Turbidity is an important index of water quality.In order to rapidly and accurately measure turbidity of water

a new type of hand-hold immersion laser turbidimeter is designed.The method of measuring turbidity is based on transmitted light mode.The 850nm low power laser-diode with photo diode（PD） is used as light source. The laser-diode generates modulated beams at 1 kHz controlled by transmitting circuit.The light intensity is kept constant by the PD BPW82 photodiode receives the light beam transmitting from the samples.The DC restoration circuit is used to eliminate inteference of low-frequency background light.Turbidity signal is obtained through precise rectification circuit and filtering circuit from the modulating signal.The design of circuit is simplified and logarithmic amplifier is left out due to the operation ability of microprocessor.Calibration algorithm and storing methods of floating point number is given.