What should NOT be used as the sole medium for pressure testing?

Using air or other gases as the sole medium for pressure testing is not advisable due to the inherent risks associated with gas compression. Unlike liquids, gases are compressible, which can lead to unpredictable reactions in the event of a sudden release of pressure. This can result in explosive decompression or insufficient pressure control during the testing process.

On the other hand, water, oil, and steam are generally considered safer alternatives for pressure testing. These media do not have the same level of compressibility as gases, allowing for a more controlled and consistent test. Water, for example, is often used because it is incompressible, providing a clear indication of leaks under pressure. Oil can also be used in specific applications, particularly in systems designed to handle it, while steam is effective in applications where temperature and pressure play a critical role, such as in certain types of boilers or heating systems.

Choosing a medium like air increases the potential for safety hazards, making it unsuitable as the only option for pressure testing.