Appliance and process for the conversion of gases by the controlled variation of gas quantities. A known volume (V1) is enclosed by 4 valves and is provided with a pressure sensor. The total gas quantity is determined before the gas passes into a sampling volume. The gas flow into the volume (V1) is throttled in order to achieve a volume filling dependent on the valve-opening time. A controlled quantity change can be achieved by adding or pumping off the gas in steps.
