Abstract: The specification discloses a boundary detector for locating the boundary between two substances having different thermal absorption characteristics. In one embodiment, the detector monitors the level of a coolant in a vessel in which case the upper surface of the coolant constitutes the detected boundary. An elongate sensor is constructed of a plurality of segments of different material connected end-to-end to form a series of thermocouple junctions. This sensor is disposed across the boundary in the two substances, and a segmented heater cable is used to heat selected ones of the thermocouple junctions. The sensor and the heater cable are electrically insulated by thermally conductive material that will transfer heat to the substances in which the sensor is disposed, and because of this heat transfer, the temperatures of the heated junctions are, in part, dependent upon the substance in which it is located. By monitoring the voltage potential across the sensor, the boundary may be detected and located.