Method for operating a stirring device and a fermenter

A method for automatically operating a digester which is at least partially filled with a substrate, the method comprising:

• a control device operating an agitating device at a plurality of different speeds of rotation at a first height position in the digester, and capturing an amount for a required power of the agitating device at the plurality of different speeds of rotation to determine a first characteristic load curve of the substrate;
• the control device displacing the agitating device in height to a second height position in the digester;
• the control device operating the agitating device at a plurality of different speeds of rotation at the second height position in the digester, and capturing an amount for a required power of the agitating device at the plurality of different speeds of rotation to determine a second characteristic load curve of the substrate;
• performing at the first and the second height position an agitating cycle, respectively, each comprising:
• prescribing, by the control device, a target speed of rotation;
• operating, by the control device, the agitating device at an actual speed of rotation corresponding to the target speed of rotation;
• capturing, by the control device, an actual measurement value which is characteristic of a torque of the agitating device at the actual speed of rotation;
• deriving, by the control device, from the actual measurement value an actual characteristic value of an applied torque of the agitating device;
• comparing, by the control device, the actual characteristic value derived at the first height position against a target characteristic value of the substrate resulting from the first characteristic load curve at the target speed of rotation and the actual characteristic value derived at the second height position against a target characteristic value of the substrate resulting from the second characteristic load curve at the target speed of rotation;
• and controlling, by the control device, the agitating device based on results of comparison.