An equipment for simultaneous nuclei and biological activity measurements in deep sea water

Cavitation, a main cause of noise on ship propellers, is a phase change phenomenon due to pressure decreasing instead of temperature increasing. This phenomenon is very sensitive to the water quality (dissolved gases, tension) and particularly, to the existence of microbubbles in the water. Those bubbles, typically a few micrometers in diameter, are named cavitation nuclei. They are filled with non-condensible gas and act as promoters during the cavitation inception phase. Nuclei counting techniques are now well known in laboratories. Three main classes can be distinguished : optical (holography, phase Doppler), acoustic or hydrodynamic (venturi). Since 1980, test facilities and more particularly water tunnels have been designed with microbubbles injectors and systems for nuclei content evaluation, however, to respect similarity laws, and to acquire a better knowledge of the behavior of nuclei at sea, it is necessary to realize measurements in situ with various climatic conditions. This kind of experiment has been realized in the recent past using a venturi, acoustic or optical methods. Some studies tried to associate the existence of micro-bubbles with biological activity. In the present program an attempt is made to correlate biological activity and nuclei distribution in sea water. This particular paper describes the instruments used to realize this program and presents, as examples, the preliminary results obtained during evaluation tests