Anaerobic degradation processes take place in the absence of oxygen. A septum sealed bottle nozzle fills the head space above the sample with inert gas. When anaerobic degradation has taken place, the dissolved CO2 can be driven off and then removed from the head space by means of a CO2 absorber. The resulting pressure difference is proportional to the CO2 concentration; the remaining overpressure is proportional to the methane concentration. The degradation process can be conveniently observed in the “pressure” operating mode.