Abstract: A differential pressure detector is employed for measuring the liquid level in a pressurized vessel. The differential pressure detector senses the level in a pressurized vessel through a variable head column and a constant head cold reference column. In order for the differential pressure detector to operate accurately, the head of the liquid in the cold reference column is maintained constant. An ambient reference column is in communication with the cold reference column. When the height of the liquid in the cold reference column drops, or the pressure of the pressurized vessel drops, liquid in the ambient reference column will flow into the cold reference column to maintain the head of the liquid in the cold reference column. When the height of the liquid level in the cold reference column is elevated, the excess liquid in the cold reference column overflows into the pressurized vessel.