A static expansion method is performed by expanding a volume of a testing gas from V0 to V0+V1 between a second chamber of the volume V1 which is connected to an upstream side of a measurement chamber and a first chamber of the volume V0 which is connected to an upstream side of the second chamber, wherein the first camber is in communication with the second chamber via a first valve, wherein the second chamber is in communication with the measurement chamber via each of a second valve and an orifice or porous plug, respectively. When the first valve is opened and the second valve is closed, the testing gas flows from the first chamber via the second chamber into the measurement chamber only through the orifice or porous plug.