The lockup pressure test verifies the pressure at which?

The lockup pressure test specifically verifies the pressure at which the regulator locks up when no propane is flowing. This is a crucial aspect of ensuring that the system operates safely and efficiently. During the test, if propane is not flowing through the system, the regulator should maintain a certain pressure without allowing gas to escape. Achieving the correct pressure indicates that the regulator is functioning properly and that the system is sealed, preventing leaks and ensuring safety when the system is idle. This is paramount in propane systems because any leakage could lead to dangerous situations.

The other options relate to different functions within the propane system but do not align with the purpose of the lockup pressure test. For instance, testing for when a tank is full or determining the maximum capacity of a system are not relevant to the lockup test's focus on regulator performance when gas is not being utilized. Similarly, the opening of the pressure relief valve pertains to a different safety mechanism entirely, designed to release excess pressure, rather than observing the lockup state of the regulator.