A method and apparatus for quantitative DGA (Diluted Gas Analysis), providing quantitative on-line readings of gas production/consumption that directly corresponds to the extent of faults in a power transformer. This quantitative DGA method uses the oil filling of the main tank of a power transformer as a measuring capacity, enabling a physical and mathematical description of the dynamic behavior of all relevant gases in its main tank and in its conservator. A strong dynamic change of the content of a calibration gas in the oil filling of a main tank of the transformer is deliberately induced for the determination of the oil throughflow between the main tank and the conservator. Subsequently, volumetric flows of all relevant gases in a transformer are calculated from a predetermined oil throughflow, dynamic changes, and measured levels of all relevant gases that are in the main tank and the conservator.