A high-precision corrosion monitoring sensor assembly is provided for permanent mounting on a vessel to measure wall thickness of the vessel. The assembly includes an ultrasonic transducer and a delay line, which are bonded to the vessel wall a first location. The assembly is further bonded to the vessel by an adjustable fixture. The assembly includes a cross-member attached to the ultrasonic transducer, having a pivot pin at each distal end. The pivot pins are attached to a first tower and a second tower, which bond to the outer vessel wall at a second location and a third location. The position of the first and second towers can be adjusted with respect to angle and placement about the pivot pins. Fasteners are attached to the pivot pins and can be tightened to make rigid the fixture, thereby allowing configuration of the assembly for use on flat surfaces and small-diameter pipes.