The present invention pertains to a plumb bob device for determining the bed height of bulk material in a shaft furnace. The winding drum 1 for unwidning and rewinding a cable 5 or a chain with a plumb bob 6 suspended at its end is driven by a hydraulic motor 3. A displacement transducer 9 is driven by the shaft of the winding drum 1. The motor for driving the winding drum 1 is a so-called low-speed hydraulic motor 3. The displacement transducer 9 sends a signal associated with the actual measured depth to a pressure-proportional valve 10 directly, i.e., without an electrical control device. This valve presets a plumb bob holding pressure associated with the actual depth for the hydraulic motor 3.
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