The rotating measuring blade projecting into the vessel is driven by a gearmotor.When the bulk goods heap up to the level of the blade, this prevents the blade from turning and it comes to a standstill.A second switch turns the motor delayed off.Should the level of the bulk goods drop and the measuring blade can turn freely, a spring brings the motor back into its original end position. At the same time the motor is turned on again and the signal switch is reset. The return torque turns the fitted motor back from its end position and actuates the signal switch by a switching arm.