a method of calibrating a pneumatic conveying apparatus wherein , with the material supply interrupted , a conveying pressure is numerically integrated and the reduction in the filling weight occurring in a specific period of time is determined . before the beginning and at the end of the integration it is determined whether the filling weight is falling approximately evenly , i . e ., whether any disturbing dynamic forces are overlying the filling weight . such a calibration method permits alterations in the conveying capacity during the calibration .