A compromise between a reliable phase-diameter relationship and a good imaging of the control volume is necessary for the application of the PDA-instrument when measuring cavitation nuclei. The phase-diameter relationship for bubbles and particles were investigated theoretically, the imaging of the control volume experimentally. Further PDA-measurements were conducted for Latex spheres of different diameters as scattering objects in water and single air bubbles in transparent plastics. The concentrations and diameters measured by the PDA agreed very well with the results obtained by other methods. An investigation of the scattering from background particles in water was conducted with the PDA in the small HSVA cavitation tunnel. Also, nuclei measurements were performed with a gassing system in the tunnel. In 1972 the ITTC-Cavitation Committee defined specifications for the control of the free air content in routine and research cavitation tests. The tests completed so far show that the PDA-instrument fulfills these specifications for the major part.