NFPA 54 National Fuel Gas Code Practice Test

The key idea is that piping tests for gas systems use an inert, noncombustible gas to safely check for leaks. Nitrogen fits this role well: it’s readily available, inexpensive, and, because it’s inert, won’t react with the fuel gas or create a flammable or explosive atmosphere if a leak is present. Helium can be used for highly sensitive leak detection, but it’s costly and not the standard choice for routine pneumatic tests. Argon is also inert but even more expensive and not commonly used for general testing. Oxygen, by contrast, would pose a safety risk because it can support combustion and accelerate oxidation if leaks occur. So, nitrogen is the preferred testing medium for piping tests to keep the procedure safe while effectively revealing leaks.