In a stirred cylindrical tank, disks and ring disks were used to divide the tank into four stages. Saturated solution of KCl was used as tracer and a special system of conductivity meter and computer was used to collect data. Influences of different parameters of the structure and operation to residence time distribution (RTD) were measured. The key parameters to back mixing were the gap between the ring disk and the tank wall, the inner diameter of the ring disk and outer diameter of the disk, while the s...