Ultrasonic flow meters with the accuracy of ±1% were employed to measure the flows of heat transfer fluid. The biogas production rate was recorded by LMF-1 type wet gas flow meters (with an accuracy of ±1%) located on the biogas outlet pipes of each digester. Since the most concerned parameters are the temperature and the biogas production rate, and no further calculations were processed in this paper, so there is no need to conduct a further error analysis for the experimental test.