Before charging an R-11 refrigeration system with liquid, what vacuum pressure must be achieved?

When charging an R-11 refrigeration system with liquid, it is essential to achieve a sufficient vacuum pressure to ensure that the system is free from moisture and air. The correct vacuum pressure to aim for is 30 inches of mercury. Achieving this vacuum level effectively removes most of the non-condensables and moisture from the system, which is critical for maintaining the system's efficiency and preventing damage.

Moisture can lead to the formation of acids and other corrosive substances within the refrigeration system, and air can cause pressure imbalances and reduce efficiency. Therefore, a vacuum of 30 inches of mercury creates an environment suitable for the introduction of R-11, allowing it to function optimally without contaminating factors.

The other options, while they represent vacuum levels, do not provide the level of assurance needed for proper system operation. Lower vacuum pressures may not adequately eliminate all moisture and air, which can compromise the system's function and longevity.