Abstract: A fluid sensor for use within the gastro-intestinal tract of a human being is disclosed. The sensor includes a sensing coil which is immersible in the sample fluid of the gastro-intestinal tract; a signal generator in electrical with the sensing coil for applying an electrical current pulse to the sensing coil; a signal receiver in communication with the sensing coil for measuring an electrical reflection relative to said electrical current pulse; and a data processor for receiving the electrical reflection and for calculating data representative of at least one property, such as pH of the sample fluid based on the electrical reflection. The fluid sensor can also include a reference coil for calibrating the sensing coil. The sensor coil and reference coil can be encapsulated in a swallowable pill shell. The sensor coil can also function as an antenna for transmitting and receiving signals to/form a remote location.