce the liquid level (NE) thereof.