Which test medium should never be used for pressure testing gas piping?

The reason oxygen should never be used as a test medium for pressure testing gas piping is primarily due to its highly reactive nature. Oxygen can support combustion and significantly increase the risk of fire and explosion, particularly around fuel gases like propane or natural gas. When performing pressure tests, safety is paramount, and the use of oxygen poses serious hazards compared to inert gases.

In contrast, nitrogen and helium are commonly used as they are inert and do not react with natural gas. Carbon dioxide, while not as ideal as nitrogen or helium, is less dangerous than oxygen. Thus, the choice of test medium is crucial for maintaining safety during gas piping pressure tests, which underscores why oxygen is unacceptable as a test medium.