Abstract: Apparatus for determining levels of immiscible fluids in a vessel that includes a transmission line sensor that extends vertically within the vessel for contact with fluids within the vessel. A generator is coupled to the sensor for transmitting along the sensor a first electrical signal that continuously sweeps a predetermined frequency range. A voltage detector is coupled to the sensor for providing a second electrical signal that varies as a function of the amplitude of electrical energy reflected at the sensor from the various electrical impedance discontinuities along the sensor, including particularly interfaces between and within immiscible fluids surrounding the sensor. The frequency spectral characteristics of the second electrical signal are analyzed to determine the spatial distribution of electrical impedance along the sensor, from which the levels of immiscible fluids within the vessel are determined.