A fluid detector includes a piezoelectric element that outputs a first ultrasonic signal in response to an input electrical signal and a lens with an upper portion and a lower portion. A housing is integral with an outer surface of the wall of a container and defines a cylindrical central bore for receiving the lens. The piezoelectric element is coupled to the upper portion of the lens so that the lens focuses the first ultrasonic signal toward the wall so that it enters the wall. An ultrasonic detector is disposed adjacent the outer surface of the wall, for receiving a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected by the presence or absence of fluid. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal.