In metal processing, metal cleanliness is a crucial process parameter for product manufacturing and recycling and, therefore, on-line ultrasonic methods to evaluate the cleanliness of molten metals are highly desired. In order to establish a procedure and to investigate the optimum system configurations, a water simulation system of ultrasonic detection has been constructed in this thesis. Measuring the concentrations, counting the numbers and measuring the size distributions of the polyvinyl chloride (PVC) particles in water is performed with the ultrasonic pitch-catch configuration. Ultrasonic signals scattered by the particles are detected and the single particle movement is successfully identified. This study verifies that the water simulation system can be employed to evaluate the cleanliness in water.  Ultrasonic probes directly associated to the system sensitivity are crucial for ultrasonic techniques. In this thesis, several materials and shapes of metallic buffer rods are investigated experimentally using an ultrasonic longitudinal wave transducer (UT). Furthermore, the ultrasonic wave patterns of different cross-section shapes of rods are investigated by scanning a small ball bearing immersed in water with pitch-catch mode. Finally, evaluation and recommendations for the application of these high performance metallic buffer rods is presented in this thesis