In this paper, an experimental system was constructed by a vacuum pump, a suction room, two sets of ball
valve and sieve of longitudinal. In the system, the fry was sucked from head. In order to hold invariable suction
velocity, the suction pressure of the system was adjusted by tank of pressure control. By using two sets of laser
sensor set up outside wall of a measuring pipe (or U-shaped ditch), the body length of the fry inside the pipe (or
U-shaped ditch) was measured when the fry drained from the system. The ability of the laser wave to pass
through pectoral fins, the effects of the existence of water, diameter of the measurmg pipe and the fry velocity
on the accuracy of measuring fry body length were discussed. Measuring error caused by laser wave diffusion
was also analyzed. The results were as follows.
1. When fry body width (13 -18mm) was nearly equal to the imer diameter (20mm) of a measuring pipe,
the measurement of fry body length was most accurate as the laser wave could pass through the pectoral fin.
2. Whether the velocity of the fry inside a pipe flowing water rose or not, measuring accuracy didn't drop.
When measuring fry body length, it was necessary to adjust the velocity of the fry inside the U-shaped ditch to
below 2m/s, because measuring accuracy dropped if the velocity rose to 2m/s.
3. As this system had only a minimal measuring error (0.0959mm) caused by laser wave diffusion, it could
be considered an effective method for measuring fry body length.