Proof Pressure test. This is carried out when the required thickness of all the pressure parts has not been accurately calculated or is in doubt. Its objective is to demonstrate the integrity of the pressure equipment. Proof pressure testing should only be carried out hydraulically and the pressure applied gradually until the specified test pressure is reached or until significant yielding of any part of the pressure equipment occurs. A method to determine significant yield is contained in British Standard 5500: 1997, amended 1998, Specification for unfired fusion-welded pressure vessels.