Hydrostatic tests are conducted under the constraints of either the industry's or the customer's specifications. The vessel is filled with a nearly incompressible liquid - usually water or oil - and examined for leaks or permanent changes in shape.
Red or fluorescent dyes are usually added to the water to make leaks easier to see. The test pressure is always considerably more than the operating pressure to give a margin for safety, typically 150 percent of the design pressure. An example would be if a cylinder was rated to DOT-2015 PSI it would be tested at around 3360 PSI to ensure maximum usage and to provide more safety. Water is commonly used because it is almost incompressible (compressible only by weight, not air pressure), so will only expand by a very small amount should the vessel split. If high pressure gas were used, then the gas would expand to perhaps several hundred times its compressed volume in an explosion, with the attendant risk of damage or injury. This is the risk which the testing is intended to mitigate.