Why a half-inch pipe measures nothing like half an inch
Measure a half-inch iron pipe and you will find no dimension that is actually half an inch. Early pipes of that name did have a half-inch bore, but thick walls. As metal got better, walls thinned while the outside stayed the same so new pipe could still join old, and the name simply stayed behind.
Pipes are among the oldest pieces of plumbing engineering. The Pyramid of Sahure in Egypt, completed around the 25th century BC, had a temple drained by more than 380 metres of copper piping. Pipes carry far more than water: gas and oil across continents, powders and bulk solids through food plants, steam through high-pressure factories. Because a hollow tube is much stiffer for its weight than a solid bar, pipe also turns up as scaffolding, handrails and support frames.
Modern metal pipe grew out of gunmaking. During the Napoleonic Wars, Birmingham gunsmiths tried rolling iron musket barrels, and one of them, Henry Osborne, perfected a workable method in 1817. Around 1820 he began turning out iron tubes for gas and sold some to the gas-lighting pioneer Samuel Clegg. Early steel pipes were riveted, then clamped with H-shaped bars, until welding took over by the early 1930s.
There are still competing ways to make a pipe. Seamless pipe is made by forcing a solid billet over a piercing rod so no weld is needed, which long made it the trusted choice for high pressure. Since the 1970s, better materials, process control and inspection have let properly specified welded pipe, rolled from plate or coil and joined along the seam, replace seamless in many jobs, often more cheaply and to tighter tolerances. Engineers also distinguish pipe from tube: pipe is sized by a nominal diameter and a wall thickness schedule, while tube is usually sized by its actual measurements.
The sizing systems remain charmingly odd. For pipe smaller than 14 inches, the nominal size is only a rounded label, so a 2-inch steel pipe really measures about 2.375 inches across the outside. A higher schedule number means thicker walls and a smaller bore within the same outside diameter. Copper went the same way: a 1930s one-inch copper pipe was named for its bore, and modern thinner walls have made that bore merely nominal too.
Source: Pipe (fluid conveyance)
