Final answer:
For regular pressure tests of pressure vessels, a liquid is commonly used because it is relatively incompressible, allowing for the even transfer of pressure and easy detection of leaks, yielding consistent and reliable results.
Step-by-step explanation:
All pressure vessels must be pressure tested on a regular basis, and the fluid used for these tests is commonly a liquid. This is because liquids are relatively incompressible and can effectively transfer pressure evenly throughout the vessel. Testing with a liquid also allows for the easy detection of leaks. When compared to gases, liquids will typically not compress significantly under pressure, providing a more accurate measurement of the vessel's integrity. It is important to use a liquid because it provides more consistent and reliable results when compared to other fluids like gases that may compress too much or oils that may have varying viscosities affecting the pressure readings.