Abstract: A liquid level instrumentation measurement system for measuring the level of liquid under high pressure in a pressure vessel containing liquid and vapor with non-condensable gases. The system includes a reference leg outside the pressure vessel, a condensing chamber at the upper end of the reference leg defining a vapor space in which vapor condenses to keep the reference leg filled with liquid, a variable leg connected near the bottom of the pressure vessel, and a differential pressure detector connected to both the variable leg and the reference leg for generating a signal proportional to the level of liquid in the pressure vessel. A vent outlet is disposed in the condensing chamber to which a vent leg is connected. The vent leg may be disposed between the vent outlet in the condensing chamber and the variable leg or between the vent outlet and the head vent line which connects the pressure vessel to a main vapor line.