Which material should never be used to leak-test flammable or oxidizing gas cylinders?

Using petroleum to leak-test flammable or oxidizing gas cylinders is highly inappropriate due to the properties of petroleum itself. As a flammable substance, petroleum poses an increased risk of ignition when in contact with flammable gasses. If there were a gas leak and petroleum were present, it could create a hazardous situation, potentially leading to explosions or fires.

Alternatives such as soap solutions or water are generally safe for leak testing. Soap solutions can effectively identify leaks through visible bubbles that form when gas escapes, while water can also indicate leaks through the formation of bubbles. However, care must be taken when using water with certain types of gases or cylinders. Always prioritize safety by choosing non-flammable materials for such testing scenarios.