A method of setting a flow coefficient for a fluid pipe of given cross-sectional area, comprising the steps of: obtaining an optimal approximate line using a number n of consecutive sets of data points (Xi, Yi), each point (Xi, Yi) comprising a flow velocity value (Xi) along the fluid pipe and a corresponding flow coefficient value (Yi), of all flow velocity data points stored in a flow velocity data memory section for storing flow velocity data (Xi) measured by a flow velocity measurement section, and reference flow rate data (Qa) stored in a reference data memory section, said reference flow rate data with a measured flow velocity value (Xi) and the given cross-sectional area allowing to determine the flow coefficient value (Yi); selecting sets of data points among the n consecutive sets of data points so that the error of the data points in each set with respect to the optimal approximate line are within a predetermined range of error Er; setting a region by a flow coefficient calculation section, said region corresponding to a finite interval of flow velocity values including flow velocity values corresponding to neighboring data points (Xi, Yi) interpolated by said optimal approximate line; and storing a calculated flow coefficient value (Kc) in a flow coefficient memory section, said calculated flow coefficient value (Kc) being determined by the flow coefficient calculation section from the optimal approximate line; and wherein when a type of a fluid changes from a first fluid to a second fluid, the flow velocity value (Vm(air)) of the first fluid is multiplied by a fluid-type-dependent constant so as to calculate a flow velocity value of the second fluid and thus so as to convert the flow coefficient for the first fluid to a new flow coefficient for the second fluid; wherein the constant is a flow velocity ratio Rv which is obtained by dividing the flow velocity value (Vm(Gas, 0.80)) of the second fluid by the flow velocity value (Vm(Air, 0.80)) of the first fluid at the same flow coefficient. obtaining an optimal approximate line using a number n of consecutive sets of data points (Xi, Yi), each point (Xi, Yi) comprising a flow velocity value (Xi) along the fluid pipe and a corresponding flow coefficient value (Yi), of all flow velocity data points stored in a flow velocity data memory section for storing flow velocity data (Xi) measured by a flow velocity measurement section, and reference flow rate data (Qa) stored in a reference data memory section, said reference flow rate data with a measured flow velocity value (Xi) and the given cross-sectional area allowing to determine the flow coefficient value (Yi); selecting sets of data points among the n consecutive sets of data points so that the error of the data points in each set with respect to the optimal approximate line are within a predetermined range of error Er; setting a region by a flow coefficient calculation section, said region corresponding to a finite interval of flow velocity values including flow velocity values corresponding to neighboring data points (Xi, Yi) interpolated by said optimal approximate line; and storing a calculated flow coefficient value (Kc) in a flow coefficient memory section, said calculated flow coefficient value (Kc) being determined by the flow coefficient calculation section from the optimal approximate line; and wherein when a type of a fluid changes from a first fluid to a second fluid, the flow velocity value (Vm(air)) of the first fluid is multiplied by a fluid-type-dependent constant so as to calculate a flow velocity value of the second fluid and thus so as to convert the flow coefficient for the first fluid to a new flow coefficient for the second fluid; wherein the constant is a flow velocity ratio Rv which is obtained by dividing the flow velocity value (Vm(Gas, 0.80)) of the second fluid by the flow velocity value (Vm(Air, 0.80)) of the first fluid at the same flow coefficient.