The gas meter consists of two vessels, the plastic one (26 cm diameter) placed upside down inside the metal one (40 cm diameter). A metal tube (1 cm diameter) was sealed to the metal vessel bottom, and it was bent up from the central point to a height of 50 cm. The biogas line from the digester was connected to this tube. The metal cylinder was then filled with water, up to the height of the metal tube. Biogas production is measured by the displacement of the plastic vessel. By knowing this height (displacement) and the diameter of the plastic cylinder, the volume of biogas produced is calculated.