Waste paper or pulp bales are transported on a conveyor belt to the pulper, where they are broken down by adding process water. A stirrer speeds up to separate fibers. Difficult process conditions exist in the pulper: falling bales cause severe pressure shocks, the stirrer creates vortexes. Besides that, foreign substances like wire, glass or sand enter the process along with the waste paper and have an extremely abrasive effect on the interior of the vessel. To ensure an automatic process flow, the level measuring system must establish the ratio of waste paper/pulp to process water. In addition, a possible jamming of the bales on the conveyor belt must be detected.