Heat-exchanger tubes should not be confused with steel pipe or other types of pipe that are extruded to steel pipe sizes. The outside diameter of heat-exchanger tubes is the actual outside diameter in inches within a very strict tolerance. Heat-exchanger tubes are available in a variety of metals including steel, copper, brass, 70-30 copper-nickel, aluminum bronze, aluminum, and stainless steel. They are obtainable in several wall thicknesses defined by the Birmingham Wire Gauge (B.W.G.), which is usually referred to as the B.W.G. or gauge of the tube. Table 2-1 shows tube outside diameter and B.W.G. gauge commonly used.