For dimensioning and planning plants and apparatuses as well as optimizing, the collection of rheological data is indispensable. Characterizing bio-suspensions rheologically is often impossible or can be done only with insufficient precision because of their physical properties. Using a special large-scale tube viscometer that has been developed at Hanover University of Applied Sciences and Arts, rheological data from bio-suspensions were collected directly in agriculture biogas plants without any pretreatment. The sizes measured are then evaluated and corrected, so that the final absolute viscosity data can be used.