A method comprising: providing a reference leak generating device which is connected to an upstream side of a measurement chamber, wherein the reference leak generating device comprises a first chamber connected to the measurement chamber through an orifice or a porous plug having a molecular flow conductance C and a pressure to establish molecular flow conditions which are known in advance, and characterized in that the method further comprises: determining a pressure p 1 of testing gas which is introduced into the first chamber by a static expansion method one or more times, wherein the pressure p 1 is set so that the testing gas flowing through the orifice or the porous plug satisfies the molecular flow conditions, and obtaining a leak rate of a reference leak at the pressure p 1 in accordance with a product of C and p 1, wherein the static expansion method is performed by expanding a volume of the testing gas from V 0 to V 0 + V 1 between a second chamber of the volume V 1 which is connected to the upstream side of the measurement chamber and the first chamber of the volume V 0 which is connected to an upstream side of the second chamber, wherein the first chamber 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 the orifice or porous plug, respectively, and wherein, 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. providing a reference leak generating device which is connected to an upstream side of a measurement chamber, wherein the reference leak generating device comprises a first chamber connected to the measurement chamber through an orifice or a porous plug having a molecular flow conductance C and a pressure to establish molecular flow conditions which are known in advance, and characterized in that the method further comprises: determining a pressure p 1 of testing gas which is introduced into the first chamber by a static expansion method one or more times, wherein the pressure p 1 is set so that the testing gas flowing through the orifice or the porous plug satisfies the molecular flow conditions, and obtaining a leak rate of a reference leak at the pressure p 1 in accordance with a product of C and p 1, determining a pressure p 1 of testing gas which is introduced into the first chamber by a static expansion method one or more times, wherein the pressure p 1 is set so that the testing gas flowing through the orifice or the porous plug satisfies the molecular flow conditions, and obtaining a leak rate of a reference leak at the pressure p 1 in accordance with a product of C and p 1, wherein the static expansion method is performed by expanding a volume of the testing gas from V 0 to V 0 + V 1 between a second chamber of the volume V 1 which is connected to the upstream side of the measurement chamber and the first chamber of the volume V 0 which is connected to an upstream side of the second chamber, wherein the first chamber 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 the orifice or porous plug, respectively, and wherein, 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.