Heat production rates of chemical reactions or of biological cultures are a valuable signal, able to provide crucial information about the system under investigation. Commercially available bench-scale calorimeters reach a sensitivity of approximate to 100 mW/l. This sensitivity is usually sufficient for investigations in the field of reaction engineering or for safety studies, For the investigation of processes that exhibit only small exothermic or endothermic effects on the observation of the growth of biological material, the sensitivity of the calorimeter needs to be improved. This paper describes the modifications undertaken to reach a sensitivity of a few milliwatts per Liter with an RCl from Mettler-Toledo.