Abstract: With increasing awareness of environment protection, more and more scholars fix their attention on the natural fish damage caused by hydraulic machinery. The gas access system composed of a pressure vessel, a vacuum pump and an air compressor is used to simulate the pressure changing courses in hydraulic machinery, and the digital signal system composed of a pressure sensor, an amplifier, an A/D card and a computer is used to get the pressure-time data. After each group of experiment, fishes injured by pressure gradient are checked and anatomized carefully. In present studies, it is demonstrated that fishes are severely injured under different pressure gradients during subatmospheric conditions, and the maximum pressure gradient safe for fishes passing through hydraulic machinery is concluded, which can provide reference databases for the design of new environmentally friendly hydraulic machinery.

Key words: Fish damage, Hydraulic machinery, Pressure gradient, Simulating experiment, Subatomospheric condition