Abstract: A sensor for detecting an interface of a liquid in a container comprises a transmitter and a receiver of ultrasonic waves, each situated outside the container, and coupled to the wall of the container by a strip waveguide. The transmitter is arranged to cause surface acoustic waves to propagate through a portion of the wall, and the receiver is arranged to detect waves derived from the transmitter. When the liquid is adjacent to the portion of the wall, mode conversion of the surface acoustic waves in the wall of the vessel into compression waves in the liquid causes a change in the energy of waves reaching the receiver.

Abstract: An ice detector to detect a lyaer of ice on a thin solid sheet 14, such as the skin of an aircraft wing, even in the presence of water, comprises two ultrasonic transducers attached to the sheet. The first transducer 10 is adapted to cause propagation of ultrasonic waves through the sheet 14 having their predominant component parallel to the surface of the sheet 14. Such waves will dissipate energy into an ice layer adhering to the surface, but not into air or a liquid layer. The second transducer 30 is adapted to detect the waves propagating in the sheet and to give a signal representative of their amplitude. A discriminator 34 responsive to the signal detects the presence of an ice layer on the surface.