An ultrasonic device for detecting concentrations of solids or positions of solid/liquid interfaces in liquid media, particularly Bayer process liquids. The device comprises at least one piezoelectric element provided with electrodes; electrical circuit elements connected to the electrodes for conveying electrical signals, in use, to and from control apparatus; an ultrasound-transparent window, made of glassy carbon, positioned adjacent to the or each piezoelectric element; a protective matrix, preferably of epoxy resin, embedding and surrounding said the or each piezoelectric element and the electrodes while allowing ultrasonic waves to pass through the or each adjacent window to or from the piezoelectric element, and an external protective cover, preferably comprising an elastomer that is resistant to caustic alkali and has a smooth and non-porous surface, forming an exterior of the device except at the or each window. The use of glassy carbon for the windows makes the probes resistant to corrosion and scaling without impeding the effectiveness of the device. Further improved scaling resistance is provided by an elastomeric material preferably used as the external protective cover.