The piezoelectric grain loss detection has been established for mobile machinery, and is also suitable for process monitoring of grain pre-cleaning. For this purpose, an acceleration sensor measured vibrations of the grain impact of a baffle plate and converted it into an electronic signal which is proportional to the loss. A variety of pre-cleaners, that on the market today, are not electronically controlled or regulated, and a feedback from the loss to the control unit is not provided. The characterization of the sensor system was performed on the new test rig with a grain dose unit. Numerous measurements with different species of crop verify a deviation in the relevant range of less than 3 %.