Abstract: A sensor apparatus for detecting sludge in a tank e.g. an oil storage tank, wherein sonar tarnsmission and reception is effected by a transducer (29) whose acoustic center is coincident with the center of curvature of a liquid filled part spherical sonar dome part (15) of a housing (13) for the apparatus. The acoustic pressure waves consequently pass through the dome part of the housing in a direction normal to its surfaces thus minimizing velocity, amplitude and phase changes of the waves as they pass to and from the transducer, and avoiding blind spots in the area monitoried by sensor apparatus. Preferably, the apparatus further includes an arrangement (31, 33, 35, 37) within the housing for angularly moving the transducer (29) about two orthogonal axes which intersect at said center of curvature.