Abstract: A method and apparatus for monitoring and indicating whether a tracheal tube is located in the trachea or esophagus during an endotracheal intubation of a human or animal is disclosed. In a calibration mode, the resonant frequency of the tube is determined by generating a signal at a plurality of frequencies and finding the signal which propagates with the greatest amplitude. This :indicates the resonant frequency. When the tube is inserted into the patient, the resonant frequency of the system shifts, thus the amplitude of the signal changes. Based on known characteristics of the trachea and esophagus, the amount of change is dictated by whether the tube has been inserted into the trachea or esophagus. Alternatively, the device can continually determine the resonant frequency and use this to monitor the level of liquid in a tank.