A method for operating a tissue processor is described. During the operation of a tissue processor a reagent is fed from a container into a retort of the tissue processor, which retort does not contain any tissue samples. Thereafter, the reagent is returned into the container. During feeding, returning and/or in between feeding and returning a measured value is detected by a sensor of the tissue processor. This value is representative of a concentration of the reagent. During this feeding, this returning and/or in between no tissue sample is placed in the retort.