A fuel level monitoring system for a fuel system having a primary fuel tank and a secondary fuel tank includes a fuel level sensor that is disposed within the secondary fuel tank and that generates a first signal. There are two methods that have been demonstrated by experiment and supported by theory. In the case when parametric mixer is used to generate frequency combs with small frequency pitch, the phase correlation of the input waves can be achieved by electro-optical modulator and a single master laser. The system further has a valve assembly associated with the duct which valve assembly normally closes off the duct, but which valve assembly is opened by flow of liquid along the fill-pipe into the tank, such that vapor in the tank may be drawn along the duct from the ullage space by the reduced static pressure in the region of reduced cross-sectional area of the fill-pipe.