A method and apparatus for determining the theoretical internal diameter of a tube of unknown internal diameter comprising an air regulation system comprising a pressure regulator and a volumetric flow meter for obtaining an actual volumetric flow rate through the tube of unknown internal diameter. Calculating a theoretical volumetric flow rate for a range of tubes of known internal diameters. Calculating the theoretical internal diameter of the tube of unknown internal diameter, wherein the actual volumetric flow rate obtained from the apparatus is interpolated with the respective theoretical volumetric flow rates of the tubes of known internal diameter to obtain the theoretical internal diameter of the tube of unknown internal diameter.