The in-situ measurement method of the turbidity of different heights in a vessel was proposed by using a multi sensor, consisting of infrared light emitting diodes and phototransistors. The experimental results showed that the turbidity was able to represent the crystal concentration under the experimental conditions covered in this work. By changing the impeller speed adequately according to the measurement results of the turbidity, an attempt was made to control the crystal size and its distribution and a satisfying result was obtained.